The impact of organ motion on intestine doses and complication probabilities in radiotherapy of bladder cancer.

BACKGROUND AND PURPOSE: The intestine is an organ at risk during irradiation of tumours in the abdomen and pelvis, and it is therefore of interest to predict the risk for complications when planning the treatment. However, this organ displays considerable temporal variations in volume and shape. The aim of this investigation was to investigate the uncertainties caused by organ motion in dose-volume histograms (DVHs) and normal-tissue-complication probabilities (NTCP's). PATIENTS AND METHODS: Between 6 and 8 weekly repeat CT scans were acquired for 10 patients with muscle invading urinary bladder cancer. The intestine was delineated in all scans, and the coordinates of the outlines were transferred to the planning CT using the appropriate transformation. Using the actual treatment plan, the DVHs for each of these 6-8 instances of the intestine as well as the corresponding NTCP estimates were calculated. Also, for each patient, a 3D matrix was created that contained the number of scans where the intestine occupied the voxels represented by the elements of the matrix. From this matrix additional information about the organ movements were extracted. RESULTS: The mean values (across scans for individual patients) for the volume receiving at least 30.8 Gy, V30.8, ranged from 77 to 336 cm3, from 52 to 250 cm3 for V49.5 and from 38 to 243 cm3 for V53.5. The corresponding relative standard deviations were 0.45, 0.45, and 0.51, respectively. The relative standard deviations (over repeat scans for each patient) had ranges 0.065-0.45, 0.10-0.53, and 0.10-0.54 and the mean relative deviations were 0.20, 0.24, and 0.26, approximately half the magnitude of the variation between the mean values for the patients. For 6 out of 10 patients, the volume occupied by the intestine in only one of the CT scans was larger than the volume occupied in all CT scans, thus illustrating the very mobile nature of this organ. CONCLUSIONS: The movements of the small intestine cause large uncertainties in the DVH and calculated NTCP for the individual patient, and the usefulness of dose constraints for this organ may be questioned. Still, the inter-patient variation was larger, and it may be that the DVH can be useful for judging which patients have the greatest risk for radiation injury.