Daily Ultrasound Imaging for Patients Undergoing Post-Prostatectomy Radiotherapy Predicts and Ensures Bladder and Rectum Dosimetric Endpoints

Abstract

Patients who receive radiotherapy (RT) for prostate cancer are routinely set up for daily treatment through radiographic means. Dose volume histograms define dosimetric impact to the bladder and rectum based on their initial size at the time of CT simulation. We set out to establish a data-driven process, via daily bladder ultrasound (US) imaging and comparative cone-beam CT (CBCT), that define bladder volume required to meet V40/65 constraints prior to placing a patient on the treatment table. This is a single institution retrospective study of 20 patients who received post-prostatectomy RT (PPRT) in 2019. Each patient received a daily US prior to treatment to confirm bladder volume within +/-20% of the volume recorded at CT simulation. Three times a week, CBCT was performed on these patients. Retrospectively, we contoured the bladder and rectum on each CBCT registered. Contours were transferred to the planning CT and a session dose was recorded corresponding to the “contours-of-the-day”. A composite dose for all fractions was calculated utilizing CBCT mean volumes. A mixed effect model was used to estimate trajectory slopes of radiation exposure with OAR volume increase. Slope differences by V40/65 for prostate fossa (PF) and pelvic lymph nodes (PF/pLN) were tested using a 3-way-interaction term with Bonferroni correction for multiple comparison error. For the 20 patients, 390 CBCT scans had corresponding US – 10 received treatment to PF and 10 received RT to the PF/pLN. Pre-defined organ at risk constraints for the bladder were V65<50%, V40<70%, and V65<35%, V40<55% for the rectum. The CBCT bladder volume [76-578cc] was greater than the pre-treatment bladder US [87-466cc] due to volume filling during the time interval between US:CBCT measurements (r = 0.8±0.05). Mixed model detected a statistically significant 3-way interaction (p<0.01) for bladder volume and V40/65; this same interaction was not observed for the rectum. Both PF and PF/pLN patients showed improvement in V40/65 with an increase in bladder volume – PF patients had slightly improved results. For PF patients, bladder constraints are met when the US volume is >108 cc and for PF/pLN patients when the US bladder volume is >200 cc. Rectal filling showed no association with CBCT volume. Daily US of the bladder prior to PPRT allows for dosimetric predictions prior to daily treatment. This translates into fewer CBCT for the patient and improved throughput on the machine. This technique is easy to institute in routine clinical practice and ensures OAR volumetric constraints are met based on daily US measurements.