A94: Target coverage during image-guided radiotherapy for prostate cancer – how relevant is the initial rectal volume?
1Mount Vernon Cancer Centre, Rickmansworth, UK,2Genesis Cancer Care, Queensland, Australia,3Cancer Partners UK, Elstree, UK
Rectal distension on the planning CT scan is a predictive factor for biochemical failure following prostate radiotherapy, for patients treated without volumetric image guidance [1-3]. We investigated the variation and impact of rectal volume during modern image-guided radiotherapy (IGRT) with daily cone-beam CT (CBCT) verification.
844 CBCT verification images from 20 patients undergoing radical prostate IGRT were included. Patients were treated with 74 Gy in 37 fractions using 7-field intensity-modulated radiotherapy. Daily online CBCT soft tissue matching was used for treatment verification. CBCT images were imported into the treatment planning system, and volumes were contoured manually. A 7 mm margin in all directions was used around the clinical target volume (CTV). Soft tissue match shifts were applied, and contours superimposed on the planning CT. Dose-volume parameters were assessed for the CTV, rectum and bladder.
Rectal volume during the course of radiotherapy showed major fluctuations from the volume on planning CT scan. Re-imaging during daily verification reduced this variation. Mean rectal volume during treatment was higher than the planning CT volume in the majority of cases. In contrast, the volume of contoured CTV showed minimal variation from planning CT. There was moderate correlation between the rectal volume on planning CT scan and the range of rectal volume variation during treatment (R = 0.46, p < 0.05). However, high rectal volumes on planning CT were not associated with a reduction in CTV coverage during treatment (Spearman's rho -0.05, p > 0.05). On unpaired analysis of treatment fractions, there was an association between increased rectal volume and low bladder volume with a reduced CTV coverage. However, there was no such association on mixed model analysis when inter-patient variation was incorporated.
Rectal volume on the planning CT scan is not predictive of CTV coverage during prostate IGRT with daily CBCT imaging.