Beam Characteristics and stopping power ratios of small radiosurgery fields

摘要

Small megavoltage (MV) photon fields < 3x3cm~2 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 10c10 cm~2 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 10x10~2 cm 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 10x10 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 10x10 cm~2 respectively. Although the changes of mean energy are > 20% for photons and > 5% for electrons between 4 mm field and 10x10 cm~2, 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 10x10 cm 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.