Changes in penile bulb dose when using the Clarity transperineal ultrasound probe: A planning study.

PURPOSE: The Clarity system allows monitoring of intrafraction target organ movements in external beam radiation therapy of prostate cancer by using transperineal ultrasound. The probe positioning at the perineum could lead to a compression and shift of the penile bulb (PB) toward the high-dose region. Dose to the PB has been reported to be associated with the risk of posttreatment erectile dysfunction. This planning study reports on PB translations and changes in volume and dose when applying the transperineal ultrasound probe. METHODS AND MATERIALS: For 10 patients treated with external beam radiation therapy for prostate cancer between 2013 and 2014, a planning computed tomography scan with and without the ultrasound probe in place was acquired. The planning target volume and organs at risk including the PB were contoured in the computed tomography scan with and without the probe. Radiation therapy plans for both scenarios were calculated. In a second step, for planning with the probe in position, an additional objective for improved sparing of the PB was introduced. RESULTS: The median PB volume was 5.5 mL (range, 3.8-7.1 mL) without the probe and 3.5 mL (range, 2.0-5.8 mL) with the probe. The median shift of the PB was 1 mm in the posterior (range, 3 mm posterior-2 mm anterior) and 6 mm in the superior direction (range, 0-14 mm superior), with no relevant shift of the prostate. The median mean dose in 95% of the PB was 34.1 Gy (range, 6.0-50.4 Gy), 48.3 Gy (range, 7.2-56.8 Gy), and 39.4 Gy (range, 5.6-51.3 Gy) for plans without probe, with probe, and with probe and additional planning objective, respectively. CONCLUSIONS: Dose to the PB increased when using the transperineal probe. After introducing an additional plan-optimization objective for PB sparing, dose-volume parameters were below Quantitative Analyses of Normal Tissue Effects in the Clinic thresholds for all but one patient.

A dosimetric comparison of real-time adaptive and non-adaptive radiotherapy: A multi-institutional study encompassing robotic, gimbaled, multileaf collimator and couch tracking.

PURPOSE: A study of real-time adaptive radiotherapy systems was performed to test the hypothesis that, across delivery systems and institutions, the dosimetric accuracy is improved with adaptive treatments over non-adaptive radiotherapy in the presence of patient-measured tumor motion. METHODS AND MATERIALS: Ten institutions with robotic(2), gimbaled(2), MLC(4) or couch tracking(2) used common materials including CT and structure sets, motion traces and planning protocols to create a lung and a prostate plan. For each motion trace, the plan was delivered twice to a moving dosimeter; with and without real-time adaptation. Each measurement was compared to a static measurement and the percentage of failed points for γ-tests recorded. RESULTS: For all lung traces all measurement sets show improved dose accuracy with a mean 2%/2mm γ-fail rate of 1.6% with adaptation and 15.2% without adaptation (p<0.001). For all prostate the mean 2%/2mm γ-fail rate was 1.4% with adaptation and 17.3% without adaptation (p<0.001). The difference between the four systems was small with an average 2%/2mm γ-fail rate of <3% for all systems with adaptation for lung and prostate. CONCLUSIONS: The investigated systems all accounted for realistic tumor motion accurately and performed to a similar high standard, with real-time adaptation significantly outperforming non-adaptive delivery methods.