Purpose: The RefleXionTM X1 radiotherapy machine is a novel combination of positron emission tomography (PET) and computed tomography (CT) subsystems in the same gantry with a 6MV linac, with the intent of delivering biology-guided radiotherapy treatment (BgRT). Small treatment beam penumbra is desirable for small clinical targets. This study evaluated the focal spot size influence on treatment beam penumbra using a Monte Carlo (MC) technique.
Methods: A reference field of 10 cm x 2 cm at source to axis distance (SAD) of 85 cm was modeled and simulated using the BEAMnrc MC package. The electron beam incident on the X-ray target was empirically modeled with a Gaussian function and the FWHM of the beam for the linac head model was varied between 0.4 and 2.0 mm in steps of 0.2 mm. Dose distributions were calculated in a water phantom of 60 x 30 x 22 cm³ using the DOSXYZnrc MC code. The effect of X-ray beam spot size on percentage depth dose (PDD), penumbra width, and field width based on profiles at depth of 15 mm and profiles in the air at SAD=85 cm was evaluated.
Results: The variation in X-ray beam spot size did not have a significant effect on the PDD, with an average difference < 0.34% for PDD at depth of 100 mm. Penumbra width increases as X-ray beam spot size increases (R=1, P<0.01), while the field width is not affected by the variation of X-ray beam spot size.
Conclusion: The beam spot size has a sizeable effect on penumbra width. This study shows that a small X-ray beam spot is preferred for small treatment fields due to the resulting narrow penumbra width.
Funding Support, Disclosures, and Conflict of Interest: Research supported by RefleXion Medical.