ESTRO 2023 - Abstract Book

S1812

Digital Posters

ESTRO 2023

Conclusion Our treatment approach ensures a high median dose to the GTV even when the tumor is located close to the thoracic wall. We believe this approach can deliver higher GTV doses compared to only prescribing a dose to the PTV, when dose escalation to the GTV is actively pursued even for tumors close to the thoracic wall. Even though we are planning on the mid-position CT, we show delivery of a high dose on all phases, confirming that this approach is robust with respect to respiratory motion. We believe that this practice of prescribing a dose for the GTV, but ensuring that demands are met for both GTV and PTV, is a relevant and reproducible strategy to use.

PO-2040 Dosimetric Impact of Tools in Optimization of RapidArc Plans for Prostate Cancer

C. de Almeida Ribeiro 1 , A. M. Froner 2 , B. D. Frohlich 3

1 Università Cattolica del Sacro Cuore, Medical Physics, Roma, Italy; 2 Santa Casa de Caridade de Bagé, Radiation Oncology, Bagé, Brazil; 3 Hospital de Clínicas de Porto Alegre, UFRGS, Radiation Oncology, Porto Alegre, Brazil Purpose or Objective Optimization tools were introduced in inverse-planning treatment planning systems(TPS) to increase the accuracy of the dose verification process, such as the Aperture Shape Controller (ASC) and MU Limit (ML) tools. The ASC has the function to limit the complexity of VMAT planning by increasing the of the MLC opening and consequently reducing the modulation of the plan and the ML controls the number of MUs that the photon optimizer produces. In this context, the aim of this study was to investigate the effect and the limits of the ASC tool and ML on the VMAT plan quality Materials and Methods Thirty prostate plans were submitted to the CHHiP protocol and generated in Eclipse (Varian, v.15.6, Acuros). They were reoptimized three times, the first with the ASC tool, the second with the MU limit tool and the third with both tools. In the first one, the “very high” penalty of the ASC was used from the options “Very low to very high” to explore the limits of its capacity. Then, the ASC were compared to the MU Limit tool, which is composed of Minimum MU (Min MU), Maximum MU (Max MU) and Strength (S). Since the scope of this study is to reduce the planning complexity, the Max MU parameter was used at a value of 50% of the reference value, while S was used at a constant 100 and without filling for the Min MU field. No other parameters of the plans were modified. The original and reoptimized plans were compared using compliance criteria for conformity index (CI), CIPaddick, heterogeneity index (HI), gradient index (GI), dose to rectum and bladder, number of MUs, PTV coverage, all obtained from Eclipse, and gamma index, which was obtained through the quality control of the plans, performed with the Electronic Portal Imaging Device (EPID) Results There was no significant change for bladder and rectal dose restrictions with the ASC tool (fig1). The ML and its combination with the ASC tool showed a significant dose increase in most cases. The only case in which there was a dose decrease was in the V60Gy restriction of the bladder for the ASC+ML technique. The gamma index, PTV coverage, CI, CIPaddick, GI and HI assessments showed comparable results and no significant degradation for the plans reoptimized with the ASC tool was observed (fig2). For both tools, significant changes were found in PTV coverage, CI and GI. The number of MUs decreased significantly in all plans of all optimization techniques. The greatest restriction of MUs found was in both tools, going from 627 MU to 338 MU.

Fig1: Bladder and rectum dose evaluation