(P023) Dosimetric Evaluation of Accelerated Partial Breast Irradiation Utilizing the ViewRay Magnetic Resonance Image-Guided Radiation Therapy System

Benjamin W. Fischer-Valuck, MD, Karl Sona, Sahaja Acharya, MD, Min Yang, Rojano Kashani, PhD, Imran Zoberi, MD, Maria Thomas, MD, PhD; Washington University

PURPOSE: External beam radiation therapy (EBRT) is a noninvasive alternative to deliver accelerated partial breast irradiation (APBI). However, due to the larger target volume, cosmetic outcomes after EBRT APBI may be inferior to more invasive techniques, such as brachytherapy. Utilizing the ViewRay magnetic resonance imaging (MRI)-guided radiation therapy system, MRI can be used for planning and daily positioning, as well as for precise targeting during treatment. This prefraction imaging allows for a reduction in the planning target volume (PTV) margin, which may improve acute skin toxicity and cosmetic outcomes compared with traditional EBRT methods.

MATERIALS AND METHODS: Ten patients with ductal carcinoma in situ (DCIS) or early-stage invasive breast cancer and negative axillary lymph nodes were treated with APBI using the ViewRay system. Prescription dose to the PTV was 38.5 Gy, delivered in 10 fractions, twice daily. The PTV for ViewRay treatment planning included a 1-cm margin around the lumpectomy cavity. Comparison three-dimensional conformal EBRT (3DCRT) plans for each patient were generated using the Pinnacle planning system and a 2-cm margin around the lumpectomy cavity. Dosimetric parameters for the PTV, ipsilateral breast, and other critical structures for each treatment plan were compared.

RESULTS: Using the ViewRay system to obtain MR imaging for setup prior to each fraction, we observed that the lumpectomy cavity could be precisely localized and aligned prior to each fraction. A decrease in the PTV margin using the ViewRay system resulted in a mean PTV volume of 85 cc compared with 177 cc for 3DCRT. The median PTV receiving 95% of the prescribed dose for both the ViewRay and 3DCRT plans was 100%. Mean volume of the ipsilateral breast receiving the prescription dose was 11.72% for ViewRay treatment plans compared with 21.56% for 3DCRT treatment plans (P < .04). Mean volume of the ipsilateral breast receiving 50% of the prescribed dose using the ViewRay treatment plan was 31.34% compared with 52.71% for the 3DCRT treatment plans (P < .02). Reductions in the mean volumes of the ipsilateral lung, heart, and contralateral breast with the ViewRay plans were also observed, although they were not statistically significant.

CONCLUSION: MRI-guided EBRT using the ViewRay system is a novel approach to deliver APBI. ViewRay APBI is noninvasive yet maintains a high degree of precision by using prefraction MR imaging, thus allowing a reduction in the PTV margin. The resultant decrease in the ipsilateral breast dose may reduce acute skin toxicity and improve cosmetic outcomes. Thus, the ViewRay system is an attractive alternative to existing APBI techniques.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org