Tissue equivalency of phantom materials for neutron dosimetry in proton therapy

Purpose: Previous Monte Carlo and experimental studies involving secondary neutrons in proton

therapy have employed a number of phantom materials that are designed to represent human tissue.

In this study, the authors determined the suitability of common phantom materials for dosimetry of

secondary neutrons, specifically for pediatric and intracranial proton therapy treatments.

Methods: This was achieved through comparison of the absorbed dose and dose equivalent from

neutrons generated within the phantom materials and various ICRP tissues. The phantom materials

chosen for comparison were Lucite, liquid water, solid water, and A150 tissue equivalent plastic.

These phantom materials were compared to brain, muscle, and adipose tissues.

Results: The magnitude of the doses observed were smaller than those reported in previous experi-

mental and Monte Carlo studies, which incorporated neutrons generated in the treatment head. The

results show that for both neutron absorbed dose and dose equivalent, no single phantom material

gives agreement with tissue within 5% at all the points considered. Solid water gave the smallest

mean variation with the tissues out of field where neutrons are the primary contributor to the total

dose.

Conclusions: Of the phantom materials considered, solid water shows best agreement with tissues

out of field.