Temporal heart dose variations during IMPT for esophageal cancer and the impact of an adaptation protocol

In intensity-modulated proton therapy (IMPT) for esophageal cancer, organ motion can cause deviations between planned and delivered dose. Owing to posterior beam arrangements, the heart is particularly vulnerable, as the distal fall-off frequently overlaps cardiac tissue. This study quantified heart dose changes during treatment, identified contributing anatomical factors, and evaluated a heart-dose-guided adaptation protocol.

We retrospectively analyzed 194 esophageal cancer patients treated with IMPT using weekly repeat CT scans (reCTs). Prescriptions were 23 × 1.8 Gy or 28 × 1.8 Gy (RBE). Plans were recalculated on each reCT to derive mean heart dose (MHD) and heart V40Gy. Dose trends were assessed alongside anatomical changes, including diaphragm baseline shift, heart position and volume, heart displacement, internal target volume (ITV), intraesophageal air, posterior subcutaneous adipose tissue, and dysphagia scores. Significant variables were included in an Elastic Net model to predict MHD changes with 10-fold cross-validation. The effect of an adaptation protocol-triggered by MHD increases >1.5 Gy or target underdosage-was evaluated in 111 patients treated after August 2022, 25 of whom underwent replanning due to increasing MHD.

Linear mixed modeling in all reCTs of 194 patients showed significant increases in MHD (0.4 Gy/week, p < 0.001) and heart V40Gy (4.1 cc/week, p < 0.001), alongside increased dysphagia scores, cranio-caudal heart displacement, reduced ITV, and diaphragm baseline shifts relative to planning CTs. Elastic Net modeling identified diaphragm shift and heart volume change as key predictors of MHD increase. In the full cohort, MHD increased by 0.7 Gy, while in the sub group of 25 adapted patients, the MHD increase was reduced from 1.5 Gy to 0.8 Gy (p < 0.01).

Heart dose increases significantly during IMPT for esophageal cancer due to anatomical changes. Heart-dose-guided adaptive replanning effectively mitigates this increase.