Transperineal ultrasound is a good alternative for intra‐fraction motion monitoring for prostate stereotactic body radiotherapy

PURPOSES: To report our experience in a prospective study of implementing a transperineal ultrasound system to monitor intra‐fractional prostate motion for prostate stereotactic body radiotherapy (SBRT). MATERIAL AND METHODS: This IRB‐approved prospective study included 23 prostate SBRT patients treated between 04/2016 and 11/2019 at our institution. The prescription doses were 36.25 Gy to the Low‐Dose planning target volume (LD‐PTV) and 40 Gy to the High‐Dose PTV (HD‐PTV) in five fractions with 3 mm planning margins. The transperineal ultrasound system was successfully used in 110 of the 115 fractions. For intra‐fraction prostate motion, the real‐time prostate displacements measured by ultrasound were exported for analysis. The percentage of time prostate movement exceeded a 2 mm threshold was calculated for each fraction of all patients. T‐test was used for all statistical comparisons. RESULTS: Ultrasound image quality was adequate for prostate delineation and prostate motion tracking. The setup time for each fraction under ultrasound‐guided prostate SBRT was 15.0 ± 4.9 min and the total treatment time per fraction was 31.8 ± 10.5 min. The presence of an ultrasound probe did not compromise the contouring of targets or critical structures. For intra‐fraction motion, prostate movement exceeded 2 mm tolerance in 23 of 110 fractions for 11 of 23 patients. For all fractions, the mean percentage of time when the prostate moved more than 2 mm in any direction during each fraction was 7%, ranging from 0% to 62% of a fraction. CONCLUSION: Ultrasound‐guided prostate SBRT is a good option for intra‐fraction motion monitoring with clinically acceptable efficiency.