MO‐G‐213AB‐01: Quantification of TomoTherapy MVCT Dose

Purpose: To quantify MVCT dose of TomoTherapy for three jaw settings, J4, J1, and J0.1, corresponding to beam widths of 7 mm, 4 mm, and 3 mm, respectively, at the isocenter plane, and three imaging modes, fine, normal, and coarse, corresponding to a couch speed of 4, 8, and 12 mm/rotation, respectively. Material and Methods: An MVCT dose engine was commissioned specifically for the MVCT beams, including updates to the fluence attenuation table (FAT), energy deposition kernel, cone profiles, and penumbrae. MVCT dose calculation was then applied on real and synthesized images of cylindrical water phantoms of diameters ranging from 5 cm to 40 cm, and the results were compared with film measurement. Result: For the J1 jaw and coarse imaging mode, the maximum difference between calculation and measurement was about 6% of the center dose. Calculation on synthesized phantoms showed that the center dose decreased almost linearly as the phantom diameter increased, and that the fine mode received twice the dose of the normal mode and three times that of the coarse mode. The maximal dose due to the helical ripple ranged from 100%∼200% of the center dose, with increasing ratios for larger phantoms (due to the larger radius), smaller jaws, and faster couch speed (the latter two yielding a higher helical pitch). For all jaw settings and couch speeds, the mean dose and average surface dose vary from 95%–115% of the center dose with increasing ratios for larger phantoms. Conclusion: An MVCT dose calculator was set up with validation through film measurement and subsequently used to calculate TomoTherapy MVCT dose for various phantom sizes under various imaging parameters. The results can serve as a reference for TomoTherapy MVCT dose.