R Cattell*, A Hsia, J Kim, X Qian, S Lu, A Slade, K Mani, S Ryu, Z Xu, Stony Brook University Medical Center, Stony Brook, NY

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  R Cattell

PO-GePV-M-132 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

ePoster Forums

Purpose: To summarize our institutional prostate stereotactic body radiation therapy (SBRT) experience using auto beam hold (ABH) technique for intrafractional prostate motion and assess ABH tolerance of 10-millimeter (mm) diameter.

Methods: Thirty-two patients (160 fractions) treated using ABH technique between 01/2018 and 03/2021 were analyzed. Planning target volume (PTV) margin was 5mm circumferential (3mm posteriorly) around the prostate. SpaceOAR (Boston Scientific, Marlborough, MA) was placed in all cases prior to treatment. During treatment, kV images were acquired every 20-degree gantry rotation to visualize 3-4 gold fiducials within prostate to track target motion. If center of fiducial fell outside tolerance, beam was automatically turned off for reimaging and repositioning. Number of beam holds and magnitude of couch shift corrections were recorded. Dosimetric differences from intrafractional motion were calculated by shifting planned isocenter.

Results: Couch correction magnitude (mean±SD) in vertical, longitudinal, lateral directions were -0.7±2.5, 1.4±2.9 and -0.1±0.9mm, respectively. For most fractions (77.5%), no correction was necessary. Number of fractions requiring one, two, or three corrections were 15.6%, 5.6% and 1.3%, respectively. Of the 49 corrections, couch shifts greater than 3mm were seen primarily in the vertical (31%) and longitudinal (39%) directions; corresponding couch shifts greater than 5mm occurred in 2% and 6% of cases. Dosimetrically, 100% coverage decreased less than 2% for clinical target volume (CTV) (-1±2%) and less than 10% for PTV (-10±6%). Dose to bladder, bowel and urethra tended to increase (Bladder: D1cc:0±8%, D10%:184±466cGy, D40%:139±241cGy, Bowel: D1cc:54±129cGy; D5cc:44±116cGy, Urethra: D0.03cc:1±1%). However, doses to the rectum tended to decrease (Rectum: D1cc:-206±564cGy, D10%:-97±426cGy; D20%:-50±251cGy; D50%:-25±97cGy).

Conclusion: Large prostate movements (>5mm) were detected in 22.5% of treatments, with predominant motion in longitudinal and vertical directions. With this motion, rectal dose tended to decrease, while bladder dose increased. With 10mm diameter ABH circular threshold, coverage loss of clinical target volume was minimal.

Keywords

Prostate Therapy, Image-guided Therapy, Radiation Therapy

Taxonomy

TH- External Beam- Photons: Motion management - intrafraction

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