The radiation oncologist's perspective on stereotactic radiosurgery.

Abstract

Unfavorable gliomas: The basis for single-fraction stereotactic radiosurgery (SRS) is largely historical in nature and rooted in conventional thinking. This is derived from the original use of SRS in the treatment of arteriovenous malformations (AVMs), where the benefit of single-fraction high-dose radiation is clearly optimal in terms of addressing AVM obliteration kinetics. However, tumor cell kinetics are not the same as AVM obliteration kinetics and therefore may not be optimally addressed by single-fraction SRS. In addition, fractionated (F) SRS, as compared to single-fraction SRS, should allow for sparing of normal tissue damage. The relatively noninvasive nature of SRS allows for the potential of exploiting the use of FSRS and also allows for consideration of delivering FSRS in a split-course fashion. This provides an additional advantage over what can normally be achieved by use of stereotactic brachytherapy, in that sterotactic brachytherapy is likely to be performed only once in the course of a patient's primary treatment. This strategy exploits tumor and/or normal tissue cell kinetics, inclusive of attempting to counteract the initial accelerated tumor growth phase pre-CEBRT(conventional external beam radiation therapy), thereby decreasing the rate of clinical tumor progression during CEBRT. This split-course design should also help to counteract the effect of accelerated tumor repopulation post-CEBRT. Our unique experience with this approach in patients with unfavorable gliomas will be reviewed. Brain metastases: While whole brain radiation therapy (WBRT) remains a standard of care in patients with brain metastases, is potential neurocognitive morbidity remains a poorly understood concern. Despite this, and with an increasing role of surgery and/or SRS in the primary management of patients with brain metastasis, recently reported experiences withholding WBRT as part of primary therapy for brain metastases have not analyzed the potential effects on neurological functional status and/or neurocognition associated with the increased risk of brain tumor recurrence seen with such a strategy. We recently evaluated the risk of symptomatic brain tumor recurrence and associated neurologic deficit in 36 patients treated for newly diagnosed unresected brain metastases treated by Gamma Knife SRS alone followed by planned observation. Among the 17 patients (47%) developing brain tumor recurrence, 71% (12/17) were symptomatic and 59% (10/17) had an associated neurologic deficit. Also of interest, the author (WFR) performed a secondary analysis of a randomized phase III study of accelerated hyperfractionation (AH) versus standard accelerated fractionation (AF) in patients with unresected brain metastases. Control of brain metastases had a significant impact on MMSE. It is only among patients with 'uncontrolled" brain metastases that a drop in MMSE score is seen. Details of these studies, along with others, will be reviewed and implications with regards to the complementary role of WBRT in patients undergoing SRS for brain metastases will be discussed.