Locoregional control in high-risk neuroblastoma using highly-conformal image-guided radiotherapy, with reduced margins and a boost dose for residual lesions

Introduction: Radiotherapy protocols for high-risk neuroblastoma (HR-NBL) vary across international studies. The purpose of this study was to evaluate the locoregional control in a national HR-NBL cohort treated with highly-conformal image-guided radiotherapy (IGRT), using reduced margins, and a boost dose for residual lesions. Materials and methods: Patients treated with radiotherapy as part of first-line HR-NBL treatment between 2015 and 2022 were eligible. To obtain clinical, internal, and planning target volumes, +0.5 cm, 4DCT-based, and + 0.3/0.5 cm margins, respectively, were added to the edited gross tumour volumes. Prescription dose was 21.6/1.8 Gy, followed by 14.4/1.8 Gy for any residual lesions measuring ≥ 1 cm³ at the time of radiotherapy planning. Intensity-modulated arc therapy was combined with daily cone beam CT-based online patient position verification. Locoregional failure (LRF) rates were compared for the presence of residual lesions < 1 cm³ vs. ≥ 1 cm³ (with/without locoregional activity on nuclear- and MRI[diffusion-weighted imaging]-scans) pre-radiotherapy, age at diagnosis, MYCN-status, [¹³¹I]mIBG therapy, response to induction chemotherapy, interval to radiotherapy onset, and metastatic site irradiation. Results: Among the 77 included patients, 34 had residual lesions (median volume: 10.0 cm³, IQR 4.8–29.9) with activity visible on 17 nuclear- and 10 MRI-scans. Five-year LRF rate was 7.8 % (95 % confidence interval 1.8–13.8), and not significantly different between those with residual lesions < 1 cm³ vs. ≥ 1 cm³ (6.4 % vs. 14.3 %, respectively, p = 0.27), or any of the other variables. All 6 LRFs (2 isolated, 4 combined) occurred < 1.5 years post-radiotherapy. Conclusion: In HR-NBL, IGRT with reduced margins and a boost dose for residual lesions ≥ 1 cm³ demonstrated excellent locoregional control, comparable to modern literature.