ALARA: In Radiation Oncology and Diagnostic Imaging Alike

March 15, 2014

In this issue of ONCOLOGY, Dr. Goske and colleagues present an excellent review of efforts to reduce radiation exposure from diagnostic medical imaging.[1]

In this issue of ONCOLOGY, Dr. Goske and colleagues present an excellent review of efforts to reduce radiation exposure from diagnostic medical imaging.[1] They outline the “Image Gently” campaign, which seeks to raise awareness of the need to adjust radiation dose in diagnostic imaging and the need for radiation protection in children; they also discuss the “Image Wisely” campaign, whose goal is to raise awareness about radiation protection in adults. These issues are particularly important in the setting of pediatric cancer.

The field of pediatric radiation oncology has long emphasized the judicious use of radiation, and has been at the forefront of the movement to reduce patient exposure to therapeutic radiation where possible. Forty years ago, a child with Hodgkin lymphoma (HL) involving the mediastinum would likely have been treated with total lymphoid irradiation to 44 Gy to a large field that included the entire chest, abdomen, and pelvis. Thanks in large part to collaborative research that has allowed rapid advances in the field of pediatric oncology, today this child would receive 15–25 Gy to a much smaller volume, with accurate calculation of dose received by adjacent tissues and organs, and with the treatment plan selected after multiple iterations had been evaluated for optimal target coverage and normal tissue constraints, in an effort to minimize radiation where it is not required. Implementation of advanced treatment modalities and new technical innovations, including intensity-modulated radiation therapy and proton therapy, requires precise medical imaging for target definition, and this has facilitated further reduction of therapeutic radiation exposure to normal tissues in children. Finally, risk stratification and identification of patients who may be able to avoid radiation therapy altogether, or to receive lower radiation doses, has further reduced dose in children where it is not needed.

While great strides have been made in pediatric oncology in reducing therapeutic radiation dose where possible, the use of diagnostic radiation as part of surveillance imaging has been less well studied, and there are no prospective randomized trials to guide decision making. Results of retrospective studies of the benefits of surveillance imaging have been mixed and vary according to both disease site and time from completion of treatment. In an analysis of Pediatric Oncology Group (POG) trial 9425, Voss et al reported patterns of recurrence and methods of detection in children with HL and found that only 8% of those who relapsed in the first year following treatment had recurrence detected by surveillance imaging alone, while 16% of patients had recurrence detected exclusively by surveillance imaging after the first year.[2] However, detection of late relapse did not affect overall survival (OS). The authors argue that CT is overused for routine surveillance in HL and should be restricted to the first year post-therapy.

There is stronger evidence supporting the use of surveillance imaging in children treated for a central nervous system (CNS) malignancy. Minn et al performed an analysis of all children who were treated on one of ten POG trials for malignant glioma, medulloblastoma, or ependymoma between 1985 and 1999 and who relapsed.[3] Approximately one-third of relapses were detected by imaging, and these were more often late relapses, defined as later than the mean relapse time of 8.8 months. Children whose relapses were detected by imaging had a significantly longer OS than those detected by clinical examination, and time to relapse was longer in those whose relapses were detected by imaging. Importantly, these POG trials primarily used MRI as surveillance imaging. While this modality is not risk-free and may require the use of anesthesia, it does not expose children to ionizing radiation. Thus, although surveillance imaging may be of value in those who have undergone treatment for CNS malignancies, radiation exposure in this setting may be generally avoided.

Nonetheless, in pediatric oncology patients undergoing treatment with radiotherapy, the amount of radiation exposure from a diagnostic scan is dwarfed by the exposure from therapeutic radiation. For example, a chest CT gives an absorbed dose of 5–10 mGy, whereas mediastinal radiation dose for Hodgkin lymphoma is typically 15–25 Gy, a 3,000-fold difference.[4,5] Even if a child were to receive 100 chest CTs over a lifetime, this would give a cumulative dose of approximately 0.5–1 Gy, still very small in comparison with therapeutic doses. When accustomed to considering radiation in these therapeutic dose levels, there may be a bias among radiation oncologists toward considering the radiation exposure from a diagnostic scan as being nearly inconsequential compared with the amount of radiation administered for therapy. However, if radiation effects do indeed follow a linear no-threshold model as discussed by Goske et al, each additional exposure incrementally increases the risk, in a linear fashion, of a radiation-induced malignancy developing, regardless of past exposure. Furthermore, other models, including those proposed by Shuryak et al and discussed in the present article, suggest that prior exposure to radiation may actually increase risks from future imaging scans.[6] Still, concerns regarding a small increase in the risk of a theoretical malignancy must be balanced against the possibility of early detection of recurrence of a known cancer, which in some situations may lead to potentially curable salvage therapy.

We agree with and support the “Image Gently” and “Image Wisely” campaigns. We further agree that approaches to diagnostic imaging should be standardized, and that the role of surveillance imaging in post-treatment pediatric cancer patients should be investigated in randomized clinical trials. However, we foresee potential obstacles to accrual in these studies, since both physicians and families have individual biases with respect to the importance of imaging studies, as well as the estimation of risk related to diagnostic imaging. We also believe in the ALARA principle-keeping radiation dose “as low as reasonably achievable”-whose value in the therapeutic setting has been demonstrated through the development and implementation of advanced imaging techniques that allow for the treatment of smaller volumes, with lower doses, and using risk-adapted therapy. Still, while continuing the movement toward reducing radiation exposure where possible, we must not be afraid to continue to use radiation when it is appropriate and necessary, in both diagnostic and therapeutic settings.

Financial Disclosure: The authors have no significant financial interest in or other relationship with the manufacturer of any product or provider of any service mentioned in this article.

References:

1. Goske MJ, Frush DP, Brink JA, et al. Curbing potential radiation-induced cancer risks in oncologic imaging: perspectives from the ‘Image Gently’ and ‘Image Wisely’ campaigns. Oncology (Williston Park). 2014;28:232-43.

2. Voss SD, Chen L, Constine LS, et al. Surveillance computed tomography imaging and detection of relapse in intermediate- and advanced-stage pediatric Hodgkin’s lymphoma: a report from the Children's Oncology Group. J Clin Oncol. 2012;30:2635-40.

3. Minn AY, Pollock BH, Garzarella L, et al. Surveillance neuroimaging to detect relapse in childhood brain tumors: a Pediatric Oncology Group study. J Clin Oncol. 2001;19:4135-40.

4. Huda W. Radiation doses and risks in chest computed tomography examinations. Proc Am Thorac Soc. 2007;4:316-20.

5. Shrimpton PC, Hillier MC, Lewis MA, Dunn M. National survey of doses from CT in the UK: 2003. Br J Oncol. 2006;79:968-80.

6. Shuryak I, Sachs RK, Brenner DJ. Cancer risks after radiation exposure in middle age. J Natl Cancer Inst. 2010;102:1628-36.