Abstract
Objective:Radiotherapy for prostate cancer does not explicitly take into account daily variation in the position of the rectum. It is important to accurately assess accumulated dose (DA) to the rectum in order to understand the relationship between dose and toxicity. The primary objective of this work was to quantify systematic (Σ) and random (σ) variation in the position of the rectum during a course of prostate radiotherapy.Methods:The rectum was manually outlined on the kilo-voltage planning scan and 37 daily mega-voltage image guidance scans for 10 participants recruited to the VoxTox study. The femoral heads were used to produce a fixed point to which all rectal contours were referenced.Results:Σ [standard deviation (SD) of means] between planning and treatment was 4.2 mm in the anteroposterior (AP) direction and 1.3 mm left–right (LR). σ (root mean square of SDs) was 5.2 mm AP and 2.7 mm LR. Superior–inferior variation was less than one slice above and below the planning position.Conclusion:Our results for Σ are in line with published data for prostate motion. σ, however, was approximately twice as great as that seen for prostate motion. This suggests that DA may differ from planned dose in some patients treated with radiotherapy for prostate cancer.Advances in knowledge:This work is the first to use daily imaging to quantify Σ and σ of the rectum in prostate cancer. σ was found to be greater than published data, providing strong rationale for further investigation of individual DA.
Highlights
In their study, if the planning rectum volume was $96 cm[3], the upper anterior rectal wall shifted posteriorly during treatment, but if the planning volume was,96 cm[3], the upper rectal wall shifted anteriorly during treatment
Variation in axial size of the rectum We found that the median rectal radius was 1.3 mm greater for contours drawn on the MV scans than those on the kV scans, making it 1.1 times as large
This work shows that the rectum could be tracked from day to day on the MV image guidance CT scans and is the first step towards building a model of this organ for calculation of DA
Summary
If the planning rectum volume was $96 cm[3], the upper anterior rectal wall shifted posteriorly during treatment, but if the planning volume was ,96 cm[3], the upper rectal wall shifted anteriorly during treatment. Lebesque et al[5] found that a large rectum at planning was predictive of a large difference between planning and treatment rectal volumes. Rectal distension at planning has been shown to correlate with an increased risk of biochemical and local relapse of prostate cancer
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