(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Helical tomotherapy treatment times are large, variable, and non-intuitive in relation to the prescribed dose. It would be useful to describe a helical RayStation beam as equivalent to a fixed, broad, radiotherapy beam; amenable to calculation by a measurement-free, pseudo-classical method.
Methods: In the proposed model, dose is expressed as the product of output, time, TMR, and net transmission through the couch, the MLC, and the jaws. Output and couch factor are properties of the accelerator. Treatment time and MLC modulation are reported in RaySation plans. Jaw transmission is taken as the ratio of the field length setting to the irradiated length, calculable from values in the plan report. A parametrized, published expression is used for TMR, based on an approximate field size implied by the target’s volume and mean depth implied by the patient’s cross-sectional area.
Results: For the initial series of 16 Tomotherapy plans (11 patients), the dose calculated by this simplistic model were compared to the prescribed dose. In all cases, dose agreement was within 20%. In 5 cases, agreement was within 5%.
Conclusion: Modeling a volumetric, helical tomotherapy plan as an intuitively-equivalent broad-beam point dose calculation, using no measured data, neither for the plan nor the treatment unit, is an over-simplification which radically suppresses the true complexity of helical tomotherapy. Yet, it results in significant agreement in calculated dose. Only one-third of cases showed the 5% level of agreement expected of a full and sufficient verification method. With further refinement, this might be improved. However, the level of agreement observed is sufficient to be useful, particularly as this method creates an intuitive connection to the classical methods as well as to the clinically tangible characteristics of the plan. This approach shows promise.
Funding Support, Disclosures, and Conflict of Interest: COI - RaySearch employment and/or stock ownership by authors.
Keywords
Not Applicable / None Entered.
Taxonomy
TH- External beam- photons: General (most aspects)
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