P181] Establishing margins from clinical to planning target volume for low-risk prostate cancer radiotherapy: A multi institutional study

Abstract

Purpose The aim of this study was to determine the appropriate margins from clinical target volume (CTV) to planning target volume (PTV) in low risk prostate cancer irradiation, in terms of image guided radiotherapy (IGRT) protocol from three radiotherapy (RT) centers. Methods Twenty patients with low risk prostate cancer were selected from each RT center for retrospective review. Prescribed dose of 78 Gy in 39 fractions was delivered with volumetric modulated arc therapy (VMAT). Pretreatment preparation was applied in accordance with institutional protocol. Cone beam computed tomography (CBCT) was daily co-registered with planning Computed Tomography (CT), using two registration protocols: bone match and prostate (soft tissue) match. Based on this data, set up errors and inter-fraction prostate motion were calculated in three RT centers. Literature data for delineation errors and intra-fraction motions were used to determine CTV-PTV margins for different imaging protocols (online 2D/2D bone match and online CBCT or 2D/2D prostate fiducial markers match). Results The standard deviation of the systematic set up errors was in the range of 1.6–3.0 mm and for random set up errors 1.8–3.0 mm. The overall mean prostate movement, relative to bone anatomy, in anterior/posterior (A/P), inferior/superior (I/S), and left/right (L/R) direction, respectively was for: 1st RT center 0.5 ± 1.9 (1 SD), 0.1 ± 0.8 and 0.1 ± 0.7 mm, 2nd RT center 0.1 ± 1.4 (1 SD), 0.1 ± 1.4 and 0.1 ± 1.5 mm, and. 3rd RT center 0.6 ± 2.2 (1 SD), 0.6 ± 2.2 and 0.3 ± 1.3 mm. According to the IGRT modalities and frequency of use, the calculated CTV-PTV margins of A/P, I/S and L/R directions, respectively were in the range of: -for online 2D/2D bone match 7.4–9.1, 7.4–10.1 and 5.5–7.0 mm, respectively, -for online CBCT or 2D/2D prostate fiducial markers matching: 5.7, 7.0 and 5.0 mm, respectively. Conclusions In low risk prostate cancer irradiation, CTV-PTV margins less than 10 mm could not be used safely without daily CBCT prostate based soft tissue matching, due to the increased risk of missing the CTV. Generally smaller, precise margins followed with daily IGRT and online corrections, may influence the local control and lower toxicity rates.