A treatment planning investigation into the dosimetric effects of systematic prostate patient rotational set-up errors.

The purpose of this study was to investigate the potential dosimetric effects of systematic rotational setup errors on prostate patients planned according to the RTOG P-0126 protocol, and to identify rotational tolerances about either the anterior-posterior (AP) or left-right (LR) axis, under which no correction in setup is required. Eight 3-dimensional conformal radiation therapy (3D-CRT) treatment plans were included in the study, half planned to give 7020 cGy in 39 fractions (P-0126 Arm 1) and the other half planned to give 7920 cGy in 44 fractions (P-0126 Arm 2). Systematic rotations of the pelvic anatomy were simulated in a commercial treatment planning system by rotating opposing apertures in the opposite direction to the simulated anatomy rotation. Rotations were incremented in steps of 2.5 degrees to a maximum of +/-5.0 degrees and +/-10.0 degrees about the AP and LR axis respectively. Dose distributions were evaluated with respect to the planning objectives set out in the P-0126 protocol. For patients on Arm 2 of the study, maintaining the prescribed dose to 98% of the PTV was found to be problematic for superior-end-posterior rotations beyond 5.0 degrees . The results also show that maintaining a rectal dose less than 7500 cGy to 15% of the volume can become problematic for cases of small rectal volume and large superior-end-anterior rotations. We found that setting rotational tolerances will depend on which Arm of the protocol the patient is, and how well the initial plan meets the protocol objectives. In general, we conclude that for rotations about the AP axis, no tolerance level is required; however, cases presenting extreme rotations should be investigated as routine practice. For rotations about the LR axis, we conclude that a tolerance level for patients on Arm 2 of the protocol should be set at +/-5.0 degrees . This tolerance represents the systematic setup error which would require correction if a variation to the initial plan was deemed unacceptable.