A systematic review of prostate bed motion and anisotropic margins in post-prostatectomy external beam radiotherapy

•Prostate bed motion can cause target volume miss in radiotherapy.•The largest prostate bed motion occurs in the anterior-posterior direction.•The magnitude of motion varies between the superior and inferior prostate bed.•Anisotropic CTV to PTV margins are recommended for post-prostatectomy. Prostate bed (PB) motion may lead to geographical miss of the target volume in post-prostatectomy radiotherapy (RT). Optimal clinical target volume (CTV) to planning target volume (PTV) margins prevent geographical miss and unnecessary irradiation of normal tissue. There is little data available informing appropriate CTV to PTV margins in the post-prostatectomy setting. The purpose of this review was to quantify the inter-fraction and intra-fraction motion of the PB and draw a conclusion regarding the use of anisotropic CTV to PTV margins for post-prostatectomy RT treatment. A search of PubMed and EMBASE databases was carried out using keywords (prostate bed [Title/Abstract]) AND (motion [Title/Abstract]). All study types assessing inter-fraction and/or intra-fraction motion of the PB based on imaging of soft tissue anatomy were included. Data on patient preparation, immobilisation, and image guidance was abstracted from the included studies. Magnitude of PB motion along with the estimated CTV to PTV margins calculated was also tabulated. Quality of studies was assessed using the MINORS tool. Seventeen studies were included in the analysis. The largest magnitude of inter-fraction PB motion occurs in the anterior-posterior direction. This motion is attributed to the influence of the bladder and rectal volume on the PB. The PB moves independently of bone and the magnitude of motion varies between the superior and inferior portions of the prostate bed. Anisotropic CTV to PTV margins are appropriate for use in the post-prostatectomy setting and their implementation for treatment planning purposes are warranted based on the evidence reviewed.