Multiadaptive Plan (MAP) IMRT to Accommodate Independent Movement of the Prostate and Pelvic Lymph Nodes

Daily prostate displacement can be determined by registering the implanted fiducial markers from the planning CT with the daily verification images. However, the detected rotational shifts may not be corrected due to practical reasons. The question is what the dosimetric impact is if the rotational shifts are not corrected and what the dosimetric gain is if the rotations are compensated with translational shifts. The aim of the study is to quantify geometric and dosimetric effects of rotational shifts with and without corrections. Five patients, with 43 kilovoltage cone beam CT (KV-CBCT) and implanted markers, were selected for this retrospective study. The prostate, bladder, and rectum were manually contoured on each CBCT by a physician. Two methods were applied to register markers from CBCTs with those from the planning CT: one method allowed both translational and rotational shifts to align the markers first and then zero out the rotation, mimicking the frequent clinical practice in situations without a six-degree treatment table; the other method allowed only translational shifts to align markers, partially compensating the prostate rotations by translational shifts. Center of mass distance (CMD) and Overlap index (OI) were evaluated as geometric indices for these two registration methods. Their dosimetric effects were also analyzed by comparing the dose coverage of the prostate clinical target volume (CTV) and the possible reduction of the planning margin. We found that prostate rotational shifts dominated in the left-right axis, with respective systematic and random