A surface-conforming flexible collimator for improving electron radiotherapy

Objective.We aim to simplify the traditional method of patient-specific skin collimation and to improve the precision of electron radiotherapy treatment with a new surface-conforming collimation system.Method.A surface-conforming collimation device for electron radiotherapy, consisting of parallel steel cables, was created. The cables move freely relative to each other and can be fixed and reshaped with a clamp system along the length of the device. The cables do not completely fill the space; thus, their packing fraction was determined to characterize dosimetric effects. The cable collimator was tested with a 10 × 10 cm cone with four electron energies (6, 9, 12, and 15 MeV) at three SSDs (100 cm, 105 cm, and 110 cm). Radiochromic film was placed at the surface and at the maximum dose depth (d_max) for each energy. Penumbral width, defined as the distance between 20% and 80% of the maximum dose, was measured for each energy and depth.Results.The largest improvement with respect to traditional collimation was seen for 6 MeV with 110 cm SSD at the surface. The penumbra at the surface for the conventional collimator in this case was 18.7 mm, while the cable collimator penumbra was 1.4 mm. The smallest improvement was seen for 15 MeV with 100 cm SSD atd_max(2.7 cm), where the penumbra for the traditional collimator was 11.1 mm, while the cable collimator penumbra was 9.8 mm. In all cases, the cable collimator reduced penumbral width. Surface dose inhomogeneities due to scatter were most pronounced at higher energies. The packing fraction was measured to be 0.551 ± 0.016.Significance.The prototype shows significant improvement in penumbra compared to conventional collimation. The system aims to serve as a replacement for current skin collimation techniques and could be used with Cerrobend applicators for additional surface blocking.