Hebb M, Winter J, Champagne K, McCurdy B, Zhu H, Petropoulos L
Cancer Care Manitoba, Winnipeg, Manitoba.
IMRIS, Winnipeg, Manitoba.
Med Phys. 2012 Jun;39(6Part6):3665. doi: 10.1118/1.4734890.
To demonstrate the feasibility of a novel radiotherapy-compatible cableless radiofrequency (RF) coil for an MR-guided radiation therapy(MRgRT™) system that employs a movable MRI system. This coil technology will expedite clinical workflow by eliminating need for coil connections and cables, allowing RF coils to remain in place for treatment.
We quantified radiation transmission factors and surface dose changes for aluminum (Al), copper (Cu) and FR4 substrate typical of RF coils using a Varian Trilogy linear accelerator. Ion chamber measurements were performed by applying 6 MV fields of various sizes through sheets of each material and metal-substrate combination. These measurements were repeated with the materials on a standard treatment couch. Material optimization provided input to build a prototype cableless coil and representative coil segment for surface dose measurements.
We observed expected patterns for radiation field sizes between 1 cm × 1 cm and 25 cm × 25 cm and expected variation with off-axis distances < 12 cm. All metal-substrate combinations had transmission factors > 0.996, the lowest being Cu-FR4. Relative surface dose increases were similar for Cu-FR4 (3.9) and Al-FR4 (3.3) combinations. Couchtop relative surface dose increases were much greater than coil materials alone and did not increase substantially with the addition of coil materials (couch-only = 6.6, couch with Cu-FR4 = 7.3). Relative surface dose increase was 6.23 for prototype coil segment with capacitor, but the capacitors are not in the primary beam path for the coil design.
Results indicate surface dose effectsare the dominant consideration in RF coil design for MRgRT. Similar Cu and Al surface dose effects suggest Cu is a viable coil inductor material for this application. Given that coil material contribution to surface dose is small compared to the couchtop material it is feasible to keep this cablelesscoil in place during radiation treatment. This work has received research personnel support from IMRIS.
