ESTRO 2022 - Abstract Book

S843

Abstract book

ESTRO 2022

1080) for WT, 2arcs and 3arcs VMAT respectively. No statistically significant difference was found between 2arcs and 3arcs VMAT but the MU of VMAT groups were significantly higher than WT. As illustrated in table 1, statistically significant differences were found in all PTV and OAR dosimetric parameters (p<0.01) apart from heart V17Gy (p=0.83) between all three techniques. For multiple pairwise comparisons, WT group achieved statistically significant poorer target volumes and better OARs sparing than the VMAT techniques (p<0.01); there were no dosimetric benefits of 3arcs VMAT over the 2arcs technique.

Conclusion This study suggested that VMAT techniques had dosimetric advantages in target volume coverage over WT; 2arcs VMAT technique has been suggested to be the optimum technique compared to 3arcs due to its shorter treatment beam on time. Caution should be made about post RT toxicities to thyroid and oesophagus with VMAT techniques for breast IMC RT due to the expense of larger low dose coverage to surrounding OARs.

OC-0951 heart sparing in treatment planning for esophageal cancer VMAT: at what cost for lung dose?

M. de Boer 1 , A. Arents-Huls 1 , R.G.J. Kierkels 1 , P.M. Jeene 1 , K. Muller 1

1 Radiotherapiegroep, Department of radiation oncology, Arnhem, The Netherlands

Purpose or Objective Traditional treatment planning for esophageal cancer was mainly concentrated on target coverage and lung dose reduction while accepting relatively high dose to the heart. Recently a normal tissue complication probability (NTCP) model based indication protocol for treatment modality selection (i.e. photon or proton therapy) has been implemented in the Netherlands, in which a predicted 2 year overall survival (OS) increase of 5% is required to select for proton therapy. The NTCP model uses the volume of the GTV and the neo-adjuvant therapy status as clinical factors. The mean heart dose (MHD) is the only dosimetric parameter in the NCTP model. Consequently, a lower MHD directly leads to a higher predicted overall survival. Lowering MHD leads to an increase in lung dose with a possible increase of pulmonary toxicity. In this study, we investigated the trade-off between heart and lung dose sparing in photon radiotherapy for esophageal cancer patients. Materials and Methods At least five plans with varying heart and lung doses were optimized for seven esophageal cancer patients treated with volumetric arc therapy in 23 or 28 fractions of 1.8 Gy (patient characteristics in table 1). While maintaining adequate target coverage (planning target volume D95 ≥ 98%), the following plans were created: (1) optimal lung sparing; (2) optimal heart sparing; (3) a range of plans, each with a step-wise increase of approximately 1 Gy mean lung dose (MLD), while lowering the MHD as much as possible. For all plans the estimated predicted 2 year overall survival was derived from the NTCP model. Also, plan robustness was assessed by recalculation of the plan on the inhale and exhale phase of the 4DCT. Results In total, 41 plans were optimized with varying lung and heart dose levels. All plans except the maximum heart sparing plans were sufficiently robust for breathing motion. As compared to the maximum lung sparing plan, the MHD could be reduced by approximately 9.5 Gy for a 1 Gy increase in MLD. On average, approximately 3 Gy increase in MLD resulted in 15.8 Gy reduction of MHD. Increasing the MLD more than 3 Gy did not lead to substantial better heart sparing in our patients (figure 1). The mean predicted 2 year overall survival of the maximal lung sparing plans was 38% (range 23–60%). Increasing the