Pathologic complete response (pCR) rates and outcomes after neoadjuvant chemoradiotherapy with proton or photon radiation for adenocarcinomas of the esophagus and gastroesophageal junction

Background: Pathologic complete response (pCR) after neoadjuvant chemoradiotherapy (nCRT) is associated with improved survival in patients treated for esophageal cancer. While proton beam therapy (PM) has been demonstrated to reduce toxicities with nCRT, no data comparing pCR rates between modalities exist to date. We investigated pCR rates in patients with distal esophageal/GEJ adenocarcinomas undergoing trimodality therapy with nCRT-PBT or photon-based nCRT with the hypothesis that pathologic responses with PBT would be at least as high as with photon therapy. Methods: A single-institutional review of patients with distal esophageal adenocarcinoma treated with trimodality therapy from 2015-2018 using PBT was completed. PBT patients were matched 1:2 to patients treated with photons. Chi square and two-sample t-tests were utilized to compare characteristics, and the Kaplan Meier method was used to estimate oncologic endpoints. Results: Eighteen consecutive PBT patients were identified and compared to 36 photon patients. All patients received concurrent chemotherapy; 98% with carboplatin/paclitaxel. Most patients were male (91%) and White (89%); median age was 62 years (range, 31-76 years). Median radiation dose in both cohorts was 50.4 Gy (range, 41.4-50.4 Gy); all courses were delivered in 1.8Gy fractions. Age, gender and race were well balanced. Patients treated with PBT had a significantly higher pre-treatment nodal stage (N) and AJCC 7th edition stage grouping (P=0.02, P=0.03). Despite this, tumoral and nodal clearance and pCR rates were equivalent between cohorts (P=0.66, P=0.11, P=0.63, respectively). Overall pCR and individual primary and nodal clearance rates, overall survival (OS), locoregional control (LRC), and distant metastatic control did not significantly differ between modalities (all P>0.05). Major perioperative events were balanced; however, there were 5 (14%) perioperative deaths in the photon cohort compared to 0 (0%) in the proton cohort (P=0.06). Conclusions: The use of PBT in trimodality therapy for distal esophageal adenocarcinoma yields pCR rates comparable to photon radiation and historical controls. Pathologic responses and oncologic outcomes in this study did not differ significantly between modalities despite PBT patients having higher AJCC stages and nodal disease burdens.