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Effectiveness of Daily Endorectal Balloon for Post-Prostatectomy Patients Undergoing Pencil Beam Scanning Proton Therapy

L Yin*, N Vapiwala, D Sutton, X Ding, S Swisher-McClure, V Bui, C Deville, Z Tochner, S Both, Univ Pennsylvania, Philadelphia, PA

SU-E-J-146 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Pencil beam scanning proton therapy was found more sensitive to organ motion as compared to conventional IMRT. Significant target under dose may be caused by interfractional tumor motion. The objectives of this study are to investigate the role of endorectal balloon (ERB) in daily patient setup throughout the course of proton therapy and its dosimetric impact.

Methods:
Ten consecutive post-prostatectomy patients enrolled on a prospective IRB approved institutional study were analyzed. Patients underwent CT/MRI simulation and treatment with daily ERB. Six T2-MRI verification scans were performed during the treatment course. A pencil beam scanning proton therapy plan was generated for each patient. Organ at risks and CTVs were contoured by physician on each of the weekly MRI scans. The MRI scans were subsequently rigidly fused to the CT simulation images in order to simulate 2 possible daily kV-kV patient setups: 1) alignment to bony anatomy and 2) alignment to pelvic bone in cranial caudal direction and ERB along the AP and lateral direction. The dosimetric impact of interfraction target motion on target coverage was compared for both scenarios.

Results:
We found that target interfraction motion in the AP direction (average 4.1mm) is consistently larger than those in the lateral and cranial caudal directions (2.9 mm and 3mm respectively) most likely due to variations in bladder volume. ERB can be used to further improve the patient bony alignment(~1mm), in AP and lateral directions. Overall when both ERB pelvic bony structures are used for patient alignment, we observed a median 4.03% decrease in V98% of CTV as compared to 8.27% when only bony alignment is used.

Conclusion:
In post-prostatectomy patients undergoing pencil beam scanning proton therapy, daily patient setup via bony alignment and ERB can stabilize the target and maintain the dose coverage within 5% relative to the initial treatment plans.

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