Changes in applicator positions and dose distribution between high dose rate brachytherapy fractions in cervix carcinoma patients receiving definitive radiotherapy

This study examines the change of applicator geometry and its effect on rectal/rectum (R) and bladder (B) doses, and obtained radiobiological equivalent doses (RED), between each high dose rate (HDR) brachytherapy (BT) fraction in cervical carcinoma patients. BT using a tandem (T) and two ovoids (O) is included, and any discrepancies in applicator positions among the fractions were calculated. Whether the change of applicator position had an effect on the calculated R and B doses was analysed. Furthermore, the relationship between the size of tumour, the magnitude of displacement and the change in R and B doses was also investigated. Lastly, the changes in R and B RED were noted. The average magnitude of displacement was between 2.0 mm and 16.9 mm, showing time trend. There was no relationship between tumour size and the magnitude of discrepancy of Left O, Right O, T, R, B, and neither change in R and B doses (p>0.05). The mean differences of R and B doses were between 49-78 cGy, and 70-84 cGy, respectively. The magnitude of discrepancy and changes in doses showed no correlation (p>0.05). There were no significant differences in REDs for bladder (p = 0.8) and rectum (p = 0.2). In conclusion, there were significant differences in the applicator positions R and B and R and B doses among the fractions, which confirm the necessity of treatment planning in each HDR BT fraction. However, the total calculated R and B REDs did not show a remarkable difference.