Organ-specific low-dose assessment in pediatric radiotherapy using nanodot OSL and NTCP modeling: Application to medulloblastoma.
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BACKGROUND: In pediatric radiotherapy, controlling low-dose exposure is a major challenge, particularly in the treatment of medulloblastoma, where the long life expectancy of young patients makes optimization of radioprotection crucial. PURPOSE: This study provides a comprehensive and quantitative assessment of low-dose exposure to normal tissues across the entire radiotherapy workflow, with a particular emphasis on the contribution of imaging procedures (planning computed tomography (CT) and repeated cone beam computed tomography (CBCT) acquisitions) in addition to therapeutic irradiation. The cumulative impact of these steps is evaluated together with the associated risks using Normal Tissue Complication Probability (NTCP) modeling. METHODS: A pediatric anthropomorphic phantom was equipped with optically stimulated luminescence (OSL) dosimeters to measure doses delivered to normal organs during the planning CT, 15 CBCT scans for positioning, and simulated treatment plans with different radiotherapy techniques. NTCPs were calculated for critical organs based on the dosimetric data from imaging and treatment planning. RESULTS: Each imaging acquisition delivered between 3 and 7 mGy per organ, with the cumulative dose from CT and CBCT remaining consistently below 0.5 Gy, representing less than 1% of the prescribed therapeutic dose. Although small compared to therapeutic irradiation, this contribution remains biologically relevant when considering cumulative low-dose exposure in pediatric patients. Dosimetric comparison of treatment techniques showed that volumetric modulated arc therapy (VMAT) significantly reduced both the mean organ dose and the NTCP compared to three-dimensional conformal radiation therapy (3D-CRT): heart dose decreased from 16 Gy (NTCP 30%) to 6.6 Gy (NTCP 0.4%), and thyroid dose from 27.1 Gy (NTCP 12%) to 8.7 Gy (NTCP < 1%). Some organs, such as the brain and chiasma, remained highly exposed, reflecting the dosimetric constraints of comprehensive tumor coverage. CONCLUSION: By systematically quantifying and integrating imaging doses with therapeutic irradiation, this study underscores the often underestimated role of imaging in cumulative exposure during pediatric craniospinal irradiation. The results provide a solid foundation for tailoring radioprotection strategies and emphasize the need to reconsider planning and follow-up protocols to deliver safer and more targeted treatments for children.