Daily Ultrasound Imaging for Patients Undergoing Postprostatectomy Radiation Therapy Predicts and Ensures Dosimetric Endpoints

Patients who receive radiation therapy (RT) for prostate cancer are routinely positioned through radiographic means. We set out to establish a data-driven process that defines bladder volume required to meet V40/65 constraints using daily bladder ultrasound (US) and comparative cone beam CT (CBCT) before placing a patient on the treatment table. This was a single institution retrospective study of 20 patients (390 CBCT scans) who received postprostatectomy RT. Each patient received a daily US before treatment. CBCT alignment was performed 3 times a week. The bladder and rectum were contoured on each CBCT and a session dose was recorded. A mixed-effect model was used to estimate trajectory slopes of radiation exposure with organs-at-risk 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 the 20 patients, 10 received treatment to PF and 10 received RT to the PF/pLN. Predefined bladder constraints were V65 < 50%, V40 < 70%, and rectal constraints were V65 < 35%, V40 < 55%. The CBCT bladder volume (76-578 cm3) was greater than the pretreatment bladder US (87-466 cm3) due to volume filling between measurements (r = 0.8 ± 0.05). Mixed model detected a statistically significant 3-way interaction (P < .01) for bladder volume and V40/65. Both PF and PF/pLN patients showed improvement in V40/65 with an increase in bladder volume. For PF patients, bladder constraints were met when the US volume was >108 cm3 and for PF/pLN patients when the US bladder volume was >200 cm3. Rectal filling showed no association with CBCT volume. Daily US of the bladder before postprostatectomy RT allows for dosimetric predictions before daily treatment. This should translate into fewer CBCT for the patient and improved machine throughput. This technique is easy to institute and ensures organs-at-risk volumetric constraints are met based on daily US measurements.