Comparison of 2D vs. 3D dosimetry for Rotte 'Y' applicator high dose rate brachytherapy for medically inoperable endometrial cancer.

The purpose of our research is to compare CT-based volumetric calculations and International Commission on Radiation Units and Measurements (ICRU) reference-point estimates of radiation doses to the target volume, bladder, rectum, and sigmoid colon in patients with carcinoma of the endometrium treated with high dose rate intracavitary Rotte 'Y' applicator brachytherapy (HDRB). Eleven patients with cancer of the endometrium were treated with CT-compatible HDR intracavitary Rotte 'Y' applicators (Nucletron) and underwent post-implant pelvic CT scans with applicators in place. All patients were treated using orthogonal radiography-based planning. The dose was prescribed to uterine point (a point located 2 cm below the center of a line drawn between the tips of the two ends of the Rotte applicator extending laterally from the tandem by half the maximum uterine width), Point A, and 0.5 cm depth along the upper 3 cm vagina. CT-images were transferred to the PLATO treatment planning system version 14.2.6 and retrospectively planned for volumetric calculations. The clinical target volume (CTV) included the entire uterus, cervix, and upper 3 cm of vagina. The volumes of organs at risk (OAR) were digitized. Dwell positions were identified and registered in both the uterine tandem for each patient. For those receiving HDRB alone, the prescribed dose was 7 Gy x 5 fractions. Patients who were treated following external beam radiation therapy (EBRT) received 4 Gy x 5 fractions. The interfraction interval was 6-8 hours. The DVHs were computed for the CTV, bladder, rectum, and sigmoid colon. To compare doses of OARs, 1.0 cc, 2.0 cc, and 5.0 cc volumes receiving the highest dose were calculated from DVHs. 3D optimization was done to improve target coverage and decrease dose to critical organs and compared with the 2D orthogonal radiograph-based plan. The mean of percentage of prescribed dose +/- S.D to 1 cc, 2 cc, and 5 cc of the OARs of interest were as follows: Rectum 44 +/- 21%, 39 +/- 18%, and 33 +/- 15%; bladder 104 +/- 36%, 91 +/- 31%, and 73.9 +/- 24%; and sigmoid 124 +/- 35%, 109 +/- 30%, and 89 +/- 25%, respectively. The corresponding dose to ICRU 38 bladder and rectal points were 98 +/- 55% and 50.5 +/- 32%, respectively. The mean dose to uterine point and point A were 99 +/- 1.7% and 98 +/- 3%, respectively. The mean CTV volume was 160 +/- 89 cc with the percentage of volume getting 100% and 90% of the dose being 62 +/- 12% and 68 +/- 12% with 2D plan versus 57 +/- 8% and 67 +/- 8.9% with 3D plan. The dose to critical organs were reduced with 3D optimization for rectum, bladder, and sigmoid by 5.6% (p = 0.04), 20.6% (p = 0.02), and 26.8% (p = 0.005), respectively. Compared to the 3-D volume dose, the prescription points overestimated the dose to the target volume. The under-dosing was because of inability of two channel applicator to cover volumes in the region of the cervix and vagina. The dose to sigmoid colon was high and attention should be given to the sigmoid dose at the time of treatment planning. 3D planning helped in reducing the dose to the critical organs without compromising target coverage. Correlations with outcome are needed to better define the role of 3D dosimetry in treatment planning.