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Whole abdomen radiotherapy (WART) is used to treat pediatric malignancies with a high risk of peritoneal spread. WART with X-Ray (XRT) based intensity-modulated radiation therapy (IMRT) has been reported to reduce acute toxicities compared to conventional radiotherapy (CRT). Proton beam therapy (PBRT) may further reduce toxicities. We compare treatment logistics, dosimetry, and outcomes for WART with PBRT and XRT. A retrospective chart of pediatric patients receiving WART at a single center from 2014 to 2023 was performed. Dosimetric parameters, treatment delays, replanning needs and outcomes were compared between patients receiving XRT and PBRT. T-tests were used for statistical analysis where appropriate. 13 patients (7 male) with Wilms tumor (WT) (n = 11) or embryonal rhabdomyosarcoma (ERMS) (n = 2) were included. Median age at RT was 5.8 years (range 1.4-11.0). Nine WT patients received 10-17 Gy in 7 fractions and two received 18-20 Gy in 12-13 fractions. The ERMS patients received 30-36 Gy in 20 fractions. Median time from simulation to RT start was 5.5 days (range 1-8) for XRT and 8.4 days (range 5-13) for PBRT (p = 0.09). No significant differences were observed for mean dose to kidneys, femoral growth plates, vertebrae, bladder, liver, heart or V8.5 Gy and V15Gy for the liver between XRT or PBRT. The 6Gy volume in the kidneys approached significance, PBRT plans indicated significant bilateral femoral head sparing (p = 0.027). Median RT duration was 9 days (range 8-30). Of the seven PBRT patients, two required mid-treatment replanning, and two switched mid-PBRT to IMRT due to significant daily bowel content variability. Two PBRT patients had unanticipated interruptions during RT: one for replanning and a 2nd for management of increased ascites. All 6 XRT (5 IMRT, 1 3D XRT) patients completed their treatment as planned with no interruptions. Median follow-up duration post-treatment was 29.4 months (range 2.7-101.2). At last follow-up, 1 patient died of disease, 1 was undergoing treatment, and 11 had no evidence of disease recurrence or progression. IMRT and PBRT WART achieved at least comparable abdominal organ-at-risk sparing in pediatric patients. PBRT may offer some dosimetric advantages, such as better sparing of femoral heads. PBRT requires more planning time, verification scans, and replans. We note that careful evaluation of treatment set up and bowel content surveillance is necessary to insure appropriate planning and delivery of PBRT for WART. Individualized modality selection balancing efficacy, availability, and efficiency is warranted. Further study of long-term outcomes and strategies to reduce PBRT delivery complexity is needed. [ABSTRACT FROM AUTHOR]

Metaiodobenzylguanidine (MIBG) scintigraphy is standard for detecting metabolic tumor burden and radiotherapy (RT) target delineation in pediatric neuroendocrine neoplasms. However, MIBG scans are only 50% sensitive, with poor localization and spatial resolution. This is problematic for 3D RT planning, resulting in treating unnecessarily large volumes or inadvertent omission of areas that need RT. Most relapses occur in sites seen at baseline but not irradiated due to satisfactory chemotherapy response per MIBG. This suggests occult disease below the detection threshold of MIBG. We initiated a non-randomized prospective single institution trial of patients ≤30 years old with high-risk neuroendocrine neoplasms to assess the utility of FDA-approved 68Ga-DOTATATE PET/CT in defining RT clinical target volume (CTV) compared to MIBG. Patients were enrolled at initial diagnosis or later in treatment for primary metastatic or recurrent disease. Standard multimodality therapy was given before consolidative RT. RT volumes were standard-of-care and determined by post-induction MIBG. 68Ga-DOTATATE PET scans were obtained at the time of RT planning. An institutionally approved unplanned interim analysis was performed after RT delivery. This included clinically blinded review of PET scans by 2 board certified Nuclear Medicine Radiologists and comparison to baseline scans and MIBG-determined CTVs. Patterns of failure will be assessed during an ongoing 2 year follow up. Eight patients with neuroblastoma of median age 3.5 years (all <18 years old; 4 female; 7 white) were analyzed. One patient enrolled after recurrence and had previous RT. All others enrolled during initial treatment. Results of PET scan analysis are shown on the table. The average (range) SUV max was 6.7 (2.5-33) for positive scans (n = 6/8). At the time of interim analysis, only 2/8 patients are 2 years post-RT. Both are without evidence of relapse, including 1 with untreated PET-avid sites (n = 4). Three relapses occurred <2 years post-RT, with 1 instance of radiographic progression in an untreated PET-avid site during maintenance therapy. Results from this interim analysis suggest 68Ga-DOTATATE may identify lesions not detected with MIBG. Although at least 1 participant in our ongoing study may have benefitted from utilization of 68Ga-DOTATATE PET/CT for CTV delineation, multiple patients with untreated avid sites have not experienced relapse. Further efforts are required to define clinically meaningful findings on 68Ga-DOTATATE PET. [ABSTRACT FROM AUTHOR]