ESTRO 2023 - Abstract Book

S1842

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ESTRO 2023

shape and were based on DuPey Pinnacle acetabular shell, Pinnacle Marathon 35ID/58, Summit 7 titanium femoral component, with Biolox delta ceramic head. Hypo-fractionated RT plans (60Gy in 20 fractions) were created, using photon optimizer in Eclipse (v15.6.06) by a single planner using a highly prescribed approach derived from previous SOH work and an assigned titanium alloy material for the prostheses. All plans were required to meet the CHiiP trial goals in addition to local defined goals. Two VMAT prosthesis avoidance techniques were investigated, a commercial approach, Avoid Structure – Entry (ASE) and a user-defined approach, Upper Constraint on Prosthesis PRV (UCPP). The pre-SOH plans were compared to the post-SOH plans, using V37Gy, V46Gy, V50Gy, V55Gy, and V60Gy for rectum, the primary structure of interest, and various metrics for bladder, penile bulb, CTV, PTV and BODY-PTV. Results SOH resulted in statistically significant reductions, as per Wilcoxon signed rank test, in all rectal metrics, except V37Gy, for both unilateral and bilateral prostheses scenarios, regardless of avoidance technique. In the unilateral prosthesis setting, 11/13 patients had 50%+ reduction in V55Gy, while 10/13 patients had a 50%+ drop in V55Gy in the setting of bilateral prostheses. Furthermore, the mean and standard deviation of the rectal metrics for all scenarios simulated are in line with the values reported in the pivotal trial by Mariados et al. No significant change in bladder, penile bulb, CTV and PTV doses was noted between the pre- and post-SOH plans. Both avoidance techniques resulted in a statistically significant increase in bladder dose compared to baseline plans (no prosthesis), however, the increase was statistically less with the ASE compared to UCPP technique. However, the ASE technique resulted in a statistically significant increase in all rectal metrics, except V60Gy and PTV V63Gy in the bilateral hip setting. Conclusion The use of SOH appears to result in statistically significant benefits in terms of rectal dose despite the presence of simulated unilateral and bilateral hip prostheses, regardless of avoidance technique. However, both avoidance techniques resulted in a statistically higher bladder dose compared to baseline while the UCPP resulted in statistically higher rectal doses, except V60Gy, in the context of bilateral hip prostheses, making ASE a better avoidance technique for VMAT treatment. 1 Complejo Hospitalario Universitario de Cartagena-Servicio Murciano de Salud, Servicio de Radiofísica y Protección Radiológica, Cartagena, Spain Purpose or Objective The purpose of this work is to preliminarily evaluate the use of Varian HyperArc (HA) as a planning and treatment strategy for whole brain radiotherapy with hippocampal avoidance (WBRTHA) in the radiotherapy department at the Complejo Hospitalario Universitario de Cartagena (CHUC). Materials and Methods 14 patients treated in Clinacs for WBRTHA by the standard VMAT technique (TP), 4 coplanar 6MV arcs, were retrospectively planned for the TrueBeam HD-MLC linac (TPTB) and using the newly commissioned HA technique (1 full+3 non-coplanar 6FFF arcs, maximum 1400MU/min). The OAR brain, hippocampi, lenses, optic nerves, and brainstem were contoured on MRI and CT datasets. The median of the PTV, defined as the brain excluding the 3mm expanded region from hippocampi, was prescribed 25 Gy (12 cases) or 30 Gy (2) in 10 fractions. The plans were evaluated using as dosimetric parameters the PTV volume that receives 105% and 95% of the prescribed dose (V105% and V95%), the doses to the 98% (D98%) and 2% (D2%) of the PTV, the maximum (Dmax) and minimum (Dmin) doses to PTV, in addition to the mean doses, the D98% and D0.1cc to the hippocampi and lenses, and finally the Dmax to brainstem and to both optic nerves. Planning time, beam-on time, MUs and modulation index were estimated, and γ - analysis results were also assessed to ensure QA. Statistical significance was performed between HA and TP and between TPTB and TP, using Wilcoxon's signed rank test with a significance level of α =0.05. Results All plans satisfied the local dosimetric acceptability criteria. The HA plans demonstrated significant improvements in the PTV dosimetric quality that included a reduction in the parameters V105% (-1.1±2.0%), D2% (-0.2±2.7%), Dmax (-0.6±3.4 Gy) and an increased Dmin of 2.0±4.5 Gy and V95% of 0.5±1.0 %, compared with TP (Figures 1 to 2). A significant decrease in OAR doses was also found for the HA plans, as shown by the reduction in the Dmin to both hippocampi of -0.6±1.3 Gy, in the mean doses to the hippocampi (-0.3±0.8 Gy) and to both lenses (-0.6±0.7 Gy). HA plans resulted in significantly shorter planning and beam-on times with regards to TP (TPTB), on average by 52.4 minutes (26.8) and 1.8 minutes (2.0), respectively. The MUs and modulation index were lower for HA compared with TP (75.4MU and 0.4cGy/MU) and TPTB (105.5MU and 0.5cGy/MU). No significant differences were found between QA results based on γ -metrics. PO-2066 Whole brain radiotherapy with hippocampal avoidance using Varian HyperArc: Preliminary study M. Martinez Albaladejo 1 , V. Puchades Puchades 1 , J. Suárez Arteaga 1 , D. Ramos Amores 1 , A. Serna Berná 1