Prostate Helical Tomotherapy: A semi-empirical estimation of the scaling factor based on 2D approximating field

Background: In Helical Tomotherapy (HT), the scaling factor (SF) is the time in seconds that each leaf viewing a target would need to be open to deliver the prescribed dose. The SF is patient-specific and is used to calculate the rotational period of the gantry, and the total treatment time (TTT) of the HT. The SF is generally difficult to estimate. Currently, it takes about one hour to fully optimize a prostate HT plan and to calculate the corresponding TTT. The aim of this study is to develop a method for estimation of the SF directly using a patient-specific approximating function. Materials and Methods: The SFs of ten randomly selected patients were used to build the approximation model. For the entire group of patients the PTV1 ranged from 57 to 396 cm3 for PTV1 margins from 2 to 10 mm. The discrete data for every patient is represented by an individual function, SF=f (k× PTV1). The values of the function were rescaled to a special unit which represents the target volume irradiated with the prescribed dose per second. The values were normalized with two “geometric” parameters, namely, the target-to-target and the body-to-body ratios. After the normalization, the function for every patient was ordered in the file by the volume of the prostate and seminal vesicles. Results: For prostate HT planning, it was found that the planning target volume (PTV1) has a higher impact on the SF values than the size of the patient's bodies. The function SF=f (k×PTV1) was found smooth and continuous over the given interval. The rescaled and normalized functions for all patients were represented as delimiters of a 2D field. Conclusion: The method proposed for determination of the SF and TTT for HT prostate planning covers PTV1 of four margins and a volume of prostate and seminal vesicles ranging from 42.8 to 161 cm3. Using these approximations, the TTTs for a second group of patients were determined in several minutes with deviation ranging from −2.8% to +7.1% compared to that of the TTTs calculated using the HT planning system. Iran. J. Radiat. Res., 2010 7 (4): 177-185