Imaging dose and secondary cancer risk in image-guided radiotherapy of pediatric patients

Background: Daily image-guided radiotherapy (IGRT) can contribute to cover extended body volumes with low radiation dose. The effect of additional imaging dose on secondary cancer development is modelled for a collective of children with Morbus Hodgkin. Methods: Eleven radiotherapy treatment plans from pediatric patients with Hodgkin's lymphoma were retrospectively analyzed, including imaging dose from scenarios using different energies (kV/MV) and planar/cone-beam computed tomography (CBCT) techniques. In addition to assessing the effect of imaging dose on organs at risk, the excess average risk (EAR) for developing a secondary carcinoma of the lung or breast was modelled. Results: Although the variability between the patients is relatively large due to the different target volumes, the additional EAR due to imaging can be consistently determined. For daily 6MV CBCT, the EAR for developing a secondary cancer at age 50 is over 3 cases per 10 PY (patient-years) for the female breast and 0.7-0.8 per 10 PY for the lungs. This can be decreased by using only planar images (<1 per 10 PY for the breast and 0.1 for the lungs). Similar values are achieved by daily 360° kV CBCT (0.44-0.57 per 10 PY for the breast and 0.08 per 10 PY for the lungs), which is again reduced for daily 200° kV CBCT (0.02 per 10 PY for the lungs and 0.07-0.08 per 10 PY for the breast). These values increase if an older attained age is considered (e.g., for 70years, by a factor of four for the lungs). Conclusions: Daily imaging can be performed with an additional secondary cancer risk of less than 1 per 10 PY if kV CBCT is applied. If MV modalities must be chosen, a similar EAR can be achieved with planar images. A further reduction in risk is possible if the imaging geometry allows for sparing of the breast by a partial rotation underneath the patient.