Hematologic Toxicity of Single Posterior Pencil-Beam Scanning Approach for Esophageal Cancer

Abstract

Proton beam therapy has been shown to decrease heart and lung dose in esophageal cancers. Our group has previously published on the feasibility and safety of a single posterior field pencil-beam scanning (SPF-PBS) approach that has the advantage of maximally sparing heart and lung, but potentially at the cost of increased dose to the vertebral bodies (VBs). In this single-institution retrospective study, we assessed the hematologic toxicity and associated dosimetric factors with SPF-PBS. Eighteen patients treated with SFP-PBS and concurrent carboplatin and paclitaxel for locally advanced esophageal cancer between 2015 and 2016 were analyzed. Lymphocyte, neutrophil, and total leukocyte values while under treatment were recorded and graded per the CTCAE v4.03 toxicity scale, and the neutrophil-to-lymphocyte ratio (NLR) were also recorded. VBs were contoured 4 cm superior and inferior to the volume receiving at least 45 Gy (PTV45). VB mean dose and VB volumes (cc) receiving at least 5, 10, 20, 30, 40 and 50 Gy were calculated (V5-V50). A receiver-operator characteristic (ROC) analysis was performed for univariate correlation between incidence of grade ≥3 neutropenia / lymphopenia and VB dose-volume parameters. Spearman rank correlations were computed between NLR and dose variables. The rates of grade 3 toxicity were 38.9% (leukopenia), 22.2% (neutropenia), and 62.5% (lymphopenia); grade 4 toxicity rates for the same groups were 0%, 5.6%, and 18.8%, respectively. There were no cases of grade 3 or 4 anemia or thrombocytopenia. There was a significant correlation between neutropenia and VB mean dose (AUC = 0.83, p = 0.034). No statistically significant correlations were seen between lymphopenia and VB mean dose or V5-V50 (p > 0.069), nor were there correlations between NLR and VB dose (p > 0.067). The SPF-PBS approach for esophageal cancer yields hematologic toxicity rates comparable with previously published studies of IMRT and multiple field proton beam therapy. The possible increase in dose to the VBs in this setting may not be clinically relevant to hematologic toxicity, or may be balanced by the potential to spare more heart and lung tissue.