Helical Tomotherapy-Based STAT Stereotactic Body Radiation Therapy: Dosimetric Evaluation for a Real-Time SBRT Treatment Planning and Delivery Program

Stereotactic body radiation therapy (SBRT) treatments have high-dose gradients and even slight patient misalignment from the simulation to treatment could lead to target underdosing or organ at risk (OAR) overdosing. Daily real-time SBRT treatment planning could minimize the risk of geographic miss. As an initial step toward determining the clinical feasibility of developing real-time SBRT treatment planning, we determined the calculation time of helical TomoTherapy-based STAT radiation therapy (RT) treatment plans for simple liver, lung, and spine SBRT treatments to assess whether the planning process was fast enough for practical clinical implementation. Representative SBRT planning target volumes for hypothetical liver, peripheral lung, and thoracic spine lesions and adjacent OARs were contoured onto a planning computed tomography scan (CT) of an anthropomorphic phantom. Treatment plans were generated using both STAT RT "full scatter" and conventional helical TomoTherapy "beamlet" algorithms. Optimized plans were compared with respect to conformality index (CI), heterogeneity index (HI), and maximum dose to regional OARs to determine clinical equivalence and the number of required STAT RT optimization iterations and calculation times were determined. The liver and lung dosimetry for the STAT RT and standard planning algorithms were clinically and statistically equivalent. For the liver lesions, "full scatter" and "beamlet" algorithms showed a CI of 1.04 and 1.04 and HI of 1.03 and 1.03, respectively. For the lung lesions, "full scatter" and "beamlet" algorithms showed a CI of 1.05 and 1.03 and HI of 1.05and 1.05, respectively. For spine lesions, "full scatter" and "beamlet" algorithms showed a CI of 1.15 and 1.14 and HI of 1.22 and 1.14, respectively. There was no difference between treatment algorithms with respect to maximum doses to the OARs. The STAT RT iteration time with current treatment planning systems is 45 sec, and the treatment planning required 3 iterations or 135 sec for STAT RT liver and lung SBRT plans and 7 iterations or 315 sec for STAT RT spine SBRT plans. Helical TomoTherapy-based STAT RT treatment planning with the "full scatter" algorithm provides levels of dosimetric conformality, heterogeneity, and OAR avoidance for SBRT treatments that are clinically equivalent to those generated with the Helical TomoTherapy "beamlet" algorithm. STAT RT calculation times for simple SBRT treatments are fast enough to warrant further investigation into their potential incorporation into an SBRT program with daily real-time planning. Development of methods for accurate target and OAR determination on megavoltage computed tomography scans incorporating high-resolution diagnostic image co-registration software and CT detector-based exit dose measurement for quality assurance are necessary to build a real-time SBRT planning and delivery program.