BMI grouping showed the strongest correlation with dose effects : CTDI of optimal STD versus optimal SWIM positioning was 3.17 mGy versus 2.46 mGy ( subgroup 1 ) , 5.47 mGy versus 3.97 mGy ( subgroup 2 ) , 7.35 mGy versus 5.96 mGy ( subgroup 3 ) and 8.71 mGy versus 8.18 mGy ( subgroup 4 ) .