Small-Cell Lung Cancer, Mesothelioma, and Thymoma

April 16, 2009

As discussed in chapter 6, there are two major subdivisions of lung cancer: small-cell lung cancer (SCLC), for which chemotherapy is the primary treatment, and non–small-cell lung cancer (NSCLC). SCLC is decreasing in frequency in the United States, with recent data showing it represents only 14% of lung cancers. This chapter provides information on the staging and prognosis, pathology and pathophysiology, treatment, and follow-up of long-term survivors of SCLC and concludes with brief discussions on mesothelioma and thymoma.

As discussed in chapter 6, there are two major subdivisions of lung cancer: small-cell lung cancer (SCLC), for which chemotherapy is the primary treatment, and non–small-cell lung cancer (NSCLC). SCLC is decreasing in frequency in the United States, with recent data showing it represents only 14% of lung cancers. This chapter provides information on the staging and prognosis, pathology and pathophysiology, treatment, and follow-up of long-term survivors of SCLC and concludes with brief discussions on mesothelioma and thymoma.

Chapter 6 provides information on the epidemiology, etiology, screening and prevention, and diagnosis of lung cancer in general and covers NSCLC and carcinoid tumors of the lungs.

SMALL-CELL LUNG CANCER

Staging and prognosis

The TNM staging system, used for all NSCLC patients, does not predict well for survival in SCLC patients and is generally not utilized in SCLC, except for surgical staging (see chapter 6, Table 1). Rather, SCLC is usually described as either limited (M0) or extensive (M1), although these general terms are inadequate when evaluating the role of surgery. Patients with SCLC who have stages I–III disease, excluding those with a malignant pleural effusion, are classified as having limited disease. These patients constitute approximately one-third of all SCLC patients. The remaining SCLC patients fall into the extensive-disease category, which includes any patient with a malignant pleural effusion or any site of distant disease, such as the brain, liver, adrenal gland, bone, and bone marrow.

The staging of lung cancer must be conducted in a methodical and detailed manner to permit appropriate therapeutic recommendations and to allow comparison of treatment results from different institutions.

Stage is commonly reported as either clinical or pathologic. The former is based on noninvasive (or minimally invasive) tests, whereas the latter is based on tissue obtained during surgery (see chapter 6).

The most important prognostic factor in lung cancer is the stage of disease. Within a given disease stage, the next most important prognostic factors are performance status and recent weight loss. The two scales used to define performance status are the Eastern Cooperative Oncology Group (ECOG) performance status system and the Karnofsky performance index (see Appendix 1). In short, patients who are ambulatory have a significantly longer survival. Those who have lost ≥ 5% of body weight during the preceding 3 to 6 months have a worse prognosis.

Pathology and pathophysiology

SCLC tends to present with a large central lung mass and associated extensive hilar and mediastinal lymphadenopathy. Clinically evident distant metastases are present in approximately two-thirds of patients at diagnosis. Additionally, data from autopsy examination indicate micrometastatic disease in 63% of patients who died within 30 days of attempted curative resection of SCLC. Thus, it is a systemic disease at presentation in the majority of patients.

SCLC is a small, blue, round cell tumor that is primitive and undifferentiated at the light microscopic level. Electron microscopy demonstrates its neuroendocrine derivation by the presence of dense core granules. The immuno-histochemical evidence of neuroendocrine derivation includes positive staining for chromogranin, synaptophysin, and other proteins. The APUD (amine precursor uptake and decarboxylation) machinery present in the dense core granule leads to the production of biologically active amines and promotes the synthesis of polypeptide hormones such as ADH and ACTH. Paraneoplastic syndromes due to hormone excess result. The most common of these syndromes, syndrome of inappropriate antidiuretic hormone secretion (SIADH), occurs in approximately 10% of patients with SCLC. Hypercortisolism and a Cushing's-like syndrome are more rare, seen in only 1% to 2% of patients.

Treatment

TREATMENT OF DISEASE LIMITED TO LUNG PARENCHYMA

Surgery

The majority of patients with SCLC present with advanced-stage disease. In the 5% to 10% of patients whose tumor is limited to the lung parenchyma, very often the diagnosis is established only after the lung mass has been removed. If, however, the histology has been determined by bronchoscopic biopsy or fine-needle aspiration and there is no evidence of metastatic disease following extensive scanning, examination of the bone marrow, and biopsy of the mediastinal lymph nodes, resection should be performed. Adjuvant chemotherapy is recommended because of the high likelihood of the development of distant metastases following surgery.

The surgical approach in SCLC is similar to that used in NSCLC: A lobectomy or pneumonectomy should be followed by a thorough mediastinal lymph node dissection. Tumor resection in SCLC should be limited to patients who have no evidence of mediastinal or supraclavicular lymph node metastases. Recent data suggest that patients with SCLC, presenting as a solitary pulmonary nodule and proven pathologically to be stage I, have a 5-year survival rate of ~70% when treated with resection and adjuvant chemotherapy.

TREATMENT OF DISEASE LIMITED TO THE THORAX

Approximately one-third of SCLC patients present with disease that is limited to the thorax and can be encompassed within a tolerable radiation portal. In early studies in which either radiation therapy or surgery alone was used to treat such patients, median survival was only 3 to 4 months, and the 5-year survival rate was in the range of 1% to 2%. The reason for the failure of these therapies was both rapid recurrence of intrathoracic tumor and development of distant metastasis.

Chemotherapy

During the 1970s, it became apparent that SCLC was relatively sensitive to chemotherapy. Various combination chemotherapy regimens were used to treat limited SCLC. Although none of the regimens was clearly superior, median survival was approximately 12 months, and the 2-year survival rate was approximately 10% to 15%. It appears that maintenance chemotherapy adds little to survival in patients with limited SCLC.

Chemotherapy plus thoracic irradiation

One of the major advances in treating SCLC in the past 15 years is the recognition of the value of early and concurrent thoracic chemoradiation therapy. This advance was clearly facilitated by the increase in therapeutic index when PE (cisplatin [Platinol]/etoposide) chemotherapy is given with thoracic irradiation, as opposed to older anthracycline- or alkylator-based regimens. Although the major impact from this approach is improved locoregional control, there are also hints from randomized trials that early control of disease in the chest can also reduce the risk of distant metastasis.

An intergroup trial directly compared once-daily with twice-daily fractionation (45 Gy/25 fractions/5 weeks vs 45 Gy/30 fractions/3 weeks) given at the beginning of concurrent chemoradiation therapy with PE. Initial analysis showed excellent overall results, with median survival for all patients of 20 months and a 40% survival rate to 2 years. With a minimum follow-up of 5 years, survival was significantly better in the twice-daily than in the once-daily irradiation group (26% vs 16%). The only difference in toxicity was a temporary increase in grade 3 esophagitis in patients receiving twice-daily radiation therapy.

Outcomes for patients with limited-stage SCLC have improved significantly over the past 20 years. In an analysis of phase III trials during this period, median survival was 12 months in the control arm in 26 phase III studies initiated between 1972 and 1981, compared with 17 months in studies between 1982 and 1992 (P < .001). Five studies demonstrated a statistically significant improvement in survival in the experimental arm compared with the control arm. Interestingly, all five studies involved some aspect of thoracic radiation therapy (three trials compared chemotherapy alone vs chemoradiation therapy; one compared early with late radiation therapy; and one compared daily vs twice-daily thoracic radiation therapy). Similarly, data from the Surveillance, Epidemiology, and End Results (SEER) database demonstrate that the 5-year survival rate has more than doubled from 1973 to 1996 (5.2% vs 12.2%; P = .0001).

Current recommendations Although important questions remain as to the optimal radiation doses, volumes, and timing with regard to chemotherapy, a reasonable standard is to deliver thoracic irradiation concurrently with PE chemotherapy (cisplatin [60 mg/m2 IV on day 1] and etoposide [120 mg/ m2 IV on days 1–3]). An attempt is made to integrate thoracic irradiation as early as possible, during cycle 1 (or 2). Fried et al performed a meta-analysis evaluating early versus late timing of radiation therapy in limited-stage SCLC. Earlier radiation therapy was defined as prior to 9 weeks after initiation of chemotherapy versus late radiation therapy (≥ 9 weeks). Seven trials (n = 1,542 patients) were included in the analysis. They reported a small but significant improvement in 2-year overall survival for early versus late radiation therapy (5.2%; P = .03). This finding is similar to the benefit of adding radiation therapy or prophylactic cranial irradiation to chemotherapy. A greater difference was evident for the subset of patients receiving early rather than late hyperfractionated radiation therapy and platinum-based chemotherapy. Hyperfractionated accelerated fractionation should be considered, given the results of the intergroup 0096 trial. An intergroup phase III study is under way to compare twice-daily radiation therapy to 45 Gy versus once-daily with radiation therapy to a higher dose (70 Gy) versus a modified regimen combining these strategies.

Irradiation can be incorporated sequentially with chemotherapy; however, this approach appears to be inferior to early concurrent therapy and should be reserved for use in those for whom concurrent approaches are predicted to be excessively toxic. Takada et al reported on a randomized trial of concurrent versus sequential thoracic radiotherapy in combination with PE in over 200 patients with limited-stage SCLC; they demonstrated a benefit to concurrent therapy, with a median survival of 27.0 months (30%, concurrent arm) versus 19.7 months (20%, sequential arm; P = .097). Thoracic radiation therapy consisted of 45 Gy over 3 weeks, starting either with the first cycle of PE in the concurrent arm or after the fourth cycle in the sequential arm.

Results of an intergroup trial indicate that radiation therapy strategies that increase biologic dose can improve local control and survival. Further exploration of accelerated fractionation or conventional doses > 45 Gy is warranted and is currently being investigated in prospective trials.

Komaki et al reported on a phase I dose-escalation trial of thoracic radiotherapy with concurrent chemotherapy (Radiation Therapy Oncology Group [RTOG] 9712). In this regimen, the initial (larger) radiation field was treated once a day, and the smaller boost field was treated twice daily, to a maximum tolerated dose of 61.2 Gy.

Movsas et al reported the results of the first Patterns of Care Study (PCS) for lung cancer in the United States. This study was conducted to determine the national patterns of radiotherapy practice in patients treated for nonmetastatic lung cancer in 1998–1999. As supported by clinical trials, patients with limited-stage SCLC received chemotherapy plus radiotherapy more often than radiotherapy alone (92% vs 5%; P < .0001). However, the median radiotherapy dose was 50 Gy, 80% at 1.8 to 2.0 Gy per fraction.Only 6% of patients received hyperfractionated (twice-daily) radiotherapy. A total of 22% received prophylactic cranial irradiation (PCI), with a median dose of 30 Gy in 15 fractions. As key studies supporting twice-daily radiotherapy in PCI and NSCLC were published in 1999, the penetration of these trials will be assessed in the next PCS lung survey.

Interestingly, Choi et al reported long-term survival data from their phase I trial assessing chemotherapy with either standard daily radiotherapy or accelerated twice-daily radiotherapy as from the Cancer and Leukemia Group B (CALGB) 8837 trial. They previously reported that the maximum tolerated dose was 45 Gy in 30 fractions for twice-daily radiotherapy and > 70 Gy in 35 fractions for once-daily radiotherapy. The 5-year survival estimated (from this phase I trial) for the twice-daily arm was 20%, versus 36% for the once-daily radiotherapy arm.

Surgery

Although surgical resection is not usually part of the standard therapy for SCLC, the Japanese Clinical Oncology Lung Cancer Study Group reported the results of a phase II trial of postoperative adjuvant PE in patients with completely resected stages I–IIIA SCLC. The 5-year survival rates (in a cohort of 62 patients) for pathologic stages I, II, and IIIA SCLC were 69%, 38%, and 40%, respectively.

The role of surgery for stage II or IIIA SCLC has evolved from a number of phase II trials and retrospective case series to include specific indications; they include resection of tumors with mixed histology (containing both SCLC and NSCLC components), salvage surgery for chemoresistant localized SCLC or local relapse after initial response to chemoradiotherapy, or second primary tumors after cure of initial SCLC. Johnson has shown that the rate of second primary NSCLC in patients treated for SCLC can be as high as 2% to greater than 10% per year.

Prospective, randomized trials are ongoing in Europe and Japan to examine the role of surgery as part of multimodality therapy for patients with stages II and IIIA SCLC.

Prophylactic cranial irradiation

Recognition that patients with SCLC were at high risk for the development of brain metastases led to the suggestion that they be given PCI to prevent the clinical manifestation of previously present but occult CNS disease. The role of PCI has been controversial. Most trials have shown a reduction in CNS relapse rates but little effect on survival with PCI. There also has been concern about the contribution of PCI to the late neurologic deterioration seen in some patients with SCLC, although recent studies show neurologic impairment in many patients with SCLC prior to any treatment.

A meta-analysis of all randomized trials of PCI in patients with SCLC who achieved a complete or near-complete response to induction chemotherapy (alone or combined with thoracic irradiation) showed a statistically significant improvement in survival in patients treated with PCI (20.7% at 3 years vs 15.3% in those not given PCI). The survival improvement with PCI was seen in all patient subgroups, regardless of age, stage of disease, type of induction treatment, or performance status. Approximately 85% of the patients included in the meta-analysis had limited di- sease, and recommendations for use of PCI have been applied generally to this subgroup. A recent randomized trial, however, suggests benefit for PCI in patients with responding extensive disease as well (see sidebar).

Model calculations from data on patterns of failure in patients achieving a systemic complete response suggest that the greatest gain in survival to be expected with PCI is in the range of 5%. To demonstrate this convincingly would require randomized trials of about 700 patients–substantially larger than trials conducted to date. However, the recent meta-analysis of randomized trials of PCI in SCLC patients achieving complete or near-complete response of systemic disease showed a survival improvement of this magnitude with PCI.

Current recommendations Patients should be offered PCI after completion of chemotherapy/chemoradiation therapy if they have clear regression of disease and a retained ECOG performance status of 0 to 2. It should optimally be integrated within 3 to 5 weeks of the last cycle of chemotherapy.

Radiation doses for PCI should probably be in the range of 25 to 30 Gy, with a daily fraction size of 2.0 to 2.5 Gy, although recent data suggest that such doses delay and reduce rates of CNS relapse but may not eliminate it; thus, higher doses may warrant exploration. Larger fraction sizes would be expected to produce greater toxicity. Smaller fractions given twice daily may reduce toxicity, and trials of this approach are under way.

TREATMENT OF EXTENSIVE DISEASE

As mentioned previously, two-thirds of SCLC patients have extensive disease at diagnosis. Without treatment, median survival in this group of patients is 6 to 8 weeks. Treatment with combination chemotherapy increases the median survival duration to approximately 8 to 10 months.

Induction chemotherapy

The combination of cisplatin or carboplatin/etoposide (see Table 1 for common dose ranges) is considered the standard of care in the United States at this time. This standard is primarily based on therapeutic index, as randomized trials have not demonstrated a survival benefit for this combination relative to the older regimen of cyclophosphamide, doxorubicin, and vincristine. The regimen is repeated at 3-week intervals for 4 to 6 courses. In North America, multiple randomized trials of newer cytotoxins replacing etoposide in a doublet with cisplatin, or added to the etoposide/platin base, have not provided a survival benefit. Japanese data that showed a 3.4-month survival advantage for irinotecan (Camptosar), as opposed to etoposide, with cisplatin were not confirmed in a trial in the US. A second North American study, reproducing the Japanese trial, has completed accrual and is pending analysis.

TREATMENT OF PERSISTENT DISEASE

Progressive SCLC is classified based on response and duration of response to initial induction therapy. Patients whose tumors do not regress or progress up to 60 to 90 days following the last cycle of chemotherapy are considered to have refractory disease. Conversely, patients whose tumors respond and who have an unmaintained progression-free interval longer than 60 to 90 days, are deemed to have sensitive relapse. This categorization is based on the probability of objective response to additional cytotoxic therapy, which is uncommon, typically less than 15%, in the case of platin-refractory SCLC.

Topotecan (Hycamtin) is the only drug approved by the US Food and Drug Administration (FDA) for the treatment of recurrent disease. Its initial indication in 1998 was in patients with sensitive relapse and was based on similar efficacy compared with an older three-drug regimen. A subsequent trial compared intravenous administration with oral topotecan capsules, documenting similar efficacy and tolerance. Most recently, a randomized trial of 141 patients, ECOG performance status of 0 to 2, compared oral topotecan with best supportive care. Patients with both refractory and sensitive disease were accrued. Median survival was nearly doubled on the topotecan arm, 26 versus 14 weeks (P = .0104), as was 6-month survival, 49% versus 26%. Benefit was seen in all subgroups analyzed, including patients with refractory cancer and an ECOG performance status of 2. Despite a low rate of response to topotecan of 7% and typical side effects, treated patients had slower deterioration of quality of life and improved symptom control. Based on these data, the FDA granted topotecan in capsule form a broad indication for treatment of recurrent SCLC in October 2007.

More limited data with irinotecan suggest its activity is probably similar to that of topotecan; however, it has never been evaluated in a randomized trial in the recurrent setting.

Other cytotoxins, known more for their efficacy in NSCLC, such as docetaxel (Taxotere) and paclitaxel, gemcitabine (Gemzar), and vinorelbine, do not have high single-agent response rates in therapy-naive SCLC and are not recognized as standard in management.

Integration of biologics in therapy

Ongoing clinical research is focused on integration of molecularly targeted therapy in an effort to make progress in treating this stubborn malignancy. At this time, data from completed trials do not indicate an active strategy with a biologic, whether in combination with induction chemotherapy, as maintenance following induction, or as single agents for recurrent disease.

High-dose chemotherapy plus bone marrow transplant (BMT) Most phase II trials using high doses of chemotherapy plus BMT appear to show no advantage to the high-dose approach over standard doses of chemotherapy.

Alternating chemotherapy regimens have been used to overcome drug resistance. In randomized trials, alternating chemotherapy regimens have shown a slight improvement in terms of median survival (4–6 weeks) when compared with a single chemotherapeutic regimen but no improvement in long-term survival.

PALLIATION OF LOCAL AND DISTANT SYMPTOMS

Radiation therapy

Many patients with lung cancer have distressing local symptoms at some point in their disease course. These symptoms may arise from airway obstruction by the primary tumor, compression of mediastinal structures by nodal metastases, or metastatic involvement of distant organs. Radiation therapy is effective in palliating most local symptoms as well as symptoms at common metastatic sites, such as bone and brain.

Doses In the United States, most radiation oncologists use doses in the range of 30 Gy in 10 fractions for palliative treatment. Data from the United Kingdom suggest that similar efficacy without greater toxicity may be achieved with more abbreviated schedules, such as 17 Gy in 2 fractions 1 week apart or single fractions of 11 Gy (see chapter 6, Table 8). Such schedules may facilitate the coordination of irradiation and chemotherapy and also may reduce patient travel and hospitalization.

Endobronchial irradiation with cobalt-60 or iridium-192 has been used to palliate symptoms arising from partial airway obstruction, including cough, dyspnea, and hemoptysis. The dosimetric advantage of being able to deliver a high radiation dose to the obstructing endobronchial tumor while sparing adjacent normal structures, such as the lungs, spinal cord, and esophagus, has clear appeal, particularly in the patient whose disease has recurred following prior external-beam irradiation. Although good rates of palliation have been reported with endobronchial irradiation, significant complications, including fatal hemoptysis, are seen in 5% to 10% of patients. Whether this represents a true treatment complication versus the underlying disease remains unclear.

Other local approaches

Endobronchial irradiation should be considered as only one of several approaches (including laser excision, cryotherapy, and stent placement) in the treatment of patients with symptomatic airway obstruction, and management should be individualized. All of these approaches are more suitable for partial than for complete airway obstruction.

Chemotherapy

Several recent trials have explored the use of chemotherapy to palliate specific symptoms in patients with lung cancer. In general, these trials have found that rates of symptomatic improvement were considerably higher than objective response rates and were not dissimilar to symptomatic response rates with local radiation therapy. Chemotherapy in the newly diagnosed patient is highly palliative for relief of symptoms related to superior vena cava syndrome, obstructive lung disease, and painful bony metastases. In the patient with recurrent disease, irradiation is more commonly associated with symptomatic relief from these localized problems. Radiation therapy remains the standard of care for even chemotherapy-naive patients with spinal cord compression or symptomatic brain metastasis.

Follow-up of long-term survivors

At present, no standard follow-up protocol exists for patients with cured SCLC or NSCLC. However, at least long-term follow-up should include serial physical examinations once the patient has reached the 5-year mark. Controversy currently exists about the value of utilizing CT scanning or even chest x-rays for the long-term follow-up of these patients.

In this vein, retrospective reviews of the literature have revealed that patients with SCLC appear to have the highest rate of second primary tumor development, as high as 30% over the course of their lifetime, with some studies reporting annual second primary tumor rates of 5% to 10%. Therefore, the concept of chemoprevention appears to have particular merit in these patients.

MESOTHELIOMA

Mesotheliomas are uncommon neoplasms derived from the cells lining the pleura and peritoneum. Currently, 2,000 to 3,000 new cases are diagnosed in the United States each year.

Epidemiology

Gender Men are affected five times more commonly than women.

Age The median age at diagnosis is 60 years.

Etiology and risk factors

Asbestos exposure The relationship between asbestos exposure and diffuse pleural mesothelioma was first reported by Wagner, who documented 33 pathologically confirmed cases from an asbestos mining region in South Africa. Selikoff and colleagues documented a 300-fold increase in mortality from mesothelioma among asbestos insulation workers in the New York metropolitan region when compared with the general population. The interval between asbestos exposure and tumor formation is commonly 3 to 4 decades.

Asbestos fibers are generally divided into two broad groups: serpentine and amphibole. The latter includes crocidolite, the most carcinogenic form of asbestos. The inability of phagocytic cells to digest the fiber appears to initiate a cascade of cellular events that results in free-radical generation and carcinogenesis.

Diagnosis

Patients with mesothelioma usually seek medical attention while the disease is limited to a single hemithorax and commonly complain of dyspnea and pain. Dyspnea results from diffuse growth of the tumor on both the parietal and visceral pleurae, which encase the lung in a thick rind. Pain is caused by direct tumor infiltration of intercostal nerves.

Chest x-ray and CT Chest x-ray demonstrates pleural thickening, pleura-based masses, or a pleural effusion. Chest computed tomography (CT) scan more accurately portrays the extent of disease and frequently reveals chest wall invasion, as well as pericardial and diaphragmatic extension.

Thoracentesis and thoracoscopy Thoracentesis and pleural biopsy usually are sufficient to establish the diagnosis of malignancy, but a thoracoscopic or open biopsy is often required to provide enough tissue to make an accurate histologic diagnosis of mesothelioma.

Distinguishing mesothelioma from other neoplasms Light microscopy is often insufficient for differentiating among mesothelioma, metastatic adenocarcinoma, and sarcoma. Immunohistochemistry and electron microscopy are frequently necessary to establish the diagnosis.

Although adenocarcinomas stain positive for carcinoembryonic antigen (CEA), Leu-M1, and secretory component, mesotheliomas stain negative for these markers. Mesotheliomas stain positive for cytokeratin, whereas sarcomas do not. Mesotheliomas have characteristically long microvilli that are well demonstrated by the electron microscope; adenocarcinomas have short microvilli.

Pathology

Mesotheliomas may contain both epithelial and sarcomatoid elements and are classified by the relative abundance of each component. Epithelial mesotheliomas are most common (50%), followed by mixed (34%) and sarcomatoid (16%) tumors. Survival for the epithelial type is 22 months,compared with only 6 months for patients with other types.

Staging and prognosis

The International Mesothelioma Interest Group has developed a staging system based on TNM descriptors. This was adopted by the AJCC (Table 2). Another commonly utilized staging system for mesothelioma, that of Butchart, is based on inexact descriptions of the extent of local tumor growth or distant metastases (Table 3). Other, more detailed staging systems based on TNM criteria have been proposed.

The median survival following diagnosis ranges from 9–21 months. Although autopsy series have demonstrated distant metastases in as many as 50% of patients with mesothelioma, death usually results from local tumor growth.

Position emission tomography (PET) scanning may prove to be a viable prognostic tool for malignant pleural mesothelioma.

Treatment

Treatment rarely results in cure and should be considered palliative.

Combined-modality treatment options include chest tube insertion and pleurodesis to control the pleural effusion. Currently, there is renewed interest in aggressive treatment that includes extrapleural pneumonectomy with concomitant resection of the diaphragm and pericardium, followed by chemotherapy and radiotherapy. Recent studies are exploring the role of postoperative intensity-modulated radiotherapy in this setting. Subtotal pleurectomy is a less extensive surgical procedure that debulks the majority of tumor, permits reexpansion of the lung, and prevents recurrence of the pleural effusion.

Chemotherapy The benefit of chemotherapy for patients who have unresectable mesothelioma was clarified in a randomized trial. This study was a single-blind, multicenter, two-arm trial with cisplatin alone in the control arm and cisplatin combined with the multitargeted antifolate pemetrexed (Alimta) in the experimental arm. The study was based on the observation that pemetrexed produced a 16% objective response rate in previous phase II evaluation. In the randomized trial, patients treated with pemetrexed and cisplatin had an estimated median survival of 12.1 months, as compared with 9.3 months in those treated with cisplatin alone. On the basis of this improvement in survival, the combination of pemetrexed and cisplatin has received an FDA indication for the treatment of unresectable mesothelioma. The same combination is undergoing further evaluation in a neoadjuvant approach in patients with resectable disease.

THYMOMA

Thymoma is a rare mediastinal tumor that occurs mainly in the anterosuperior mediastinum.

Epidemiology

Gender The tumor affects both sexes equally.

Age Thymoma is most often seen in people in the fourth and fifth decades of life.

Etiology and associated syndromes

The etiology of thymoma is unknown, and the risk factors have not been identified. Thymoma is a tumor originating within the epithelial cells of the thymus. One-third to one-half of patients present with an asymptomatic anterior mediastinal mass, one-third present with local symptoms (eg, cough, chest pain, superior vena cava syndrome, and/or dysphagia), and one-third of cases are detected during the evaluation of myasthenia gravis. Distant metastases are distinctly uncommon at initial presentation of this tumor.

In addition to myasthenia gravis, which occurs in approximately 30% of patients with thymoma, a host of paraneoplastic syndromes have been seen in association with thymoma. These other syndromes, which occur in less than 5% of patients, include pure red cell aplasia, hypogammaglobulinemia, and a variety of other autoimmune disorders.

Diagnosis

The most commonly described symptoms are pleuritic chest pain or discomfort, dry cough, and dyspnea. Physical examination may reveal adenopathy, wheezing, fever, superior vena cava syndrome, vocal cord paralysis, and other paraneoplastic syndromes.

Chest x-ray and CT scan A chest x-ray provides an initial basis for diagnosis. The location, size, density, and presence of calcification within the mass can all be determined. Comparison of the film to previously obtained films is usually helpful.

Following identification of a mediastinal mass on conventional radiography, contrast-enhanced CT scanning should be performed. CT scanning can differentiate the cystic form from a solid lesion as well as the presence of fat, calcium, or fluid within the lesion. Magnetic resonance imaging (MRI) is increasingly available for use in the evaluation of mediastinal pathology, but it is less frequently utilized than CT. MRI is superior to CT scanning in defining the relationship between mediastinal masses and vascular structures and is useful in the assessment of vascular invasion by the tumor. 

Invasive diagnostic tests CT-guided percutaneous needle biopsy specimens are obtained using fine-needle aspiration techniques and cytologic evaluation or with larger-core needle biopsy and histologic evaluation. Fine-needle specimens are usually adequate to distinguish carcinomatosis lesions, but core biopsies may be necessary to distinguish most mediastinal neoplasms. Immunohistochemical techniques and electron microscopy have greatly improved the ability to differentiate the cell of origin in mediastinal neoplasms. Most series reported diagnostic yields for percutaneous needle biopsy of 70% to 100%.

Mediastinoscopy is a relatively simple surgical procedure accomplished with the patient under general anesthesia. It is an adequate approach to the superior, middle, and upper posterior mediastinum, and most series report a diagnostic accuracy of 80% to 90%. Anterior mediastinotomy (Chamberlain approach) provides for direct biopsy of tissue and has a diagnostic yield of 95% to 100%. Thoracotomy is occasionally necessary to diagnose mediastinal neoplasms, but its indications have been largely supplanted by video-assisted thoracoscopic techniques, which yield 100% accuracy.

The most common tumors in the differential diagnosis of an anterior mediastinal tumor are lymphomas and germ-cell tumors. Immunohistochemical markers are helpful to differentiate thymoma from tumors originating from other cell types.

Pathology

Two of the most common classification schemes for thymoma are listed in Table 4. Verley and Hollman propose a classification system based on tumor architecture, cellular differentiation, and predominant cell type. Bernatz et al describe a simpler classification by presenting thymoma based onthe percentage of epithelial cells and lymphocytes. In both of these systems, thymoma with a predominance of epithelial cells is associated with a greater increased incidence of invasion and a subsequently worse prognosis.

Staging and prognosis

The staging system proposed by Masaoka et al has been widely adopted. Stage is an independent predictor of recurrence and long-term survival. The 5-year survival rates are 96% for stage I thymoma, 86% for stage II, 69% for stage III, and 50% for stage IV.

Treatment

SURGICAL TREATMENT

All patients whose tumors are potentially resectable should undergo surgery. If the patient has evidence of myasthenia gravis, a preoperative consultation with a clinical neurologist should be considered. The incision of choice is almost always a median sternotomy, which is quick and easy to make and provides excellent exposure to the anterior mediastinum and neck. Although the surgeon is considered the best judge of a tumor's invasiveness, it is often difficult to grossly separate invasion from adherence to surrounding tissue. Experience with minimally invasive approaches (such as transcervical thymectomy) is growing; however, until longer term data become available, sternotomy should still be considered the standard surgical approach.

Complete resection of thymoma has been found to be the most significant predictor of long-term survival. Several studies have examined the extent of surgical resection on survival and disease-free survival rates. In 241 operative cases, Maggi and colleagues found an 82% overall survival rate in those whose tumors underwent complete resection and a 26% survival rate at 7 years in those undergoing biopsy alone. Other investigators reported similar results in surgical patients. Therefore, regardless of stage, tumor resectability is one of the important predictors of treatment outcome.

RADIATION TREATMENT

Thymomas are generally radiosensitive tumors, and the use of radiation therapy in their treatment is well established. It has been used to treat all stages of thymoma, either before or after surgical resection. General agreement exists regarding the postoperative treatment of invasive thymoma (stages II and III). The value of adjuvant radiation therapy for invasive thymomas is well documented and should be included in the treatment regimen regardless of the completeness of tumor resection.

CHEMOTHERAPY

Chemotherapy has been used in the treatment of invasive thymomas with increasing frequency during the past decade (Table 5). The most active agents appear to be cisplatin, doxorubicin, ifosfamide, and corticosteroids. Combination chemotherapy has generally shown higher response rates and has been used in both neoadjuvant and adjuvant settings and in the treatment of metastatic or recurrent thymomas. CAP or CAPPr (cyclophosphamide, Adriamycin [doxorubicin], Platinol [cisplatin], and prednisone) regimens have been used in neoadjuvant and/or adjuvant settings. These regimens have also been used for recurrent thymoma.

Unresectable thymoma

Advanced-stage (III/IVA) thymomas are usually difficult to remove completely. Multidisciplinary approaches, including induction chemotherapy followed by surgical resection, postoperative radiation therapy, and consolidation chemotherapy, have been reported.

Induction chemotherapy consists of cyclophosphamide (500 mg/m2 IV on day 1), doxorubicin (20 mg/m2/d, continuous infusion, on days 1–3), cisplatin (30 mg/m2/d IV on days 1–3), and prednisone (100 mg/d PO on days 1–5), repeated every 3–4 weeks for 3 courses. Twenty-two evaluable patients were consecutively treated from 1990 to 2000 in a prospective phase II study at M. D. Anderson Cancer Center. After induction chemotherapy, 17 of 22 patients (77%) had major responses, including 3 complete responses.

Twenty-one patients underwent surgical resection. All patients received postoperative radiation therapy and consolidation chemotherapy. With a median follow-up of 50.3 months, overall survival rates at 5 years and 7 years were 95% and 79%, respectively. The rate of disease progression-free survival was 77% at 5 and 7 years. The multidisciplinary approaches to unresectable thymoma appear to be promising.

SUGGESTED READING

ON SMALL-CELL LUNG CANCER

Blackhall FH, Shepherd FA: Small cell lung cancer and targeted therapies. Curr Opin Oncol 19:103-108, 2007.

De Ruysscher D, Pijls-Johannesma M, Bentzen SM, et al: Time between the first day of chemotherapy and the last day of chest radiation is the most important predictor of survival in limited-disease small-cell lung cancer. J Clin Oncol 24:1057-1063, 2006.

Fried DB, Morris DE, Poole C, et al: Systematic review evaluating the timing of thoracic radiation therapy in combined modality therapy for limited-stage small-cell lung cancer. J Clin Oncol 22:4837-4845, 2004.

Glisson BS: Recurrent small cell lung cancer: Update. Semin Oncol 30:72-8, 2003.

Hanna N, Bunn PA Jr, Langer C, et al: Randomized phase III trial comparing irinotecan/cisplatin with etoposide/cisplatin in patients with previously untreated extensive-stage small-cell lung cancer. J Clin Oncol 24:2038-2043, 2006.

Movsas B, Moughan J, Komaki R, et al: Radiotherapy (RT) Patterns of Care Study (PCS) in lung carcinoma. J Clin Oncol 24:4553-4559, 2003.

O’Brien ME, Ciuleanu TE, Tsekov H, et al: Phase III trial comparing supportive care alone with supportive care with oral topotecan in patients with relapsed small-cell lung cancer. J Clin Oncol 24:5441-5447, 2006.

Slotman B, Faivre-Finn C, Kramer G, et al: Prophylactic cranial irradiation in extensive small-cell lung cancer. N Engl J Med 357:664-672, 2007.

ON MESOTHELIOMA

Flores RM, Zakowski M, Venkatraman E, et al: Prognostic factors in the treatment of malignant pleural mesothelioma at a large tertiary referral center. J Thorac Oncol 2:957-965, 2007.

Hazarika M, White RM Jr, Booth BP, et al: Pemetrexed in malignant pleural mesothelioma. Clin Cancer Res 11:982-992, 2005.

ON THYMOMA

Huang J, Rizk NP, Travis WD, et al: Feasibility of multimodality therapy including extended resections in stage IVA thymoma. J Thorac Cardiovasc Surg 134:1477-1483, 2007.

Kim ES, Putnam JB, Komaki R, et al: A phase II study of a multidisciplinary approach with induction chemotherapy, followed by surgical resection, radiation therapy, and consolidation chemotherapy for unresectable malignant thymomas: Final report. Lung Cancer 44:369-379, 2004.