Stereotactic body radiotherapy of prostate cancer: influence of intrafraction motion of prostate on final dose distribution

Purpose: To determine impact of various planning tumour volume (PTV) margins on final dose distribution in patients with maximal intrafraction prostate displacement. Materials: We analyzed data of 15 prostate cancer patients with maximal (значения) prostate intrafraction displacement registered during stereotactic body radiotherapy (SBRT) — 5 fractions of 7.25 Gy. Prostate displacement was determined by cone beam CT that was performed before and after each fraction. Modeling of dose distribution considering prostate displacement was performed for every fraction by isocenter shift. Final dose distribution was calculated as the sum of all fractions and was evaluated for PTV with various margins: 5-5-5-3, 3-3-3-1, 5-5-5 and cropped rectum. Results: The dosimetric impact of maximal intrafraction prostate motion was minimal for target coverage even for PTV with minimal margins (3 in all directions and 1 mm — posterior): CTV V100% varied from 97.5 to 100%. Prostate displacement reduced the dose to the rectum in all but minimal doses were detected for PTV 3-3-3-1: average D2cc — 31.87 Gy (28.41–34.05). For PTV 3-3-3-1 average bladder V100% volume slightly increased from 3.67 (SBRT plan) to 4.87 cc (simulation of intrafraction prostate displacement). Conclusions: The dosimetric impact of maximal intrafraction prostate motion was minimal for target coverage and doses in rectum and bladder even for PTV with minimal (3-3-3-1) margins.