Room: AAPM ePoster Library
Purpose: A case study of a coughing patient underwent prostate SBRT to investigate the motion amplitude and its impact on dose delivery error.
Methods: A prostate patient with three gold fiducial markers inserted in the prostate was treated with VMAT SBRT to deliver 36.25 Gy in 5 fractions. Excessive streaking artifact was noticed in the CBCT images. To investigate the cause, raw projections of the CBCT were exported and analyzed using an in-house program. The positions of each marker were automatically identified from the projection images. The prostate motion was determined as the average of the traces of three markers. To study the dosimetric impact, the measured prostate motion was synchronized with the delivery of VMAT plan. The dose of each beam segment was translated according to the motion when delivering this segment. Delivered dose was estimated as the summation of segment doses translated by motion. DVHs to the targets and organs at risk from the estimated dose were compared with the planned DVHs.
Results: Prostate moved inferiorly with motion amplitude more than 1 cm when the patient coughs. Coughing caused PTV3625 V95 to decrease from 95.1% (planned) to average of 93.8% [range 91.5% to 94.8%]. There was a direct linear relationship between the percentage of time prostate motion exceeded 3 mm in any direction to loss of PTV V95 (R2=0.97). Rectum dose was not changed (Rectum D0.03cc reduced by 0.02+/-0.24 Gy per fraction, p=0.84), neither does bladder dose (Bladder D0.03cc reduced by 0.09+/-0.35 Gy per fraction, p=0.60).
Conclusion: The presence of motion aggravated the streaking artifact caused by fiducial markers in CBCT. Coughing reduced target coverage without significantly changing normal tissue dose.