Cavity check.

Top Line: There are several technical challenges with stereotactic radiosurgery (SRS) after resection of a brain metastasis.
The Study: First is the relatively large and irregular target volume. Second is the fact that the target often contains more normal brain tissue than one would include for intact brain mets. When recent randomized trials showed lower than expected local control after single-fraction post-op SRS, some raised concerns that both dose and margins were inadequate. A good alternative is considering fractionated SRS. What planning goals should one use, though, when doing 5-fraction SRS in the post-op setting to reduce the risk of radiation necrosis? This report analyzes 52 patients treated with post-op, frameless, LINAC-based SRS. The planning target was the resection cavity plus a 1.5 to 2 mm margin, and most patients received 6 Gy x 5 = 30 Gy. The cumulative rate of radionecrosis was 18%, increased by either doses >110% outside the cavity (CTV) or overlapping target volumes. On the contrary, there was no association between the max dose within the CTV and the risk of radionecrosis. In addition, no volumes of irradiated brain at sub-prescription doses were associated with radionecrosis risk. Does this mean you need homogeneous dosing for post-op SRS? Doesn’t sound like it. 
TBL: When doing 5-fraction post-op SRS for brain mets, keeping the volume receiving 33.5 Gy (or 111% of Rx) to 0.05 cc or less outside the cavity lowers the risk of radiation necrosis, a goal made easier without constraining the hotspot within the cavity. | Tanenbaum, Pract Radiat Oncol 2019


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