Improving the matching of photon and electron fields for Inverse Hockey Stick Technique (IHS technique)

Abstract

Background

The problem of geometrical aspects of field matching was investigated very extensively in 80’s and earlier. In the 90’s accelerators were equipped with asymmetric jaws that solve the problem in most cases. However, it is still not possible to have homogeneous dose distribution in the matching region if two beams with different penumbra are matched, which is the case if a photon beam and an electron beam are matched.

Aim

To improve the matching of photon and electron fields in IHS technique with individual block.

Materials/Methods

Three individual blocks made from Wood's alloy were designed with the angles of the lateral wall at 2.2, 6.5 and 9.0 degrees. Profiles of photon beams of the energy of 6MV were measured on a Mevatron KD2 with a PTW field analyser with diamond detector for the typical beam size used in the IHS technique (20×20 cm²). The measurements were performed for open beam and for beam modified with all blocks at depths of 1, 2, 3, 4 and 5 cm. The SSD was 100 cm. Profiles of the electron beams were also measured. Measurements were carried out for field size of 15×15 cm². The measurements were carried out for 9, 12 and 15 MeV electron beams with a PTW field analyser in the water phantom with Markus chamber, type 23343. The SSD was 100 cm. For each electron energy measurements were performed at 1, 2, 3, and 4 cm at depths up to 80% of distal isodose depth, i.e. at 1, 2 and 3 cm, at 1, 2, 3 and 4 cm, and at 1, 2, 3, 4 and 5 cm for 9 MeV, 12 MeV and 15 MeV, respectively. The penumbra width was obtained. The measured data were saved in digital form. By means of spreadsheet the sum of dose distributions of each electron field and each photon field was calculated.

Results

The penumbra of the photon beams was changed by application of the individual block. The larger the angle of the lateral wall of the block, the larger was penumbra width. For the largest angle block the penumbra reached almost 14mm at 40mm depth. For blocked fields the 50% profile did not coincide with the light field edge. The larger the angle of the lateral wall of the block, the more the 50% profile value was moved outside of the light field edge. Application of the modified block decreased the difference between the maximum and minimum dose in the matching region by about 10%.

Conclusions

The proposed technique of modifying the photon beam penumbra allows the dose distribution in the join-up/overlap region to be improved.