Interfraction Variations in Organ Filling and Their Impact on Dosimetry in CT Image Based HDR Intracavitary...

Since anatomical and geometric variations occur with every fraction, planning, and dose optimization is necessary for every fraction of high-dose rate intracavitary brachytherapy of carcinoma cervix. In this study, we have tried to quantify the differences in doses to organs at risk (OAR) for each fraction of brachytherapy.

One hundred and seventy computed tomography datasets of cervical cancer patients receiving intracavitary brachytherapy at our institution between January and April 2015 were analyzed. The volumes of the high-risk clinical target volume and OAR contoured were recorded for every insertion. Dose-volume histograms were generated and D90 and D100CTV and D0.1, D1, and D2cc were recorded for bladder, rectum, and sigmoid for each insertion.

Sixty-one percent had a decrease in bladder volume in the second fraction, 35% had an increase in bladder volume and 4% had no change in bladder volume. There was a strong positive correlation between increase in volume and dose (D2cc), which was statistically significant, r_s = 0.441, P = 0.013. Nearly 49.4% of patients had an increase in rectal volume during the second fraction. 45.9% had decrease in rectal volume during the second fraction. There was a positive correlation between the increase in volume and dose (D2cc), which was statistically significant, r_s = 0.393, P = 0.010. About 63.5% of the patients had a decrease in sigmoid volume during the second fraction, whereas 30.6% had an increase in volume and 5.9% had no change in volume.

First, this study emphasizes the importance of imaging and planning for every fraction of brachytherapy to quantify the exact doses to the target and OAR s. Second, it is important to follow a uniform bladder protocol for every fraction, and adequate bowel preparation is needed for every fraction to minimize the interfraction variations. Finally, it also opens the realm of an adaptive planning strategy in cervical cancers which are known for rapid tumor regression during radiotherapy.