Dosimetric feasibility of hypofractionation for metastatic bone/bone marrow lesions from paediatric solid tumours

Background and purpose: The aim of this study was to determine the feasibility of hypofractionated schedules for metastatic bone/bone marrow lesions in children and to investigate dosimetric differences to the healthy surrounding tissues compared to conventional schedules. Methods: 27 paediatric patients (mean age, 7 years) with 50 metastatic bone/bone marrow lesions (n = 26 cranial, n = 24 extra-cranial) from solid primary tumours (neuroblastoma and sarcoma) were included. The PTV was a 2 mm expansion of the GTV. A prescription dose of 36 and 54 Gy EQD2_α/β=10 was used for neuroblastoma and sarcoma lesions, respectively. VMAT plans were optimized for each single lesion using different fractionation schedules: conventional (30/20 fractions, V_95% ≥ 99%, D_0.1cm³ ≤ 107%) and hypofractionated (15/10/5/3 fractions, V_100% ≥ 95%, D_0.1cm³ ≤ 120%)_. Relative EQD2 differences in OARs D_mean between the different schedules were compared. Results: PTV coverage was met for all plans independently of the fractionation schedule and for all lesions (V_95% range 95.5–100%, V_100% range 95.1–100%), with exception of the vertebrae (V_100% range 63.5–91.0%). For most OARs, relative mean reduction in the D_mean was seen for the hypofractionated plans compared to the conventional plans, with largest sparing in the 5 fractions (< 43%) followed by the 3 fractions schedule (< 40%). In case of PTV overlap with an OAR, a significant increase in dose for the OAR was observed with hypofractionation. Conclusions: For the majority of the cases, iso-effective plans with hypofractionation were feasible with similar or less dose in the OARs. The most suitable fractionation schedule should be personalised depending on the spatial relationship between the PTV and OARs and the prescription dose.