Lifetime attributable risk of radiation induced second primary cancer from scattering and scanning proton therapy - A model for out-of-field organs of paediatric patients with cranial cancer

Background and purpose: Proton therapy (PT) can reduce side effects for paediatric cranial malignancies. Despite the high number of paediatric patients treated with PT, radiation induced risk factors for second primary cancer (SPC) in out-of-field organs are unknown. This study estimated lifetime attributable risk (LAR) of SPC as a function of age and sex for out-of-field organs following passive scattering and scanning beam PT in paediatric brain tumours.

Materials and methods: Measured neutron dose equivalent spectra for scattered and scanning PT were sourced from literature. The physical distance of 12 measured organs from paediatric CT dataset-based phantoms (5, 9 and 13 years-of-age) were applied to Schneider et al.'s analytical model using MATLAB (R2020B) to calculate the organ-specific LAR of SPC.

Results: Scanning beam PT demonstrated smaller LAR (per 10,000 person years) of SPC compared to scattering. This was prominent for more radiosensitive organs, including the lung (320 vs 50), breast (1000 vs 150) and thyroid (350 vs 75), but not for all (i.e., rectum and reproductive organs were <10). For most organs, LAR was highest for 5-year-old females (i.e., breast LAR was 1,000 higher than for 13-year-olds), however, outliers existed for distal organs (i.e., stomach and lung).

Conclusion: There was large variation in LAR estimates of out-of-field organs based on measured neutron dose equivalents. Younger female cranial paediatric patients were found at higher risk compared to males, especially for passive scattering PT. Not all organs had improved LAR using scanning beam PT for younger age groups.