Investigation of tumor motion influence on applied dose distribution in conventional proton therapy vs. IMPT a 4D Monte Carlo simulation study

Background: in radiation treatment of moving targets located in thorax region of patient body, the delivered dose does not match with the planned treatment, resulting in some over and under dosage in the tumor volume, as a function of motion magnitude and frequency. Several efforts have been done to investigate the target motion effects on dose distribution in the target and surrounding normal tissues. Materials and Methods: in this study a spherical object undergoing periodic motion was considered as target inside a water phantom and its motion magnitude and frequency were adjusted to mimic realistic respiratory patterns. We selected a proton beam for irradiation and considered two different strategies in the simulation procedure to provide 3D target dose coverage: 1- conventional proton therapy using passive dose delivery and 2- IMPT both under respiratory gating technique. Results: in conventional proton therapy, the dose contribution within the normal tissues increases linearly at each gating window increment and in motion gated IMPT the delivered dose to the target and normal tissues strongly depends on the target and beam scanning motion interplay, that results an over and under dosage in target volume. Conclusions: In Conventional Proton Therapy, although the applied dose distribution on dynamic target volume is satisfactory at each gating window size, a significant dose is delivered to the surrounding normal tissues in comparison with same calculation in motion gated IMPT. In order to protect healthy tissues it is very important to use active spot scanning methods in dose delivery, minimized target and beam scanning motion interplay.