Evaluation of MVCT images with skin collimation for electron treatment planning

Purpose: To evaluate the accuracy of electron beam dose calculations in MVCT images containing lead alloy masks. Method and Materials: A phantom consisting of two 30x30x5 cm3 slabs of CIRS plastic water® was imaged using kVCT (GE Lightspeed-RT) and MVCT (TomoTherapy Hi·Art). Nine MVCT scans were taken with different square masks of lead alloy (Cerrobend®, density = 9.4 g·cm-3) on top of the phantom. The masks contained square apertures of 3x3 cm2, 6x6 cm2 and 10x10 cm2 and had thicknesses of 6 mm, 8 mm and 10 mm. The same collimation was simulated in the kVCT images by creating regions-of-interest (ROI) duplicating the sizes, shapes, and density of the masks. Using the Philips Pinnacle3 treatment planning system, twelve treatment plans were created for the combination of four electron energies (6, 9, 12, and 16 MeV) and the three apertures. For each plan, the mask thickness appropriate for the electron energy was used and the dose distributions calculated using the kVCT and MVCT images were compared. In uniform dose regions dose differences were calculated; in high dose-gradient regions distances-to-agreement (DTA) were measured. Results: In the uniform dose region, the maximum differences of doses in the MVCT images from doses in the kVCT images were greater than or equal to ±5% for all but one opening and energy combination. In the high dose-gradient region, more than half of the maximum DTA values exceeded 2 mm. Analysis of the MVCT images showed that the differences were largely due to two errors. First, the presence of the masks caused distortions in the MVCT numbers such that the calculated dose in the MVCT images penetrated less deeply. Second, distortion in the shape of the image of the collimation caused the calculation algorithm to scatter excess electrons into the central axis of the beam. Conclusion: The presence of Cerrobend® masks in MVCT imaging produces distortions in the CT numbers that make electron beam dose calculations insufficiently accurate for electron beam treatment planning. Supported in part by a research agreement with TomoTherapy, Inc