Dosimetric feasibility of stereotactic body radiation therapy as an alternative to brachytherapy for definitive treatment of medically inoperable early stage endometrial cancer

PURPOSE: This study was designed to evaluate the dosimetric feasibility of definitive stereotactic body radiation therapy (SBRT) for the treatment of medically inoperable early stage endometrial cancer. METHODS: CT simulation scans from 10 medically inoperable early stage endometrial cancer patients previously treated with high dose-rate (HDR) intracavitary brachytherapy were used to generate Helical Tomotherapy (HT) plans using the IMRT mode with clinical target volumes (CTVs) that included the uterus plus cervix. A prescription dose of 34 Gy in 4 fractions was used. The SBRT dosimetry was compared to the 10 prior intracavitary brachytherapy plans normalized to a standard dose. Organs at risk (OARs) evaluated were the bladder, rectum, sigmoid, femoral heads, and other bowel, including both large and small bowel. The simulation CT and daily image guidance for 4 patients treated with this technique were evaluated to assess for interfraction variation in the uterine position and effects on dosimetry. RESULTS: Compared to intracavitary brachytherapy, HT SBRT produced significantly greater overall target coverage to the uterus, boost CTV, and PTV, with exception of the V150% of the uterus. HT SBRT significantly increased dose to the rectum, bowel, and femoral heads compared to intracavitary brachytherapy, though not outside of dose tolerance limits. Review of daily image guidance for patients treated with this technique demonstrated good reproducibility with a mean overlap index of 0.87 (range, 0.74 – 0.99). CONCLUSIONS: Definitive SBRT for medically inoperable early stage endometrial cancer appears to be a feasible treatment option. Future studies are warranted to evaluate long-term clinical outcomes with this technique, compared to HDR intracavitary brachytherapy.