Beam characteristics and stopping-power ratios of small radiosurgery photon beams

Small megavoltage (MV) photon fields of dimensions less than 3 × 3 cm(2) are increasingly being used in modern radiation therapy. To our knowledge, small beam characteristics and dosimetric parameters, such as the energy spectra, particle fluence, and water-to-air stopping-power ratios (SPRs) directly affect the accuracy of small field dosimetry. This study presents the characteristics of 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 water-to-air SPRs for field sizes ranging from a small 4 mm diameter circular field to a 10 × 10 cm(2) field. It sheds light on the differences between small fields collimated by the cone accessory and X- and Y-jaws and on beam characteristics outside the primary radiation fields. In addition, we also investigated the use of an 'intermediate machine-specific-reference field' (Alfonso et al 2008 Med. Phys. 35 5179-86) to determine if the variations between a small and a reference field can be eased by introducing an intermediate 4 × 4 cm(2) field instead of a standard 10 × 10 cm(2) reference field. The Monte Carlo simulation codes BEAMnrc, DOSXYZnrc and SPRRZnrc were used in this study. 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(-1)) and the other for a high-dose rate (1000 MU min(-1)) 6 MV beam. The mean energy of photons at depths (1.5-30 cm) in water are 1.72-2.36 MeV, 1.55-1.97 MeV, and 1.44-1.74 MeV for field sizes of 4 mm diameter, 4 × 4 cm(2), and 10 × 10 cm(2), respectively. The mean energy also varies significantly for electrons at depths (1.5-30 cm): 0.99-1.25 MeV, 0.94-1.09 MeV, and 0.93-1.04 MeV for field sizes of 4 mm, 4 × 4 cm(2), and 10 × 10 cm(2), respectively. The calculated water-to-air SPRs at depths (1.5-30 cm) are 1.120-1.113, 1.121-1.117, and 1.122-1.119 for field sizes of 4 mm, 4 × 4 cm(2) and 10 × 10 cm(2), respectively. Although the differences in mean energy are > 20% for photons and > 5% for electrons between 4 mm field and 10 × 10 cm(2), the effects on the water-to-air SPRs are small (<0.5%). For detectors with responses to energy changes that are not negligible, significant energy variations between small fields and the reference 10 × 10 cm(2) field may have a significant impact on the dosimetry accuracy. However, the use of an intermediate field is capable of greatly reducing these energy variations. This study also found negligible differences in dosimetric parameters between beams with different flattening filters and different incident electron energies on the target when each has the same beam quality k(Q) values specified by %dd(10)(x).