Objective: Our previous work showed the feasibility of concurrent thermal therapy with the SURLAS (Scanning Ultrasound Reflector Linear Arrays System) and irradiation with a Helical Tomotherapy (HT) system. The current work investigates the feasibility of using MVCT simulation to perform HT treatment planning for concurrent thermal therapy with the SURLAS applicator.

Methods: In the HT system, a 6-MV linear accelerator is mounted on a ring gantry in a CT configuration for helical radiation delivery and MVCT imaging. The SURLAS is a device designed for simultaneous operation with linac-based external beam radiation.
The SURLAS was placed on a RANDO phantom simulating a patient with superficial disease. A MVCT was obtained in the HT system and transferred to a Pinnacle treatment planning station (Philips Medical Systems v.8.0). An organ at risk (OAR) was contoured at various distal depths from the deepest part of the superficial PTV. The walls and internal metallic parts of the SURLAS were contoured as well and treated as objectives to completely or directionally block radiation through them. Once all regions of interest were outlined in the MVCT, the CT-data set and regions of interest files were transferred to the HT planning station. HT optimization parameters included a jaw width of 2.5 cm, pitch of 0.287 and a modulation factor of 2.0. Prescription for one simultaneous fraction was set to 95% of the PTV to receive 4Gy.

Results: The use of MVCT eliminates metallic artifacts produced at conventional CT x-ray energies. A series of SURLAS-HT treatment plans using MVCT were generated and analyzed. The relationship between PTV homogeneity, coverage, and sparing of the OAR and the degradation of such parameters due to the SURLAS metallic parts was evaluated. Both normal and fine dose grid calculation resolution were employed. Differences in dose calculations between conventional CT and MVCT will be reported.

Conclusion: HT MVCT-based treatment planning produces clinically acceptable treatment plans when the SURLAS is positioned on a RANDO phantom. The delivery of this concurrent therapy seems feasible.