Session Item

For prostate radiotherapy, the inter- and intrafraction motion of the seminal vesicles (SV) are dependent on rectal filling. The purpose of this study was to evaluate the effect of two different rectal filling regimes on the interfractional movement of the SV. 

The interfractional movement of the SV was evaluated for 84 patients and 624 weekly CBCT scans. Two groups of prostate cancer patients with different preparational regimes were retrospectively identified and analyzed. In group A (33 patients and 219 CBCT scans) laxatives were given routinely for all patients before simulation CT and for group B (51 patients and 405 CBCT scans) no laxatives were given, unless indicated based on rectal filling on initial simulation CT. The extent and direction of the SV movement was identified by CT-CBCT evaluation with focus on the SV. It was identified if SV were outside our CTV to PTV margin (5mm in all directions except cranially 8mm). Furthermore, the number of patients that required more than one CBCT/fraction and a new treatment plan during the course of treatment were identified. 

It was possible to evaluate the SV in 89 % of the CBCT scans for the total population (Group A: 192; Group B: 365). Reasons why it was not possible to evaluate the SV was deteriorated image quality due to metal artefacts from hip prothesis and/or artifacts from gas volumes mainly in the rectum. Movement of the SV was identified in 72 % of the CBCT evaluations, and more outliers with movement > 5 mm were observed in group B (14.8 %) compared to group A (13.0 %). In total 11 (13 %) patients had SV identified to be outside the PTV margin at one or more fractions. The relative frequency was higher in group B compared to group A both in number of patients, 15.6 % compared to 9.1 %, and number of fractions, 4.9 % compared to 1.6 %. This is also reflected in the number of patients that required more than one CBCT at some point during treatment, 27 % in group B compared to 12 % in group A, and the number of patients that required a new treatment plan due to lack of SV dose coverage, 7.8 % in group A compared to 3 % in group B. 

This study on interfractional movements in prostate radiotherapy, indicate that a strict rectal filling regime with use of laxatives prior to simulation CT can reduce the clinical workload, both in terms of the number of extra CBCT scans as well as the number of new treatment plans required due to SV movements outside the PTV margins.