[Comparison of Two Image Guided Radiation Therapy (IGRT) Methods Used for Prostate Cancer Patients--CBCT and 2D-2D KV]

The IGRT notion (image guided radiation therapy) comprises all techniques enabling checking and correction of patients position directly before or during an irradiation session. In last years they became a standard in radiotherapy due to decreasing of geographical misses. The aim of our study was a comparison of two IGRT techniques--CBCT and 2D-2D kV performed for prostate cancer patients and a comparison of two immobilization systems used for them. The performed analysis comprises 3582 2D-2D kV and 2110 CBCT IGRT measurements made for 85 prostate cancer patients. Patients were irradiated using thermoplastic masks and two kinds of immobilizing plates. One subgroup of patients was treated using leg supports and second one without them. Mean, maximum, minimum and standard deviation of absolute values of shifts (cm) measured using 2D-2D kV were 0.27, 0.0, 2.8 and 0.33 respectively. For CBCT these same values were 0.31, 0.0, 4.1 and 0.33. Mean, maximum, minimum and standard deviation of real shifts values (cm) measured using 2D-2D kV were -0.01, -2.5, 2.8 and 0.43 respectively. For CBCT these same values were 0.01, -4.1, 2.3 and 0.45. Comparison of shift absolute values distributions for whole analyzed group showed statistically significant difference (p=0.001) between 2D-2D kV and CBCT with higher mean for CBCT (0.31 vs 0.27) and equal standard deviations. Statistically significant difference (p=0.000...) between distributions of measurements in z axis was found (for 2D-2D kV mean 0.16, SD=0.21, for CBCT mean 0.25, SD=0.25). Comparison of absolute shifts values distributions revealed significant difference for CBCT, for two immobilization plates--in x (p=0.001) and y axis (p=0.007). For the small plate in x axis mean was 0.22 (SD=0.2), and for the large one (with an integrated head support) 0.17 (SD=0.2). Comparison of absolute shifts values for sub- groups created dependently on the leg support use, showed significant differences in x axis for 2D-2D kV (p=0.000...); mean in the subgroup irradiated without leg supports was 0.14 (SD=0.14) and for the subgroup without them was 0.19 (SD=0.15) and in x axis for CBCT (p=0.03); mean in the subgroup irradiated without leg supports was 0.18 (SD=0.18) and for the subgroup without them was 0.23 (SD=0.22). Significant difference was also revealed for the whole group of absolute shift values for 2D-2D kV (p=0.01). For the subgroup irradiated without leg supports mean was 0.27 (SD=0.34), and for the subgroup treated with them was 0.27 (SD=0.31). Obtained results permit us to conclude that in the case of prostate cancer patients IGRT based on the bone anatomy visualization, the method of choice should be 2D-2D kV, because it allows for more precise and shorter patient position evaluation and, that during IGRT of prostate cancer patients using a simple thermoplastic immobilization system is sufficient; use of more sophisticated systems with additional supports did not improve the patient immobilization.