Room: AAPM ePoster Library
Spinal stereotactic body radiotherapy (SSRT) is a vital treatment option for malignant spinal metastatic tumors. The use of an MR-linac for SSRT as compared to a conventional linac can potentially provide improved soft tissue contrast for treatment delivery. The MR-linac geometry and beam configuration, however, can differ considerably from a conventional linac. In this work, a comparative study was performed to assess the SSRT treatment plan quality and delivery differences between a conventional linac and a 1.5T MR-linac using an anthropomorphic spine phantom.
Representative pediatric thoracic and lumbar SSRT treatment plans were generated for an anthropomorphic phantom for a conventional linac (Truebeam®) using RayStation treatment planning system (TPS), and for an MR-linac (Unity) using Monaco® TPS. Two cylindrical ionization chambers were used to measure dose within the phantom target and spinal cord regions for the plans delivered using the two linacs.
The thoracic and lumbar SSRT treatment plans formulated using Monaco and RayStation TPS were comparable when evaluated using various metrics such as D95%, R50, conformity, homogeneity and gradient indices as well as spinal cord dose constraints. The average absolute measured dose for the gross-tumor volume region of the phantom was within ±3% of the calculated plan dose for both linacs. Absolute dose measurements performed in the spinal cord insert of the phantom, which was located in a high dose gradient region, were within ±2% and ±7% of the calculated dose for the lumbar and thoracic plans respectively for the conventional linac and within ±11% and ±18% for the MR-linac.
This preliminary study found SSRT treatment planning and delivery for an MR-linac comparable to a conventional linac when using an anthropomorphic spine phantom. It was observed that improvement in methods to optimize SSRT plan adaptation was required to further lower spinal cord dose for the MR-linac.