Sep 30

SS 41 - Radiation and Cancer Physics 7: AI-Driven Imaging and Predictive Modeling

340 - Radiotherapy-Induced Acute Lymphopenia for Low and Intermediate Risk Prostate Cancer

03:30pm - 03:40pm PT

Room 20/21

Presenter(s)

Maryam Moteabbed, PhD - Massachusetts General Hospital, Boston, MA

M. Moteabbed^1,2, C. Beekman^2,3, B. Y. Yeap^3,4, D. T. Miyamoto^2,3, S. C. Kamran^2,3, H. Paganetti^2,3, and J. A. Efstathiou^2,3; ¹Brown University Health and Rhode Island Hospital, Providence, RI, ²Massachusetts General Hospital, Boston, MA, ³Harvard Medical School, Boston, MA, ⁴Department of Biostatistics, Massachusetts General Hospital and Harvard Medical School, Boston, MA

Purpose/Objective(s): Radiation-induced lymphopenia has been found to impact radiation therapy outcomes for different tumor types. We aim to investigate the incidence of acute lymphopenia for prostate cancer without nodal involvement following radiation therapy (RT) using proton and photon external beams. We study the differences between changes in absolute lymphocyte counts (ALC) and correlations with dose volume histograms of circulating lymphocytes.

Materials/Methods: 63 patients with low- or intermediate-risk prostate cancer who participated in a phase III multi-institutional controlled clinical trial and accompanying registry were consented to undergo longitudinal complete blood count with differential tests to determine the ALC at 3 time points along the treatment course. The time points were just before treatment start (baseline), just after treatment completion (post-RT), and at 3 months after the end of treatment (follow-up). Thirty percent of patients were treated with protons and 70% with photon beams. Lymphopenia was defined as ALC<1000/mL of blood (grade 3 <500/mL of blood). Circulating lymphocytes were simulated using a stochastic model of blood flow and explicit models of organ vasculature. Dose to dynamic blood particles was accumulated over treatment time based on organ dose volume histograms and modality-specific beam-on time parameters.

Results: New onset of lymphopenia (no baseline lymphopenia) occurred post-RT in 26% and 59% of proton and photon patients, respectively. Fifty and 45 percent of all post-RT lymphopenia incidences were Grade2 or larger for proton and photon cases, respectively. Only one instance of grade 3 lymphopenia occurred at follow up for each treatment modality but none post-RT. ALC was significantly reduced post-RT and at 3-month follow-up compared to baseline for both modalities (p<0.001). It partially recovered at follow-up compared to post-RT for both modalities. The median reduction of ALC post-RT was 0.49 (10³/??L) for proton (??=0.48) and 0.83 (10³/??L) for photon patients (??=0.47). At 3-month follow-up, reduced ALC persisted but improved compared to post-RT in 75% and 45% of proton and photon patients who had experienced post-RT lymphopenia, respectively. Median ALC reduction at follow-up compared to baseline was 0.55 (??=0.34) for protons and 0.56 (??=0.48) for photons. The ALC reduction post-RT (p=0.06) and at follow-up (p=0.1) compared to baseline were not statistically significantly different between RT modalities. Mean accumulated dose to the circulating blood was approximately 2.5 times larger for photons compared to protons.

Conclusion: The lymphocyte count variation patterns are similar between patients receiving proton and photon-based RT, but the fall and recovery are steeper for photons compared to protons. Higher dose per volume of circulating blood during photon RT might be responsible for the observed difference.