Session Item

SRS is a well-recognized treatment option for pts with limited intracranial disease; recent data show that this approach can be preferred also for pts with multiple BMs. Modern RT techniques applied to Linac-based SRS, have increased the precision of treatment, improving target dose distribution and reducing normal tissue doses. A dedicated mono-isocenter technique with multiple non-coplanar arcs could be performed, allowing to minimize overall treatment time (OTT) and geometrical/setup uncertainties. We compared 2 different optimization approaches in terms of isodose prescription, target coverage and organs at risk (OARs) dose sparing.

69 BMs (mean7, range4–21) were treated by Single iscocenter SRS in 10 pts. Prescribed dose (Dp) was 27Gy in 3 fr. PTV was defined by 1 mm isotropic margin from each lesion. None of PTVs was overlooking to chiasm or brainstem. Mono-isocenter VMAT plans with 5 non-coplanar arcs (couch at 0°, ±45°, ±90°) were generated for all pts, 2 different modalities of optimization –one based on mono-isocenter SRS dedicated optimization tool and the other on human experience – were compared: Automatic Lower Dose Objective (ALDO) vs Intratumor Lower dose Approach (ILA). A dose normalization of 100%Dp at 98%PTV was adopted, while D2%(PTV)<150%Dp was accepted. OARs were: chiasm, brainstem and heathy brain minus PTVs. Plan-optimizations were compared by the isodose prescription, D100% and D2% for PTVs, maximum dose for chiasm and brainstem, V_18Gy for the healthy brain, number of monitor units (MU) and OTT.

Sum of PTVs, calculated for each pts as an index of intracranial disease, had mean dimension of 5.6cc (range 3.71–10.1cc). The isodose prescription for ALDO were between 60-65%, for ILA between 75-80%. For both D100% between 25,05-27,3Gy, D2% was higher for ALDO than ILA: 38-39Gy for ALDO and33-34Gy for ILA. All plans had to respect the constraints on maximum dose to the chiasm (D0.5cc <15Gy) and to the brainstem (D0.5cc <18Gy) and no difference was highlighted by comparing ALDO-ILA. The mean of the maximum dose to chiasm was 4.7Gy and 4.4Gy for ALDO and ILA, respectively. The mean of the maximum dose to brainstem was 9.8Gy and 9.4Gy for ALDO and ILA, respectively. For all cases, the V_18Gy<30cc, as prognostic factor for radio-necrosis, was respected (range 5.4-15cc for ALDO and 6.2-20cc for ILA). MU and OTT were lower for ILA. Mean MU and OTT for ALDO were 4454 and 3 minutes, respectively; while for ILA mean of MU was 2322 and mean of OTT was 1.6 minutes.

ALDO and ILA give good results of target coverage, OARs sparing and low doses at healthy brain. ALDO uses lower prescription isodoses to obtain similar results to ILA, leading to a D2%>150%Dp, which could correlate with a higher risk of radionecrosis. In SRS planning expert hands, the clinician and physicist might choose between ALDO or ILA to customize treatments based on histology, BMs size and number, proximity to OARs, performing treatment plan optimization tailored on the pt.