The dosimetric effect of inconsistency of bladder filling states between CT simulation and IMRT treatment for prostate cancer patients

Objective: To analyze the dosimetric effect of inconsistent bladder filling states between the CT simulation and treatment for prostate cancer patients undergoing intensity-modulated radiotherapy (IMRT). Methods: A total of 42 prostate cancer patients treated with IMRT were selected. After vacuum pad immobilization and simulation CT scan, the delineation of targets and organs-at-risk (OARs), treatment planning and dose calculation were performed on treatment planning system (TPS). The cone-beam-CT (CBCT) acquired before the first treatment was registered to planning CT, on which the target contours were duplicated and OARs were delineated. After dose recalculation on the CBCT using the same plan, the dosimetric differences on the CT and CBCT were compared, including mean dose of the planning target volume (PTV), homogeneity index (HI), conformity index (CI); the mean dose, V₃₀, V₄₀, V₅₀, V₆₀, and V₆₅ of bladder and rectum respectively. Results: Relative to the bladder volumes at simulation, the patients were divided into two groups with larger (15 cases) or smaller (27 cases) bladders at the first treatment. Comparing the parameters obtained from simulation CT with that from CBCT, the differences of the following parameters were of statistical significance:the bladder volume, PTV HI, PTV CI in both groups (t = 6.838, -4.372, -3.553, -3.462, 6.380, 5.037, P < 0.05), the bladder V₃₀, V₄₀, V₅₀, V₆₀, V₆₅ (t = -5.004, -4.092, -3.124, -2.707, -2.489, P < 0.05) and rectal V₄₀, V₅₀, V₆₀, V₆₅, mean dose in the group with smaller bladders (t = -2.946, -2.643, -2.426, -3.127, -2.530, P < 0.05), and the bladder V₃₀, V₄₀, V₅₀ and mean dose in group with larger bladders (t = 5.107, 4.204, 3.777, 4.155, P < 0.05). Conclusions: For prostate cancer patients undergoing IMRT, the inconsistent bladder filling states between the planning and treatment will cause disimetric differences of targets and OARs. (authors)