Room: AAPM ePoster Library
Purpose: Investigation of the Bolus Effects from Patient Immobilization Devices for IMRT and VMAT treatment of Shallow Tumors
Methods: A RANDO body phantom was scanned using two common immobilization devices: Accuform and VacLok (both manufactured by CIVCO). BB markers were placed on the phantom surface for treatment setup and reproducibility. Treatment scans for both setups were sent to Eclipse and fused using rigid registration. The PTV volume was contoured on the phantom to lie near the surface edge. Both static IMRT and VMAT plans were created using similar optimization criteria for the PTV and normal tissue falloff. PTV coverage was minimum 95%D/95%V for both plans. A standard fractionation dose of 200 cGy was chosen for the prescription. The treatment plans were delivered on a Varian Trilogy using both immobilization devices. Optically stimulated luminescent dosimeters (Nanodots) as well as Mosfet detectors were used to record surface measurements. The measurement devices were placed in the same position on the phantom for both setups.
Results: Surface dose measurements for the static IMRT delivery were higher than the VMAT (IMRT(s)??(f)?(c)? = 33% vs VMAT(s)??(f)?(c)? = 20%). Surface dose was also higher for all measurement points with plans using the Accuform as the immobilization device, particularly with the static IMRT plan (D(S)??(f)?(c)?(A)(c)(c)?(f)??(m) = 35% compared to D(S)??(f)?(c)?(V)?(c)(l)?(k) = 30% with the Nanodots; and D(S)??(f)?(c)?(A)(c)(c)?(f)??(m) = 42% vs D(S)??(f)?(c)?(V)?(c)(l)?(k) = 39% with Mosfets).
Conclusion: The choice of patient immobilization device may influence surface dose values for patients. The use of the Accuform device showed consistently higher readings for surface dose than the VacLok. These differences were greatest with static IMRT plan delivery vs VMAT. This could have clinical significance for patients susceptible to erythema or radiation burns for tumors near the skin surface where doses could approach the prescription value. Findings warrant a more thorough investigation.
Not Applicable / None Entered.