ESTRO 2021 Abstract Book

S626

ESTRO 2021

Conclusion Compared to previous published surveys, in our survey more details about TBI were queried, and (to our knowledge, for the first time) TSI was surveyed as well. New radiotherapy technologies for TBI purposes are relatively slowly introduced, despite the fact that they have been available for quite some time now, such as CT-based- treatment techniques (VMAT), digital position verification, and MLC based organ shielding.

PD-0791 VMAT plan complexity in dose painting A.M. Acosta Roa 1 , P. Lønne 1 , M.G. Guren 2 , E. Malinen 3 , T.P. Hellebust 1 1 Oslo University Hospital, Department of Medical Physics, Oslo, Norway;

2 Oslo University Hospital,

Department of Oncology, Oslo, Norway; 3 University of Oslo, Department of Physics, Oslo, Norway

Purpose or Objective Dose painting of the metabolic tumor volume (MTV) evident on PET/CT may, compared to an iso-toxic conventional treatment plan, put additional strain on the TPS optimization algorithm and result in higher plan complexity. In this study, we investigate plan complexity of dose painting in the case of anal cancer through various parameters that may impact dosimetric accuracy to evaluate the safeness and deliverability of the 44 treatment plans (22 standard and 22 dose painting by contours) were evaluated. Plans were created retrospectively using pre- and mid-treatment 18F-FDG-PET/CT images of 11 patients with squamous cell carcinoma of the anal canal, and optimized in Eclipse RT planning system (Varian Medical Systems, Palo Alto, CA; AAA 11.0.31 algorithm). VMAT technique with two full arcs and 6 MV photons was used. Standard plans consisted of 48.6 Gy, 54 Gy and 57.5 Gy in 27 fractions to elective lymph nodes, lymph node metastases and primary tumor, respectively. Dose painting included an extra dose level of 65 Gy to the MTV. The complexity metrics (CM) studied were mean dose rate (DR), mean change in dose rate per control point (∆DR/cp), average MLC leaf travel throughout the arc per degree (LT/deg), and the Modulation Complexity Score (MCS) [1]. These were calculated and extracted with the Eclipse Scripting API V15.6. CMs for standard and dose painting plans were compared by Wilcoxon signed-rank test. Plans were evaluated by Γ analysis, comparing calculated dose from the treatment plan and dose measured by an ArcCHECK (Sun Nuclear, Melbourne, FL) 3D diode array. Plans were delivered by two dosimetrically equivalent Varian Clinac 2300IX linacs. Passing rates for 3% DD and 3 mm DTA and for 2% DD, 2 mm DTA were registered. Results Comparison of CM between standard and dose painting plans is shown in Figure 1. MCS and LT/deg for standard and dose painting plans were not significantly different, but DR and ∆DR/cp were significantly different (p = 0.031 and p = 0.004, respectively). Mean Γ pass rates from the phantom measurements for 2% DD, 2 mm DTA were 97.5% and 98.1% for standard and escalated plans, respectively (p = 0.08). For 3% DD, 3 mm DTA, the mean pass rates were 99.9% and 100% for standard and escalated plans, respectively (p = 0.05). Correlation coefficients between complexity parameters and Γ pass rates for standard and escalated regimes are shown in Figure 2A. The parameters with the strongest correlation to Γ pass rate with 2% DD and 2 mm DTA for the escalated plans were MCS and ∆DR/cp, shown in detail in Figure 2B. local dose escalation. Materials and Methods