Proton therapy reduced estimated immune-cell dose in postoperative esophageal radiotherapy

KEY POINTS

• This retrospective study included 30 patients with pT1a–T1bN0M0 esophageal squamous cell carcinoma treated after endoscopic submucosal dissection. All received postoperative radiotherapy alone to 50.4 Gy in 28 fractions, without chemotherapy or immunotherapy.
• IMRT, VMAT, intensity-modulated proton therapy, and spot-scanning proton arc therapy plans were generated for every patient using identical prescriptions and organ-at-risk constraints. Proton plans incorporated 5-mm setup and 3.5% range uncertainty across 21 scenarios.
• Complete longitudinal blood counts were available for only 14 patients. Estimated dose of radiation to immune cells was strongly associated with nadir absolute lymphocyte count, explaining approximately 73% of its variation (adjusted R² 0.727, p<0.001).
• EDRIC remained independently associated with nadir lymphocyte count after adjustment for baseline lymphocyte count (p=0.004). It outperformed mean body dose (adjusted R² 0.588), mean heart dose (0.482), and mean lung dose (0.428).
• Mean EDRIC was 3.193 with IMRT, 3.407 with VMAT, 1.802 with IMPT, and 1.404 with SPArc. Both proton techniques were significantly lower than both photon techniques, and SPArc was significantly lower than IMPT (all adjusted p<0.001); IMRT and VMAT did not differ significantly.
• SPArc also produced the lowest mean lung dose (186.6 cGy), mean heart dose (382.7 cGy), and spinal cord maximum dose (2,120 cGy). Estimated NTCP was 0.96% for pneumonitis and 10.57% for pericardial effusion, the lowest among the four techniques.
• SPArc and IMPT were planning comparisons rather than delivered treatments. The analysis was retrospective, the haematological cohort was very small, post-treatment lymphocyte recovery was largely unavailable, and no survival or immunotherapy outcomes were assessed.

CLINICAL TAKEAWAY

SOURCE

Strahlentherapie und Onkologie