(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
Tumor translational and rotational uncertainties have been identifies as a significant cause of radiation dose delivery inaccuracy. Recent six degrees of freedom (6DOF) couches have the ability to align the patient for rotational and translational movements. The objective was to determine the clinical utility of rotational corrections by quantifying rotational movements for non-cranial lesions.
Methods:
Kilo-Voltage cone beam CT (CBCT) were acquired before radiation therapy of 1522 fractions for a total of 51 patients, for prostate (15), pelvic (11), lung (15), head and neck (10) disease sites. Each site was treated using a site specific immobilization device. During the setup, patient anatomy from planning CT was automatically registered to the KV-CBCT using both the mask and clip-box. The translational and rotational shifts were generated by the image registration and executed by the 6DOF robotic couch. Both the systematic and random rotational errors were calculated as defined by Van Herk.
Results:
The maximum correctable rotational errors using the 6DOF couch is 3 degrees. The rotational error distribution were not a standard normal distribution. There were site and patient dependent significant spread in the rotational errors. For the prostate, the systematic errors were 0.33 (pitch), 0.44(yaw) and -0.25(roll) and the random errors were: 0.58(pitch), 1.25(yaw) and 0.53(roll). For Pelvis, the systematic errors were 0.31(pitch), -0.25(yaw) and -0.01(roll) and the random errors were: 0.42(pitch), 1.22(yaw) and 1.05(roll). Similar magnitude of errors were observed for the lung and Head and neck. There were more than 60% of pelvis and 25% of prostate alignments with more than 1 degree of roll rotational
Conclusion:
Site and patient dependent significant rotational corrections were observed for all anatomical sites. Rotational setup errors should be corrected for all SBRT and SRS. For non-SBRT treatments, the need for daily 6DOF alignment should be reviewed for the first five fractions.
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