Dosimetric Impact Of Rotational Setup Errors For Cervical Cancer Patients With Postoperative Volumetric Modulated Arc Therapy

Rotational setup errors cause inadequate doses to the target and normal tissues, especially elongated target. With the combination of a kilovoltage imaging device and three degrees of freedom couch, rotational setup errors can’t be corrected. Few studies have been done to investigate if the rotational setup errors for gynecologic cancer patients affect doses to the target and normal tissues. The present study evaluated the dosimetric impact of rotational setup errors for postoperative cervical cancer patients with VMAT. From twenty postoperative cervical cancer patients treated with 50.4 Gy by 28 fractions using VMAT, cone beam CT (CBCT) sets were acquired once a week. In total, 121 CBCT sets were registered to the reference CT sets based on bony landmark with treatment planning system (TPS) and rotational setup errors (pitch, yaw, and roll) were calculated. The 121 rotational CT sets and structures involving rotational setup errors were created with comprehensive solution and dose distributions were recalculated on these CT sets with TPS in the same beam parameters as the reference. For CTV and OARs (rectum, bladder, small bowel, large bowel, bone marrow), the recalculated dosimetric parameters were compared with the reference. As for the rotational setup errors for pitch, yaw, and roll, the mean ± systematic errors were 0.46° ± 1.30°, 0.02° ± 0.70°, and -0.31° ± 0.57° respectively, and the maximum errors, 3.90°, -2.61°, and -2.13° respectively. For CTV and OARs, the differences of dosimetric parameters and the correlation coefficients between the difference of dosimetric parameter and each rotational setup errors were shown in Table 1. For rectum V40 and bladder V45, the maximum differences were found more than 5.0%. CTV and most OARs, except large bowel, have strong correlation with the pitch setup error.Abstract 3481; Table 1.Difference (%)Correlation CoefficientMin.Max.PitchYawRollCTV D2-0.500.30-0.740.32-0.13CTV D98-1.590.21-0.700.16-0.02Rectum V40-8.036.19-0.910.12-0.07Bladder V45-5.1711.720.910.01-0.03Small Bowel V40-4.903.06-0.910.11-0.18Large Bowel V40-1.690.91-0.300.260.03Bone Marrow V40-1.101.570.95-0.050.00 Open table in a new tab These results indicate that rotational setup errors have a slight impact to the target coverage for cervical cancer patients with postoperative VMAT. However, it is noticeable that the pitch setup error affects dose to rectum and bladder if it is more than 1.5°.