Features of Distributions Created in a Water Phantom by Narrow Bremsstrahlung Radiation Beams from 10 to 20 MV

Abstract

The paper analyzes the features of the spatial and energy distributions of photons in depth, in radius
 and in energy for small fields created in water by narrow beams of high-energy bremsstrahlung radiation from medical accelerators and mono-directional, monoenergetic beams of photons. In particular,
 the relationship between the spatial dependences of the absorbed dose, total kerma, and ionization
 kerma is discussed. Using numerical simulation, the features of photon spectra in small fields at a
 depth of 10 cm in a water phantom for bremsstrahlung beams with maximum energies of 10, 15, 18
 and 20 MeV were determined as applied to the design of Varian accelerator heads. It is shown that, in
 contrast to wide fields, the energy distribution of photons in these fields practically does not change
 within the central region of the field and strongly softens when leaving this region. With a decrease in
 the size of small fields from 4.04.0 to 0.50.5 cm, a certain hardening of the spectra is observed in the
 central region (the average energy increases by 2–4 %) and is much more significant outside this region.
 The work also determined the values of the clinical dimensions of small fields generated by high-energy
 beams of Varian accelerators of different energies in a water phantom at a depth of 10 cm, depending on
 the settings of the collimation system.
 This work was carried out with the financial support of the Russian Foundation for Basic Research and
 SITMA within the framework of scientific project No. 18-52-34008.