Radiotherapy techniques for treating muscle invasive bladder cancer have become increasingly sophisticated and image guidance using daily cone-beam computed tomography (CBCT) enables planning target volume (PTV) coverage to be accurately assessed before every treatment. We have adopted hypofractionated radiotherapy (36Gy/6F/weekly) in our centre as an alternative for less fit patients to the traditional regime of 55Gy/20F over 4 weeks. The PTV expansion margins are 1.5cm isotropically in the 36Gy/6F/weekly protocol which are greater than those used in the radical 55Gy/20F protocol where daily CBCT is used (lateral 0.8cm, anterior 1.5cm, posterior 1.2cm, superior 1.5cm, inferior 0.8cm). Reducing the PTV margins for the 36Gy/6F/weekly cohort using daily cone beam imaging could reduce treatment-related toxicity without compromising accurate delivery.
A retrospective study of daily CBCT images acquired from patients who attended Northern Centre for Cancer Care for hypofractionated bladder radiotherapy between December 2016 and December 2017 was undertaken. Study cohort comprised 15 patients aged 71-97 years with performance status 0-2. Images were reviewed offline by senior therapeutic radiographers who performed a bony match registration followed by a soft tissue match to clinical target volume (CTV), then an assessment of whether radical PTV margins would cover the bladder.
86 images were available for review. Fourteen of the 15 patients would have been able to have treatment with the smaller radical PTV margins. One patient would not have met these margins due to a considerable change in bladder shape compared with planning CT, but this was consistent throughout 6 weeks treatment. A re-plan for this patient may have overcome this problem and radical PTV margins may have been achievable. Excluding this patient, 3 scans out of 80 (4%) would need the patient to re-empty their bladder and have an additional CBCT. 96% of scans would be able to meet radical PTV margins. Including the patient that requires a re-plan, 92% of images would meet radical PTV margins.
Image-guided radiotherapy may allow the delivery of hypofractionated palliative bladder radiotherapy using smaller PTV margins. This has potential to reduce toxicity in less fit patients. Future work will look at implementing reduced PTV margins in these patients.