Computer-based External Beam Radiotherapy Planning I: Empirical formulae for calculation of depth-doses

Abstract

An empirical method is given which may be used for the automatic computation of isodose charts by a high-speed digital computer with an attached digital plotter. The basic physical data is derived from published tissue-air ratio tables. The procedure is valid for the principal axes of rectangular fields, with and without wedges of constant slope, for heams of radiation from a 4 hlv linear accelerator, from 60Co units or in the H.V.T. range 1-3 m m Cu for diaphragm-limited fields. For oblique incidence of radiation on a surface of arbitrary shape, a correction procedure equivalent t o the 'effective S.S.D.' method is used, and a procedure is given to estimate the approximate depth-dose in the presence of internal inhomogeneities of the body.

Scatter dose for wedged fields

“…The distribution of scatter dose in a cross sectional plane, from a rectangular and non-wedged beam of 6oCo photons or high-energy x-rays, shows a limited dependence on beam energy, depth and field size, as has been demonstrated by Worthley and Cooper (1967), Bjarngard et al (1982), Brown et al (1981) and Bloch and Wallace (1979). This study shows that some of these findings carry over to wedged beams.…”

“…This includes the use of the product rule, which remains valid with about the same limitations as for non-wedged beams. The observation by Worthley and Cooper (1967) that for points within the field and in the principal plane of a wedged beam, the SDV is independent of depth and energy to within *3'/0 of total dose, is also confirmed.…”

“…Various methods are available for describing the scatter dose distribution for rectangular x-ray fields by analytic formulae and approximate expressions (Worthley and Cooper 1967, Bjarngard et a1 1982, Brown et a1 1981, Bloch and Wallace 1979. For wedged fields, the approximations that can be made and the corresponding inaccuracies are not as clear.…”