The usefulness of vinyl polysiloxane dental impression material as a proton beam stopper to save normal tissue during irradiation of the oral cavity: basic and clinical verifications.

Purpose: To evaluate the feasibility and usefulness of vinyl polysiloxane (VPS) dental impression material as a proton beam stopper for oral cavity irradiation.

Methods: VPS compounds with different base-catalyst mixture ratios were created, and the relative linear stopping power (RLSP) of each VPS compound was measured to compare with the RLSPs obtained from converted CT data. Then, a model plan was created to simulate oral cancer, and depth-dose distributions that were calculated using radiation treatment planning (RTP) were investigated by comparing the distribution with the measurements. The radioactivation of the VPS material was also measured after 2-Gy proton beam irradiations. For clinical use, a T4 gingival squamous cell carcinoma was treated using proton beam therapy with a VPS bite block. Treatment plans with and without the VPS bite block were created, and the dose-volume histograms (DVH) of the tongues were compared.

Results: Both the RLSPs and the CT numbers were constant of the ratio of VPS mixtures. The measured RLSP of the VPS was 1.51±0.01, which was approximately 4% greater than the CT-converted RLSP. In a model simulation, the measured depth-dose distribution inside the VPS dropped steeply compared to the RTP calculation, and the dose behind the VPS bite block was less than 0.1% of the prescribed dose. The equivalent dose rates for VPS immediately after irradiation were below 1 μSv∕h and reached background levels within 30 min. In clinical use, VPS reduced a 10 cc local overdose region as well as the mean dose in the tongue compared to the plan without VPS, while the DVH of the planning target volume was maintained. The onset of severe mucositis was not observed behind the VPS bite block.

Conclusions: VPS is easy to shape and reproducible. The authors succeeded in demonstrating its safety and accuracy as a proton beam stopper.