Purpose: Stereotactic body radiation therapy (SBRT) has been shown to be an effective, well-tolerated treatment for local control of tumors metastatic to the liver. Multi-institutional Phase II trials are examining 60 Gy in 3 fractions delivered by linac-based, 3D-conformal IMRT. HiArt Helical TomoTherapy is a treatment unit that delivers co-planar helical IMRT that is capable of image-guided SBRT. We hypothesized that the maximum tolerable dose (MTD) delivered to a lesion by Helical TomoTherapy-based SBRT could be predicted based on the planning target volume (PTV) and liver volume.
Methods And Materials: To test this, we performed inverse treatment planning and analyzed the dosimetry for multiple hypothetical liver gross tumor volumes (GTV) with conventional PTV expansions. Inverse planning was carried out to find the maximum tolerated SBRT dose up to 60 Gy to be delivered in 3 fractions based on the dose constraint that 700 cc of normal liver would receive less than 15 Gy.
Results: Regression analysis indicated a linear relationship between the MTD, the PTV and the liver volume, supporting our hypothesis. A predictive equation was generated, which was found to have an accuracy of +/-3 Gy. In addition, dose constraints based on proximity to other normal tissues were tested. Inverse planning for PTVs located at varying distances from the heart, small bowel, and spinal cord revealed a predictable decrease in the MTD as the PTV increased in size or approached normal organs.
Conclusions: These data provide a framework for predicting the likely MTD for patients considered for Helical TomoTherapy liver SBRT.
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