Changes in target volume during radiotherapy treatment of invasive bladder carcinoma

Ninety patients with stage T3 Nx Mo carcinoma of the urinary bladder were treated with radical megavoltage external beam radiotherapy. Planning for treatment was undertaken on a treatment planning system utilizing CT scan slices to define the target volume and patient outline. All patients underwent a second CT scan half way through their course of treatment to assess any change in target volume and the continued adequacy of the original treatment plan. Seventy-two patients (80%) had no spatial shift in target volume, but, of the 18 patients with such a shift, treatment plans were changed in seven. The majority of patients had no delay in continuing their treatment after replanning, but one patient had a gap of 5 days before restarting treatment. An analysis of the factors possibly associated with a change in target volume showed that a primary tumour at the bladder base, rather than elsewhere in the bladder, was the single most important criterion for predicting target volume changes. There was no correlation between the size of the initial tumour, or the size of the prostate gland in male patients, and the occurrence of a shift in volume outside the initial target volume. Some method of regularly assessing the continued relevance of the target volume may be needed in this group of patients to improve the precision of treatment and also improve results.