Assessment of organ-specific neutron equivalent doses in proton therapy using computational whole-body age-dependent voxel phantoms.

Proton beams used for radiotherapy will produce neutrons when interacting with matter. The purpose of this study was to quantify the equivalent dose to tissue due to secondary neutrons in pediatric and adult patients treated by proton therapy for brain lesions. Assessment of the equivalent dose to organs away from the target requires whole-body geometrical information. Furthermore, because the patient geometry depends on age at exposure, age-dependent representations are also needed. We implemented age-dependent phantoms into our proton Monte Carlo dose calculation environment. We considered eight typical radiation fields, two of which had been previously used to treat pediatric patients. The other six fields were additionally considered to allow a systematic study of equivalent doses as a function of field parameters. For all phantoms and all fields, we simulated organ-specific equivalent neutron doses and analyzed for each organ (1) the equivalent dose due to neutrons as a function of distance to the target; (2) the equivalent dose due to neutrons as a function of patient age; (3) the equivalent dose due to neutrons as a function of field parameters; and (4) the ratio of contributions to secondary dose from the treatment head versus the contribution from the patient's body tissues. This work reports organ-specific equivalent neutron doses for up to 48 organs in a patient. We demonstrate quantitatively how organ equivalent doses for adult and pediatric patients vary as a function of patient's age, organ and field parameters. Neutron doses increase with increasing range and modulation width but decrease with field size (as defined by the aperture). We analyzed the ratio of neutron dose contributions from the patient and from the treatment head, and found that neutron-equivalent doses fall off rapidly as a function of distance from the target, in agreement with experimental data. It appears that for the fields used in this study, the neutron dose lateral to the field is smaller than the reported scattered photon doses in a typical intensity-modulated photon treatment. Most importantly, our study shows that neutron doses to specific organs depend considerably on the patient's age and body stature. The younger the patient, the higher the dose deposited due to neutrons. Given the fact that the risk also increases with decreasing patient age, this factor needs to be taken into account when treating pediatric patients of very young ages and/or of small body size. The neutron dose from a course of proton therapy treatment (assuming 70 Gy in 30 fractions) could potentially (depending on patient's age, organ, treatment site and area of CT scan) be equivalent to up to approximately 30 CT scans.