Room: AAPM ePoster Library
Our standard immobilization for prostate patients undergoing MRCAT only simulation is a thermoplastic anterior mold. We evaluate a new workflow, where the patient is MR-simmed without immobilization. The immobilization mold used for treatment is machined from polystyrene foam based on the outer contour obtained from MR-sim.
Four patients went through MR-sim positioned on a pad without immobilization and received their custom machine-milled posterior mold for treatment delivery. Prescription for the 4 prostate bed and regional nodes patients were 72 Gy total of 40 fractions. Prescription for daily setup verification was bony registration using orthogonal kv images and weekly CBCT. A total of 160 setups with image-based immobilization molds were compared with 6 randomly selected prostate bed and regional nodes patients (230) setups using anterior Aquaplast immobilization. Interfractional setup reproducibility was analyzed. Difference of final online matched position and initial couch position were extracted from Aria Offline Review. Couch shift vector length (overall shift) was used for comparison along with translational shifts in each direction.
The mean of couch shift vector lengths for the 4 image-based immobilization (N=160) setups was 0.91 ±0.52 cm [range, 0 to 2.3 cm]. The mean of couch shift vector lengths for the 6 Aquaplast immobilization (N=230) setups was 0.94 ±0.45 cm [range, 0 to 3 cm]; The observed average shifts and ranges were similar in the superior-inferior, anterior-posterior and left-right directions.
The image-based immobilization workflow provides similar setup positioning for our MR-sim prostate patients without the need for covering the patient’s anterior skin surface. This opens the possibility to use surface image guidance and might offer a tattoo-less option for these patients in our clinic.
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