The maximum difference for target D90 between the reference plan and plans with applicator rotation was 0.2%, 24.2%, 30.8% for 1) circumferential, 2) anterior, and 3) lateral target, respectively. For bladder D 2cc , the maximum difference was 2.7%, 11.0%, 26.1% for 1), 2), and 3), respectively. Similarly, for rectum D 2cc , it was 3.2 %, 5.0%, 24.7%, for 1), 2), and 3), respectively. Compared to circumferential target, anterior and one-sided lateral target had larger differences. Dosimetric differences > 5% occurred at ! 15 degree rotation (or ! 2.6mm radial shift) for anterior target and ! 10 degree rotation (or !1.7mm radial shift) for lateral target. For the lateral target, there was a substantial difference between symmetric rotations clockwise and counterclockwise rotations, while anterior and circumferential target dosimetry did not differ significantly between the two directions. Conclusion: We have demonstrated significant variation in the degree of impact based on location of tumor. In order to achieve a dosemtric deviation of less than 5%, rotational error must be limited to less than 10-15%. Quality assurance measures should be taken to avoid rotation between planning and treatment.
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