Beam Characteristics and Stopping Power Ratios of Small Radiosurgery Fields

Abstract

Small megavoltage (MV) photon fields < 3×3cm² are increasingly being used in modern radiation therapy. This study presents the characteristics small photon beams and investigates the variations of energy spectra of photons and electrons as a function of field size and their effects on the stopping power ratios for field sizes ranging from a small 4 mm field to a 10×10 cm² field by using Monte Carlo techniques. The accelerator head and circular cone accessory were simulated in detail including two designs of flattening filters: one for a standard dose rate (100-600 MU/min) and the other for a high-dose rate (1000 MU/min) 6 MV beam. The mean energy of photons at depths (1.5 cm - 30 cm) in water are 1.72 – 2.36 MeV, 1.66 – 2.25 MeV, and 1.44 - 1.74 MeV for field sizes of 4 mm, 30 mm, and 10×10 cm2 respectively. The mean energy also varies significantly for electrons at depths (1.5 cm - 30 cm): 0.99 – 1.25 MeV, 0.95 – 1.14 MeV, and 0.93 - 1.04 MeV for field sizes of 4 mm, 30 mm, and 10×10 cm² respectively. The calculated water-to-air stopping power ratios at depths (1.5 cm - 30 cm) are 1.120 – 1.113, 1.121 – 1.117, and 1.122 - 1.119 for field sizes of 4 mm, 30 mm, and 10×10 cm² respectively. Although the changes of mean energy are > 20% for photons and > 5% for electrons between 4 mm field and 10×10 cm², the effects on the stopping powers are insignificant (<0.5%). However, for detectors with responses to energy changes that are not negligible, energy variations between small fields and the 10×10 cm² field may have significant impact on the dosimetry accuracy. This study also found negligible differences in dosimetric parameters between beams with different flattening filters when both have the same values %dd(10)_x.