Tumor volume regression during neoadjuvant chemoradiotherapy for esophageal cancer: a prospective study with weekly MRI

Background: Neoadjuvant chemoradiotherapy (nCRT) for esophageal cancer causes tumor regression during treatment. Tumor regression can induce changes in the thoracic anatomy, with smaller target volumes and displacement of organs at risk (OARs) surrounding the tumor as a result. Adaptation of the radiotherapy treatment plan according to volumetric changes during treatment might reduce radiation dose to the OARs, while maintaining adequate target coverage. Data on the magnitude of the volumetric changes and its impact on the thoracic anatomy is scarce. The aim of this study was to assess the volumetric changes in the primary tumor during nCRT for esophageal cancer based on weekly MRI scans. Material and methods: In this prospective study, patients with adeno- or squamous cell carcinoma of the esophagus treated with neoajduvant chemoradiotherapy according to the CROSS regimen (carboplatin + paclitaxel + 23 x 1.8 Gy) were included. Of each patient, six sequential MRI scans were acquired: one prior to nCRT, and five in each subsequent week during nCRT. Tumor volumes were delineated on the transversal T2 weighted images by two radiation oncologists. Volumetric changes were analyzed using linear mixed effects models. Results: A total of 170 MRI scans from 29 individual patients were included. The mean (+/- standard deviation (SD)) tumor volume at baseline was 45 cm(3) (+/- 23). Tumor volume regression started after the first week of nCRT with a significant decrease in tumor volumes every subsequent week. A decrease to 42 cm(3) (91% of initial volume), 38 cm(3) (81%), 35 cm(3) (77%), and 32 cm(3) (72%) was observed in the second, third, fourth and fifth week of nCRT, respectively. Conclusion: Based on weekly MRI scanning during nCRT for esophageal cancer, a considerable decrease in tumor volume was observed during treatment. Volume regression and consequential anatomical changes suggest the possible benefit of adaptive radiotherapy.