Author: Minsong Cao, Amar Kishan, Dylan P. O'Connell, Jonathan Pham, Michael Steinberg, Luca F Valle, Michael Xiang 👨🔬
Affiliation: Department of Radiation Oncology, University of California, Los Angeles 🌍
Purpose: To evaluate bladder ultrasound as a surrogate for estimating on-table bladder volume in prostate cancer patients undergoing stereotactic body radiation therapy (SBRT). The study aimed to assess the impact of pretreatment bladder volume measurement on daily treatment bladder volume and dosimetric consistency.
Methods: Nineteen pelvic patients treated with radiation therapy were analyzed in this study. Bladder ultrasound (10s acquisition) was performed before CT simulation and prior to daily treatment to assess bladder volume. A pass/fail criterion of ±25% deviation from the simulation bladder volume was used to determine whether daily treatment proceeded, or patient intervention was required (wait, drink water, etc.). Ultrasound bladder volume accuracy was evaluated in relation to CT and on-board CBCT contoured bladder volume. Inter-fraction bladder volume variation for prostate-only and prostate+lymph node (LN) SBRT patients (40Gy/5Fx;25Gy/5Fx) was compared to a prior cohort of sixty-nine treated without pretreatment bladder ultrasound. The dosimetric impact of on-table bladder volume on planning and daily bladder V20Gy≤30% constraint was evaluated.
Results: Bladder ultrasound measurements demonstrated good agreement with CT and on-board CBCT measurements (R2=0.69) with an offset of 100ml. In a sub-cohort of 6 ultrasound-guided prostate SBRT patients, ultrasound resulted in lower absolute on-table inter-fraction bladder volume change than non-ultrasound (20.4±4.5% vs 32.6±17.8%; p=0.02). All ultrasound-guided prostate SBRT patients met planning and daily bladder V20Gy constraint and showed lower daily variation than non-ultrasound (2.53±2.10% vs 3.90±2.68%; p=0.07). Retrospective domestic volume analysis indicated a minimum bladder volume threshold of 200/250ml for prostate/prostate+LN to maintain acceptable planning and daily bladder V20Gy (96%/92% vs 91%/79%).
Conclusion: Bladder ultrasound is a quick and viable surrogate for estimating on-table bladder volume in prostate cancer radiation therapy. Bladder volume threshold can improve simulation and planning consistency. Moreover, patient-specific bladder volume thresholds can be established to improve treatment delivery consistency and dosimetric outcomes.