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Impact of Cylinder Size in High-Dose Rate Brachytherapy (HDRBT) for Primary Cancer in the Vagina

H Zhang

H Zhang*, M Gopalakrishnan, P Lee, V Sathiaseelan, Department of Radiation Oncology, Northwestern Memorial Hospital, Chicago, Illinois


SU-E-T-579 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To evaluate the dosimetric impact of cylinder size in high dose rate Brachytherapy for primary vaginal cancers.
Patients treated with HDR vaginal vault radiation in a list of cylinders ranging from 2.5 to 4 cm in diameter at 0.5 cm increment were analyzed. All patients’ doses were prescribed at the 0.5 cm from the vaginal surface with different treatment lengths. A series of reference points were created to optimize the dose distribution. The fraction dose was 5.5 Gy, the treatment was repeated for 4 times in two weeks. A cylinder volume was contoured in each case according to the prescribed treatment length, and then expanded to 5 mm to get a volume Cylinder_5mm_exp. A volume of PTV-Eval was obtained by subtracting the cylinder volume from the Cylinder_5mm_exp. The shell volume, PTV-Eval serves as the target volume for dosimetric evaluation.
DVH curves and average doses of PTV-Eval were obtained. Our results indicated that the DVH curves shifted toward higher dose side when larger cylinder was used instead of smaller ones. When 3.0 cm cylinder was used instead of 2.5 cm, for 3.0 cm treatment length, the average dose only increased 1%, from 790 to 799 cGy. However, the average doses for 3.5 and 4 cm cylinders respectively are 932 and 1137 cGy at the same treatment length. For 5.0 cm treatment length, the average dose is 741 cGy for 2.5 cm cylinder, and 859 cGy for 3 cm cylinder.
Our data analysis suggests that for the vaginal intracavitary HDRBT, the average dose is at least 35% larger than the prescribed dose in the studied cases; the size of the cylinder will impact the dose delivered to the target volume. The cylinder with bigger diameter tends to deliver larger average dose to the PTV-Eval.

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