ESTRO 2024 - Abstract Book

S2235

Clinical - Upper GI

ESTRO 2024

2201

Digital Poster

Clinical impact of a dose-based adaptative protocol in proton therapy of esophageal cancer

Richard Canters, Vicki Trier Taasti, Kim Van der Klugt, Jeroen Buijsen, Gloria Vilches Freixas, Maaike Berbee

GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Department of Radiation Oncology (MAASTRO), Maastricht, Netherlands

Purpose/Objective:

In proton therapy of esophageal cancer, the adaptation rate is relatively high due to changes in anatomy or breathing pattern. In this study, we retrospectively evaluated patients that received one or multiple adaptations during treatment of esophageal cancer with intensity modulated proton therapy (IMPT). The effects of adaptation on coverage of the clinical target volume (CTV), mean heart dose (MHD), as well as the effects on expected 2 year mortality were evaluated.

Material/Methods:

We included 40 esophageal cancer patients treated at our institute with preoperative or radical chemoradiotherapy. All patients qualified for proton therapy based on a model-based selection protocol requiring normal tissue complication probability (NTCP) for two-year mortality to be 5% superior compared to photon therapy [1]. Patients were treated using a 3-beam IMPT setup, using one posterior beam, and additional posterior oblique beams on left and right. Breathing motion was accounted for by a treatment planning approach based on internal target volume (ITV), using delineated CTVs on all phases of a 4D CT. Setup uncertainty and intra fraction motion was accounted for by using a combination of 3 mm margin around the ITV (ITV_3) and 5 mm setup robustness, combined with 3% range uncertainty, followed by robust optimization. After optimization, robust evaluation was performed, with treatment plans being approved if ITV_3 D98%>94% of the prescription dose in the voxelwise minimum (VWmin) dose distribution. Each patient received weekly repeat CTs (reCTs) during treatment, on which re-delineation took place. All patients were treated according to an adaptation protocol that required both ITV D98%> 94%, as well MHD increases to be less than 1.5Gy with respect to the planning situation. To evaluate the effect of this adaptation protocol, dose was recalculated on all reCTS, after which a summed dose over all reCTs was calculated using a robust dose summation method, assuming 2 mm residual uncertainty, and 3% range uncertainty. Two dose summation scenarios were evaluated: in the first scenario, treatment using only the initial plan was assumed; in the second scenario, the actually received adaptive treatment was used in dose summation. Because reCTs were taken weekly, fractions of each plan were assigned to the closest reCT, after which a weighted summation took place. Results were compared using a paired sample t-test.

Results:

From the 40 adapted patients, 23 patients received an adaptation because of MHD increase, 8 adaptations were performed because of insufficient CTV coverage, 5 resulting from a combination of MHD increase and CTV coverage, and 5 because of technical reasons. Adaptation resulted in an average MHD reduction of 0.7Gy compared to the