Evaluation of clinical implications in the use of dose to water versus dose to medium by using NTCP and TCP models for urinary bladder tumours

Purpose: To analyze the dosimetric and radiobiological differences between dose to water versus dose to medium for patients with carcinoma of the urinary bladder.

Materials and Methods: 15 patients with cancer of urinary bladder were selected for the study. VMAT plans were generated for each patient. The dose distributions were calculated in the modes dose to water and to medium with the Monaco treatment planning system. A dosimetric comparative analysis has been made between the two modes of planning in this study. Subsequently, NTCP and TCP were determined for OARs and targets respectively.

Results: The mean dose to 2 cc of the rectum, small bowel, left and right femoral heads respectively was higher by 0.8, 1.2, 2.7, and 2.2% for the dose to water calculation. Similarly, the mean dose to D₂, D₅₀, and D₉₈ for PTV was higher by 0.4, 0.3, and 0.3% for dose to water calculation. Such small dose differences had little effect on the values of TCP and NTCP.

Conclusion: For patients with the urinary bladder there were very small differences between results between calculations carried out in dose to medium and dose to water modes.