E Huynh*, C Williams, Z Han, L Singer, M Huynh, R Mak, D Cagney, J Leeman, Brigham and Women's Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA

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  E Huynh

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WE-IePD-TRACK 3-2 (Wednesday, 7/28/2021) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]

Purpose: MR-guided adaptive radiotherapy enables visualization of the prostate and surrounding anatomy at each treatment fraction and ability to adapt the treatment plan to account for interfraction variations in anatomy. This study investigates changes in prostate volume throughout treatment and its impact dosimetrically.

Methods: Prostate cancer patients were treated with MR-guided adaptive SBRT (MRIdian, Viewray Inc.) at our institution between 06-2020 and 02-2021 to 36.25Gy in 5 fractions. The “original” plan was re-optimized at each treatment based on daily anatomy to improve target coverage and meet organ-at-risk (OAR) metrics. A “predicted” plan was generated at each fraction by re-calculating the original plan on the day-of anatomy, estimating what would have been delivered without adaptive capabilities. Dose to the PTV (V36.25) and OARs (rectum, bladder, urethra) were assessed. Differences between simulation and treatment fractions for prostate volume and PTV V36.25 were calculated.

Results: A cohort of 13 patients treated in 65 fractions was analyzed. The median prostate volume was 29.3cc (range: 16.2-104.6cc) at time of simulation. Increases in prostate volume were observed for 51/65 fractions. At the first treatment, the prostate volume was similar to simulation (median difference of -0.2cc (range: -1.5-0.49cc)). However, the median volume increased from simulation at successive fractions (fractions 2, 3, 4 and 5) by 1.1cc (range: 0.18-6.8cc), 1.2cc (range: 0.02-6.52cc), 1.0cc (range: -0.81-8.2cc) and 0.8cc (range: -1.79-7.71cc), respectively. As prostate volume increased, the predicted plan PTV V36.25 decreased by a median of -2.2% on the first fraction, and -10%, -8.9%, -6.6%, -8.4% on successive fractions. Adaptive planning improved PTV V36.25 on 54/65 fractions; remaining fractions required sacrificing PTV coverage to meet OAR metrics.

Conclusion: MR-guided prostate SBRT revealed a consistent increase in prostate volume after first fraction of treatment, which can impact PTV coverage. Adaptive re-optimization can significantly improve PTV coverage while minimizing toxicity to OARs.

Funding Support, Disclosures, and Conflict of Interest: EH, LS, RM, DC, CW have received research support from ViewRay Technologies outside the scope of this work.

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