ESTRO 2020 Abstract book

S820 ESTRO 2020

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. Conclusion 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. PO-1447 An Algorithm for 4Pi Radiotherapy Optimization applied to Accelerated Partial Breast Irradiation J. Lincoln 1 , N. Hoekstra 2 , B. Little 3 , C. Northway 1 , R.L. MacDonald 4 , C.G. Thomas 3 1 Dalhousie University, Physics & Atmospheric Science, Halifax, Canada ; 2 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 3 Nova Scotia Health Authority, Medical Physics, Halifax, Canada ; 4 Odette Cancer Centre, Medical Physics, Toronto, Canada Purpose or Objective Advantages of accelerated partial breast irradiation (APBI) include increased breast conservation rates, improved cosmetic outcomes, and shorter treatment times compared to traditional fractionation regimes (Njeh 2010). When considering hypo-fractionation, it is critical to ensure accurate dose delivery to the tumor volume, while minimizing surrounding normal tissue doses. Non-coplanar (4Pi) radiotherapy optimization has been well documented for cranial stereotactic treatments in terms of capability to reduce dose to cranial organs-at-risk (OAR) (MacDonald 2015). In this study we aimed to adapt the current optimization technique for application in APBI, and to provide the algorithmic considerations required for implementation. Material and Methods A key difference between cranial and extra-cranial treatments is that the available geographic 4Pi space decreases significantly owing to potential collisions between the treatment couch and gantry. Therefore, an anthropomorphic phantom was modified and aligned on a treatment couch (Varian Medical Systems, Palo Alto, CA) to simulate a patient undergoing APBI treatment. Collision zones were then manually measured for all couch and gantry angles using a 5 cm buffer. Previous 4Pi optimization methodologies published (MacDonald 2015) proved unsuitable considering the significantly larger OARs for APBI, and thus a new methodology was required. A ray tracing algorithm (Moeller 1997) was created to calculate beams-eye-view overlap between relevant OARs and target volume (Figure 1). The algorithm measures distances from the radiation source to entrance and exit points of the outer contour, target volume, and all OARs. These distances are then used to integrate the area under a percent depth dose curve, yielding a relative measure of volume overlap that depends on how much of an OAR is irradiated when delivering a conformal aperture to the target volume. These overlap scores for every combination of couch angle and gantry angle were plotted in maps for a test patient. A navigation algorithm (MacDonald 2015) was then used to

warranted to plot % volume outside PTV vs. dose, indentifying as showed in fig 1, if glands were preserved or not. The plans obtained using AP end eq. 1 were able to minimize the median dose of entire glands reducing risks of toxicity in accordance with guide lines (Merlotti et al. Radiation Oncology 2014); the median dose of entire glands was reduced using AP of 12%±2% and planning time was of 21±8 min vs. 68±19 min planning manually.

Conclusion AP based on linear model for parotid glands resulted a feasible strategy in SIB HNC. Linear model offered the possibility to foresee the expected toxicity in glands before to plan. AP saved planning time offering more time for planner in favour of minor tweaking steps at the end of plans. AP warranted the standardization of high quality plans and treatment outcome in SIB HNC. PO-1446 Multiple brain metastases (BMs) radiosurgery (SRS) with single isocenter: ALDO or ILA? F. Gregucci 1 , I. Bonaparte 1 , A. Surgo 1 , R. Carbonara 1 , A. Fiorentino 1 1 Miulli General Regional Hospital, Radiation Oncology, Acquaviva delle Fonti-Bari, Italy Purpose or Objective 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. Material and Methods 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. Results 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