Author: Neil A. Kirby, Parker New, Niko Papanikolaou, Holly Paschal, Karl H. Rasmussen, Daniel L. Saenz 👨🔬
Affiliation: UT Health San Antonio 🌍
Purpose: To simplify the traditional method of patient-specific skin collimation and improve the precision of electron radiotherapy treatment with a new surface conforming collimation system.
Methods: We have created a surface-conforming collimation device for electron radiotherapy, consisting of a set of parallel steel cables. They move freely relative to each other and can be fixed in shape with a clamp system along the length of the device, as well as be reused for any desired shape. The cables do not fully fill the space, so their packing fraction was also characterized for dosimetric implications. The cable collimator was tested for a 10x10 cm field with four electron energies (6, 9, 12, and 15 MeV) at two different SSDs (100 and 110 cm). Radiochromic film was placed at the surface and at dmax for each energy. Penumbral width, defined as the distance between 80 and 20 percent the maximum dose, was measured for each energy and depth.
Results: The largest improvement 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 at a depth of 2.7 cm (dmax), 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. The packing fraction was measured to be 0.55.
Conclusion: 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 can be used with Cerrobend applicators for additional surface blocking.