ABSTRACT
PURPOSE: Pencil-beam scanning proton therapy is particularly sensitive to anatomic changes, which may affect the delivered dose distribution. This study examined whether offline adaptation using on-treatment magnetic resonance imaging (MRI) scan during proton therapy could improve plan quality for pediatric patients. METHODS AND MATERIALS: Pediatric patients with at least 1 MRI scan in the treatment position (MRItx) during proton therapy between January 2017 and July 2019 were retrospectively reviewed. Patients underwent MRI and computed tomography simulation. Cases were planned with scenario-based optimization with 3 mm/3% positional/range uncertainty. Patients demonstrating anatomic change on MRItx were recontoured. The original plans were applied to the anatomy-of-the-day for dose recalculation (delivered plans). Plans were subsequently reoptimized offline, using original beam angles and dose-volume constraints (adapted plans). Delivered plans were compared with adapted plans to detect significant changes in plan quality, defined as a ≥5% decrease in the clinical target volume (CTV) receiving 95% of the prescription dose (V95) or a ≥5% increase in the dose-volume parameter used as an organ-at-risk constraint. RESULTS: Seventy-three pediatric patients were eligible, with 303 MRI scans (73 simulation and 230 MRItx scans) available for analysis. The median MRItx scans per patient was 3 (range, 1-7). Twenty patients (27%) showed anatomic change, with 11 (55%) demonstrating a significant change in delivered plan quality. Significant changes were noted on MRItx from week 2 (n = 3) or week 3 (n = 8). Seven of these 11 patients (64%) had a significantly reduced CTV V95 (median decrease, 7.6%; range, 5%-16%). Four (36%) had a significantly increased dose to the brain stem, hippocampus, and/or optic apparatus. Eight had a suprasellar low-grade glioma or head and neck rhabdomyosarcoma. CONCLUSION: On-treatment MRI was useful in detecting anatomic changes during proton therapy. MRI-based offline adaptation improved plan quality for most patients with anatomic changes. Further studies should determine the clinical value of MRI-based adaptive therapy for pediatric patients.
Subject(s)
Magnetic Resonance Imaging , Proton Therapy , Quality of Health Care , Radiotherapy, Image-Guided , Adolescent , Child , Child, Preschool , Female , Humans , Infant , Male , Radiotherapy Planning, Computer-Assisted , Young AdultABSTRACT
PURPOSE: To investigate the effect of interplay between spot-scanning proton beams and respiration-induced tumor motion on internal target volume coverage for pediatric patients. MATERIALS AND METHODS: Photon treatments for 10 children with representative tumor motions (1-13 mm superior-inferior) were replanned to simulate single-field uniform dose- optimized proton therapy. Static plans were designed by using average computed tomography (CT) data sets created from 4D CT data to obtain nominal dose distributions. The motion interplay effect was simulated by assigning each spot in the static plan delivery sequence to 1 of 10 respiratory-phase CTs, using the actual patient breathing trace and specifications of a synchrotron-based proton system. Dose distributions for individual phases were deformed onto the space of the average CT and summed to produce the accumulated dose distribution, whose dose-volume histogram was compared with the one from the static plan. RESULTS: Tumor motion had minimal impact on the internal target volume hot spot (D2), which deviated by <3% from the nominal values of the static plans. The cold spot (D98) was also minimally affected, except in 2 patients with diaphragmatic tumor motion exceeding 10 mm. The impact on tumor coverage was more pronounced with respect to the V99 rather than the V95. Decreases of 10% to 49% in the V99 occurred in multiple patients for whom the beam paths traversed the lung-diaphragm interface and were, therefore, more sensitive to respiration-induced changes in the water equivalent path length. Fractionation alone apparently did not mitigate the interplay effect beyond 6 fractions. CONCLUSION: The interplay effect is not a concern when delivering scanning proton beams to younger pediatric patients with tumors located in the retroperitoneal space and tumor motion of <5 mm. Children and adolescents with diaphragmatic tumor motion exceeding 10 mm require special attention, because significant declines in target coverage and dose homogeneity were seen in simulated treatments of such patients.
