Predicting the required thickness of custom shielding materials in kilovoltage radiotherapy beams.

Abstract

The planning and delivery of kilovoltage (kV) radiotherapy treatments involves the use of custom shielding designed and fabricated for each patient. This study investigated methods by which the required thickness of custom shielding could be predicted for non-standard shielding materials fabricated using 3D printing techniques. Seven kV radiation beams from a WOmed T-300 X-ray therapy unit were modelled using SpekPy software, and AAPM TG-61 data were used to account for backscatter and spectral effects, for incrementally increasing thicknesses of Pb, W-PLA composite and Cu-PLA composite materials. The same beams were used to perform physical transmission measurements, and the thickness of each material required to achieve 5% beam transmission was determined. While the measured transmission factors for Pb, W-PLA and Cu-PLA shielding generally exceeded the calculated transmission factors, these differences had minimal effect on the derived thicknesses of shielding required to achieve 5% transmission, where calculations agreed with measurements within 0.5mm for Pb at all available energies (70-300kVp), within 1.4mm for W-PLA at all available energies, and within 2.1mm for Cu-PLA at superficial treatment energies (70-100kVp). The incremental transmission factor calculation method described and validated in this study could be used, in combination with the conservative addition of 1-2mm of additional material, to estimate shielding requirements for novel materials in therapeutic kilovoltage beams. However, if calculated shielding thicknesses equate to 10mm or more, then additional verification measurements should be performed and the clinical suitability of the novel shielding material should be re-evaluated.