Interfractional variation of uterine position during radical RT: weekly CT evaluation

This study was performed to quantitate the positional change of the uterus during the period of radiotherapy (RT).

For 13 patients who received RT with the use of small bowel displacement system (SBDS) for uterine cervix cancer, CT scans were taken before the beginning of RT. Three more weekly CT scans were subsequently performed during the RT period. The position of the uterus on each set of CT images was defined and compared to that on the initial CT images. The difference was quantified by measuring the parameters as follows: the change of uterus border in superior (D(S)), right lateral (D(R)), left lateral (D(L)), anterior (D(A)) and posterior (D(P)) direction measured on images taken before and during treatments. The change of uterus volume was also measured, and the correlation between the uterus volume reduction and uterus mobility was tested.

The most prominent interfractional positional changes were in the cranio-caudal direction. Among the three sets of comparisons, the largest mean values for D(S), D(R), D(L), D(A) and D(P) were -0.77+/-1.87, -0.29+/-1.02, -0.3+/-1.25, -0.20+/-1.13 cm, and -0.55+/-1.21 cm, respectively. The average target volume reductions were 6.4% (p=0.1335), 11.7% (p=0.0138) and 27.2% (p=0.0192) in the 1st, 2nd and 3rd week of treatment, respectively. Significant correlation between D(S) and uterus volume reduction (p=0.002) was found.

The average interfractional positional changes of the uterus with the use of SBDS were relatively small, while the inter-patient deviations were large. The significant target volume reductions and inter-patient deviations of uterus mobility need to be accounted for when conformal radiation therapy for cervical cancer is performed.