The effects of external beam irradiation on the growth of flat bones in children: Modeling a dose-volume effect

Purpose: To model the effects of external beam irradiation on the developing flat bones of pediatric patients undergoing radiation therapy (RT) for tumors involving the musculoskeletal system. Methods and Materials: Patients with image-guided RT plans including areas adjacent to facial or pelvic flat bones underwent retrospective contouring of nontumor involved flat bones ipsilateral and contralateral to the treatment side. Radiation dose-volume information and bone volume data (initial and the most recent follow-up) were analyzed in 15 paired flat bones from 10 patients (ages 1.0-17.0 years). The models to predict bone growth after completion of RT (v post) were based on initial bone volume (v pre), the patient's age, time to follow-up (t), and the dose-volume parameter (vInt35+ ). Results: We developed a dose effects model as follows: Log (vpost / vpre ) = βtimet + βagegroupt + βdoset vInt35 . The dose-volume parameter vInt35 predicted significantly for alterations in growth in younger patients, but not for older patients. The predictability of the fitted model for relative change in bone growth improved in the younger age group with the addition of the dose-volume term vInt35 (correlation coefficient of r = 0.5510 to r = 0.6760 with the addition vInt35 ). Conclusions: Our model accurately predicted flat bone growth and is notable for the inclusion of radiation dose-volume information, which is now available in the image-guided RT era. Further refinement of this model in a prospective patient population is underway