Late radiation-related lymphopenia after prostate stereotactic body radiation therapy plus or minus supplemental pelvic irradiation

Prior studies suggest lymphopenia following radiation therapy may impact toxicity and cancer control. Chronic radiation-related lymphopenia (RRL) has been noted in prostate cancer patients treated with conventionally fractionated pelvic radiation therapy. The impact of utilizing hypofractionated high integral dose therapies such as stereotactic body radiation therapy (SBRT) on RRL is less well characterized. This prospective study sought to evaluate the impact of prostate SBRT plus or minus supplemental pelvic nodal radiation (PNI) on RRL. Between 2012 and 2023, serial serum absolute lymphocyte counts (ALCs) were measured in 226 men treated at MedStar Georgetown with robotic SBRT using the CyberKnife® (CK) (36.25 Gy in 5 fractions) alone or CK (19.5 Gy in 3 fractions) followed by supplemental PNI using VMAT (37.5-45.0 Gy in 15-25 fractions) per an institutional protocol (IRB#: 2012-1175). Baseline ALC (k/μL) was measured 1-2 hours prior to robotic SBRT and at each follow-up appointment (1, 3, 6, 9, 12, 18, and 24 months post-treatment). Lymphopenia was graded using the CTCAEv.4: Grade 1 (0.8-1.0 k/μL), Grade 2 (0.5-0.8 k/μL), Grade 3 (0.2-0.5 k/μL) and Grade 4 (