PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Treatment-related lymphopenia (TRL) is frequently associated with radiation therapy. Multiple prior studies have shown associations between TRL and inferior outcomes in various solid tumors, including patients treated with conventionally fractionated radiation therapy for lung cancer. TRL was recently shown to occur in patients with lung cancer treated with stereotactic body radiation therapy (SBRT), but the survival impact of TRL in this setting remains unknown. We seek to evaluate the impact of severe TRL in the setting of lung cancer treated with SBRT on overall survival and evaluate potential strategies to minimize TRL.
Materials/Methods: We retrospectively identified 346 patients treated with thoracic SBRT between 2008 and 2016. Of these patients, 137 had complete blood counts with differentials within 12 months preceding and following SBRT. This was further refined to exclude patients who received prior radiation therapy to the head, neck or thorax; patients who received chemotherapy; and patients with pre-treatment absolute lymphocyte counts less than 500x106 cells/L. The remaining 91 patients were included in the analyses. All patients were treated with 5 fraction regimens, following RTOG 0813 or 0915 as appropriate. We defined severe TRL as an absolute lymphocyte count less than 500x106 cells/L. Kaplan-Meier curves, log-rank analysis and cox regression were performed to assess for survival differences associated with severe TRL using statistical software.
Results: Of the 91 included patients, median age was 71.9 years, 55% were female, and 39% had central lung lesions. Planning target volumes ranged from 4.3-183 cm3. Patients with severe TRL had worse overall survival (log rank, Ӽ2=7.62, p=0.006). On univariate cox regression, minimum absolute lymphocyte count was directly related to survival, with a hazard ratio of 0.353 (p=0.002). Flattening filter free-arcs were associated with less lymphocyte depletion compared to step and shoot IMRT, with a mean difference in depletion of 211x106 cells/L between modalities (p=0.04). PTV size also had a statistically significant positive correlation with lymphopenia.
Conclusion: Severe TRL after lung cancer SBRT is associated with worse overall survival. The use of flattening filter free-arcs reduced lymphocyte depletion compared to step and shoot IMRT.
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