The impact of pelvic radiotherapy on hematological outcomes in pediatric genitourinary rhabdomyosarcoma.

379 Background: Hematological toxicities are common with radiation therapy (RT), particularly pelvic RT, which affects nearly 50% of total body hematopoiesis. This study evaluates the impact of pelvic RT on hematological toxicities in pediatric patients with pelvic genitourinary rhabdomyosarcoma (GU-RMS). Methods: Following permission, a secondary analysis was conducted on 488 pediatric subjects with intermediate-risk RMS from the ARST0531 (NCT00354835) trial. This trial compared Vincristine, Dactinomycin, and Cyclophosphamide (VAC) to VAC alternating with Vincristine and Irinotecan (VI). Both arms received RT starting in week 4. Of these, 65 patients had pelvic GU-RMS (bladder/prostate = 60 [92.3%]; female-organ = 2 [3.1%]; paratesticular = 3 [4.6%]) and received pelvic RT, while 423 received non-pelvic RT. Multivariable logistic regression models assessed hematological toxicities (anemia, leukopenia, neutropenia, thrombocytopenia, lymphopenia) during weeks 1–43, adjusting for age, race, tumor size, and chemotherapy. Primary outcomes included any hematological toxicity, and secondary outcomes focused on febrile neutropenia (FN), infectious complications (IC), and the timing of toxicity at T1 (weeks 1–15), T2 (weeks 16–30), and T3 (weeks 31–43). Results: Subjects with GU-RMS receiving pelvic RT did not have a significantly higher risk of cytopenias compared to non-GU RMS patients. Neutropenia affected 79.4% of patients, with no significant difference between GU (73.8%) and non-GU (80.4%) (OR 0.64, p=0.16). Thrombocytopenia was significantly more common in GU-RMS patients during the first 15 weeks (OR 2.79, p=0.01). FN and IC were comparable across groups. Conclusions: Hematological toxicities were similar in pediatric GU and non-GU RMS patients. Pelvic RT was linked to an early rise in thrombocytopenia, though the difference diminished over time. Understanding these toxicities is key to improving management of pediatric RMS patients. Patient characteristics and hematological toxicities outcomes: pelvic GU vs. non-GU primary. Parameter Overall Cohort, N (%)N = 488 GU, N (%)N= 65 (14.51) Non-GU, N (%)N = 383 (85.49) p-value < 10 y.o. 306 (68.30) 56 (86.15) 250 (65.27) 0.001 Male 241 (53.79) 48 (73.85) 193 (50.39) <0.001 White Race 319 (71.21) 48 (73.85) 271 (70.76) 0.487 Cytopenia OR (95% CI, p ) Anemia 159 (35.5) 24 (36.9) 135 (35.2) 0.96(0.54-1.70, 0.89) Thrombocytopenia 131(29.2) 22 (33.8) 109 (28.4) 1.49 (0.81-2.73, 0.20) Leukopenia 196 (43.75) 26 (40) 170 (44.4) 0.73 (0.42-1.27, 0.27) Neutropenia 356 (79.4) 48 (73.8) 308 (80.4) 0.64 (0.34-1.19, 0.16) Lymphopenia 105 (23.4) 15 (23.1) 90 (23.5) 0.85 (0.45-1.61, 0.61) GU – Genitourinary; OR – Odds Ratios.