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Role of Radiation Therapy in Retroperitoneal Sarcomas

Role of Radiation Therapy in Retroperitoneal Sarcomas

ABSTRACT: Historically, patients with retroperitoneal sarcomas have had a poor prognosis. Surgical resection continues to be the standard treatment for these tumors. However, their anatomic location and large size at presentation often make complete surgical resection infeasible. Even with complete gross removal of tumor, most patients will experience local failure. Adjuvant radiation therapy has been used to improve local control rates. In the postoperative setting, radiation doses to the tumor bed are limited by radiation tolerances of surrounding normal tissues. Extrapolation of data from soft-tissue sarcomas at other sites suggests that delivery of higher radiation doses, in combination with surgery, may favorably affect local control. Preoperative radiation therapy, in combination with brachytherapy or intraoperative radiation therapy at the time of surgical resection, allows for the safe delivery of higher doses of radiation than is possible in the postoperative setting. These approaches make it possible to maximize the likelihood of local control and cure while minimizing normal tissue toxicity. [ONCOLOGY 10(12):1867-1872, 1996]

Introduction

Retroperitoneal sarcomas account for approximately 15% of the
6,000 new soft-tissue sarcomas diagnosed each year. The prognosis
of patients with retroperitoneal sarcomas is poor. The rarity
of these tumors, as well as the diverse histologic subtypes and
grades, makes it impossible to come to any definitive conclusions
about optimal management from the literature. The role of radiation
in the treatment of retroperitoneal sarcomas has been controversial.
However, several key points and guiding principles can be extracted
from the available data.

Presentation and Natural History

Most patients present with an abdominal mass. Because of their
anatomic location, these tumors often grow to a tremendous size
without causing any systemic symptoms. There are reports of patients
presenting with retroperitoneal tumors as large as 40 pounds with
relatively few other symptoms.[1] Symptoms, such as weight loss,
nausea/vomiting, early satiety, and back pain, occur later in
the disease process and are related to pressure exerted by the
tumor on adjacent organs.

As opposed to soft-tissue sarcomas at other sites, in which pulmonary
metastases represent the vast majority of distant failures, retroperitoneal
sarcomas also commonly fail in the liver via access of the portal
venous system. In addition, peritoneal sarcomatosis is often seen.

One must distinguish sarcomas arising from the retroperitoneum
from those arising from abdominal viscera. The latter are much
more easily resected, are less likely to invade into adjacent
structures, and are less apt to require adjuvant therapy.

Histopathology

Soft-tissue sarcomas are derived from the primitive mesoderm.
Although the histopathology is diverse, the most common subtypes
in the retroperitoneum are leiomyosarcoma and liposarcoma.[2,3]
Other frequently seen subtypes include fibrosarcoma, malignant
fibrous histiocytoma, lymphangiosarcoma, and neurofibrosarcoma.
Despite the diversity of these tumors, histologic sub-type is
relatively unimportant and does not play a major role in overall
management.

In contrast to subtype, histologic grade does influence prognosis
and treatment. Higher-grade tumors tend to be more locally invasive
and aggressive than lower-grade tumors and have a higher incidence
of distant metastases. In soft-tissue sarcomas at all sites, histologic
grade has been shown to be the single most important prognostic
factor.[4,5] Several institutional reviews have demonstrated that
grade is also an important prognostic factor in sarcomas of the
the retroperitoneum.[2,6-8]

Diagnosis and Staging

The diagnostic work-up in a patient with a retroperitoneal mass
should include a complete blood count, serum electrolytes, serum
creatinine, and liver function tests. Computed tomography (CT)
or magnetic resonance imaging (MRI) are the best studies for evaluating
the extent of the primary tumor. These imaging studies can assess
invasion or displacement of neighboring structures, evaluate the
liver, and may help guide the diagnostic biopsy.

When the primary tumor is located in a region where one kidney
is likely to be sacrificed by surgery or damaged with radiation,
a renal scan is indicated to evaluate the function of the contralateral
kidney. Other studies, such as barium enema, upper gastrointestinal
(GI) series, or IV pyelography, are rarely necessary.

Incisional biopsy is the preferred means of obtaining a diagnosis,
because it provides sufficient tissue for accurate grading of
the tumor and provides adequate information for treatment planning.
Hemostasis should be ensured following the incisional biopsy to
prevent tumor seeding.

The work-up for metastasis should include a CT scan of the chest/upper
abdomen to rule out pulmonary and/or liver metastases.

Once a diagnosis has been established, a multidisciplinary approach
should be undertaken prior to the initiation of therapy for the
proper sequencing of each treatment modality. The precise goal
of treatment (cure or palliation) should be understood by the
treating physicians, as well as the patient.

The staging system for soft-tissue sarcomas, as defined by the
American Joint Committee on Cancer (AJCC), incorporated histologic
grade as the primary determinant in the clinical staging (Table
1
). One of the ironies of the AJCC system is that although the
stage grouping is based on tumor, there are no definitive criteria
for grading soft-tissue sarcomas. Several grading systems have
been proposed, which use a variety of histologic criteria. Trojani
et al proposed a three-grade system in which mitotic count, differentiation,
and necrosis were found to be the three necessary parameters in
attributing a tumor grade.[9] Hajdu described a two-grade system
based on differentiation, cellularity, stromal involvement, vascularity,
necrosis, and mitotic count.[10] There are no widely accepted
criteria for a tumor grade despite its importance in prognosis.

Surgery

Surgical resection continues to be the primary therapy for retroperitoneal
sarcomas. Extent of surgical resection has often been reported
as the single most important prognostic factor for these tumors.[2,6,8,11]
Complete surgical resection provides improved local control and
survival rates when compared with a more limited resection.[2,8,12,13]
Cody et al, in a review of 158 patients with retroperitoneal sarcomas
treated at Memorial Sloan-Kettering Cancer Center (MSKCC), reported
a 5-year survival rate of 40% in those patients who underwent
complete excision vs a rate of 3% in those with an incomplete
resection.[2]

It is likely that tumors with a more favorable prognosis at the
outset are also most amenable to surgical resection. Thus, the
improved prognosis associated with a complete resection probably
represents both a cause and an effect.

Despite complete surgical removal of a retroperitoneal sarcoma,
the majority of patients experience a local recurrence (Table
2
). Storm and Mahvi,[12] in a review of the literature, report
a > 90% local recurrence rate at 10 years following complete
surgical resection. This local failure is not unexpected given
the lack of anatomic barriers in the retroperitoneum and the large
size of these tumors.

Even with surgical sacrifice of such organs as the kidney, bowel,
and pancreas, the anatomic limits of the retroperitoneum prohibit
wide surgical margins. Approximately one-half of the patients
explored for these tumors are able to undergo complete removal
of all gross disease,[12] but clear pathologic margins are the
exception, as microscopic extension beyond apparent tumor borders
is the rule.

Sarcomas often have a pseudocapsule of compressed normal tissue
into which there is microscopic tumor invasion. This can lull
the surgeon into believing that the operation performed was better
than was actually the case. Unlike extremity sarcomas, where wide
margins are routinely achieved, a complete "oncologic"
resection is rarely accomplished in the retroperitoneum.

Given these limitations of surgery, it is not surprising that
a large discrepancy exists between the local recurrence rates
of extremity and retroperitoneal sarcomas after surgical resection.
The efficacy of adjuvant radiation therapy in decreasing local
failure rates following wide surgical excision of extremity sarcomas
has been well documented. Surgical excision as the sole treatment
of high-grade extremity sarcomas results in local recurrence rates
of 35% to 60%, whereas local control rates of approximately 85%
can be expected with the addition of radiation therapy.[14] The
improvement in local control with radiation therapy seen in extremity
sarcomas should also be observed in the retroperitoneum since
there is no suggestion that soft-tissue sarcomas at these sites
are biologically different.

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