Presenter(s)
K. Shin1, W. Floyd1, M. El-Jammal1, M. M. Badawy1, L. McCullum1,2, N. A. West2,3, S. Thrower4, M. Naser1, M. K. Rooney1, C. Tang1, H. Mok5, C. J. Hassanzadeh1, Q. N. Nguyen1, S. E. McGuire1, K. E. Hoffman1, S. J. Frank3, P. T. Tran1, J. Wang4, C. D. Fuller1, and S. Choi1; 1Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 2The University of Texas MD Anderson Cancer Center UTHealth Houston Graduate School of Biomedical Sciences, Houston, TX, 3Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 4Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 5Department of Genitourinary Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX
Purpose/Objective(s): Whole pelvic radiotherapy (WPRT) for prostate cancer irradiates large volumes of pelvic bone marrow, causing dose-dependent lymphopenia. However, preserving pelvic bone marrow and lymphocyte reserve is important for toxicity mitigation. We aimed to compare the incidence of Grade =3 lymphopenia between patients receiving WPRT versus prostate-only RT and identify predictive factors for severe lymphopenia.
Materials/Methods: We retrospectively identified patients diagnosed with non-metastatic prostate adenocarcinoma (2023-2024) and received definitive radiotherapy. Patients were treated with either IMRT or IMPT. We excluded those with metastatic disease, a brachytherapy boost or no blood counts from RT start to 6 months post-RT. The primary endpoint was Grade =3 lymphopenia (per CTCAE v5, the Common Terminology Criteria for Adverse Events). Fisher’s exact test and multivariable logistic regression were used to compare groups and estimate odds ratios (OR, 95% confidence interval [CI]). Secondary outcomes included overall survival (OS) and biochemical recurrence-free survival (bRFS), with patients stratified by the development of severe lymphopenia (Grade =3 vs. =2) within 6 months of RT completion. Survival was estimated using the Kaplan-Meier method and compared via log-rank test.
Results: Among 277 patients included, 225 patients received WPRT (IMRT 189, IMPT 36) and 52 received prostate-only RT (IMRT 45, IMPT 7). Grade =3 lymphopenia occurred in 49.8% (112/225) of WPRT patients versus 9.6% (5/52) of prostate-only patients (OR 9.25, 95% CI 3.51–30.93; p<0.001). In the IMRT subgroup, 53.9% (102/189) of WPRT patients versus 8.9% (4/45) of prostate-only patients experienced Grade =3 lymphopenia (p<0.01). In the IMPT subgroup, rates were 27.8% (10/36) vs 14.3% (1/7) (p=0.66). On multivariable analysis, WPRT remained the strongest independent predictor of Grade =3 lymphopenia (OR 8.81, 95% CI 3.75–25.01; p<0.001), and IMRT had higher risk than IMPT (OR 2.57, 95% CI 1.25–5.58; p=0.01). Concurrent chemotherapy and age were not significant predictors. At a median follow-up of 21.4 months, 2-year OS and bRFS rates were comparable between patients with Grade =3 versus Grade =2 lymphopenia (97.2% vs. 99.0%, p=0.2; and 98.2% vs. 97.2%, p=0.9, respectively).
Conclusion: Including pelvic nodes in the radiation field significantly increased severe lymphopenia and was the strongest predictor of Grade =3 lymphopenia. This effect was much weaker with IMPT, implying that bone marrow-sparing approaches may preserve lymphocyte counts. This study is limited by retrospective and single institution design. Future study should prospectively relate pelvic marrow dose to Grade =3 lymphopenia and the benefit of IMPT in marrow sparing RT.