Treatment margins and treatment fractionation in conformal radiotherapy of muscle-invading urinary bladder cancer.

Background and Purpose: Different treatment margins and fractionation schedules are used in conformal radiotherapy (CRT) of urinary bladder cancer. This study compared intestine and rectum dose-volume histogram (DVH) data and normal tissue complication probability (NTCP) estimates for various clinically applied margins and fractionation schedules in bladder irradiation.
Patients and Methods: Normal tissue dose distributions in fifteen bladder cancer patients treated with CRT were studied using standard three- and four-field configurations. The impact of margin width on intestine and rectum dose distributions was initially evaluated using DVH data. NTCP modelling with the probit model was used to compare the impact of choice of margin size and fractionation schedule. The analysis included margin combinations of 1.0 cm isotropic (narrow margins) and 1.2-2.0 cm non-isotropic (wide margins) and fractionation schedule alternatives of 52.5Gy/20, 55Gy/20, 57.5Gy/20 and 64Gy/32.
Results: Using wide as compared to narrow margins, the volumes of intestine and rectum receiving high doses increased by factors of approximately two and four, respectively. Similar differences between wide and narrow margins were found when calculating intestine and rectum NTCPs. The impact of margin size depended strongly on the volume effect expressed by the NTCP model parameters. With standard parameters, however, the choice of margins and fractionation schedule had a similar impact on intestine NTCPs, while for the rectum, the choice of margin had a greater impact than the choice of fractionation. For a given margin size, the intestine and rectum NTCPs for the 55Gy/20 and the 64Gy/32 schedules were comparable. For clinics using narrow margins and a fractionation of 52.5Gy/20, the NTCP modelling suggested that a change in fractionation schedule (to 55Gy/20 or 64Gy/32) or a change to wide margins would have a similar effect on the intestine NTCP predictions.
Conclusions: This modelling study documented that the choice of margins was as important as the choice of fractionation in terms of intestine and rectum DVH data and NTCP predictions.