Determination and use of scatter correction factors of megavoltage photon beams: Measurement and use of collimator and phantom scatter correction factors of arbitrarily shaped fiels with a symmetrical collimator setting

For each radiation treatment machine, the dose per monitor unit must be known at a reference point in a water-phantom under reference conditions. It varies with field size, due to: ( 1) changes in radiation scattered from the head of the treatment machine to the reference point and into the monitor chamber and ( 2) changes in the radiation scattered from the irradiated part of the phantom to the reference point. The field size dependence is usually described by the total scatter correction factor, Scp• If the field size defined by the collimator setting at the reference source-surface distance ( SSD) does not correspond with the field size at the phantom surface, Sec must be separated into two factors, the head or collimator scatter correction factor, Sc, and the phantom scatter correction factor, Sc. This is the case when a source-surface distance different from the SSD of the reference condition is used, when tissue is missing in the beam, or when shielding blocks are applied. The factor Sc describes the influence of the setting of the collimator on the total scatter correction factor; Sp describes the influence of the field size at the phantom surface, and thus of the irradiated volume on this correction f actor.

In this report, recommendations are given for the measurement of Scp, Sc and Sp in a photon beam at a reference depth of 10 cm. The reference field is defined as the open field, with a collimator setting yielding a 10 cm x 10 cm field when the SSD is set equal to the source-axis distance ( SAD) . For the determination of Sc, the use of a narrow cylindrical beam-coaxial phantom (the mini-phantom) is recommended. In this way, measurements can be performed in small fields and the disturbance of contaminating electrons, reaching the point of interest from the head of the treatment machine, becomes negligible. Construction details of the mini-phantom are described.

Furthermore, a consistent set of relations is presented for the use of these factors in dose calculations for symmetric ally collimated square, rectangular and arbitrarily shaped fields, at an arbitrary SSD. These include blocked and wedged fields. A procedure to use Sc and Sp data in the calculation of monitor units is presented. Relations are given to calculate the collimator and phantom scatter data for the reference situation from already available data, measured at a non­ reference depth and at a non-reference SSD. The influence of asymmetric set-up of the collimating jaws or multi-leaf collimators on the scatter correction factors is, however, not yet considered.

Scp, Sc and Sp data sets are presented in this report for different types of treatment machines and for a wide range of photon beams, with beam qualities ranging from 6°Co to 25 MV. Sp is shown to be a smooth f unction of the beam quality if the same reference depth of 10 cm and an SSD of 100 cm is chosen for all beam qualities. However, Sc is shown to depend on the design of the head of the linear accelerator.