Use of a PLA sleeve to remove electron enhancement in superficial X-ray therapy.

A newly installed superficial X-ray unit was found to produce enhanced electron dose at the skin surface. The ACPSEM kilovoltage dosimetry recommendations suggest using nail varnish within the treatment cones as a method to reduce this dose. In this study, a 3D PLA sleeve was produced and used as an alternative to the nail varnish for energies between 55 and 100 kV. Further, plastic wrap was also investigated as an alternative method to reduce dose. It was found that a 1 mm printed sleeve, inserted into the treatment cone sufficiently reduced the enhanced dose as measured with a thin-window Exradin chamber to within 3.3% of the dose measured with a Farmer-type ionisation chamber. The use of plastic wrap also reduced the enhanced dose, but impracticalities in its use make it non-viable for routine clinical use.

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

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 0.5 mm for Pb at all available energies (70-300 kVp), within 1.4 mm for W-PLA at all available energies, and within 2.1 mm for Cu-PLA at superficial treatment energies (70-100 kVp). The incremental transmission factor calculation method described and validated in this study could be used, in combination with the conservative addition of 1-2 mm of additional material, to estimate shielding requirements for novel materials in therapeutic kilovoltage beams. However, if calculated shielding thicknesses equate to 10 mm or more, then additional verification measurements should be performed and the clinical suitability of the novel shielding material should be re-evaluated.

The effect of fluorescence on surface dose with superficial X-rays incident on tissue with underlying lead.

An Advanced Markus chamber on the surface of solid water phantom was used to determine surface dose reduction, with either a lead or air interface, as a function of surface-interface separation (t). The beam quality dependence of dose reduction was investigated using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 superficial X-ray unit. For each beam the dose correction factor, DCF(t), namely the ratio of surface dose (t) to surface dose (t = 100 mm), was determined. Monte Carlo simulations of DCF(t) with a lead interface were compared with corresponding measured values. Simulated spectra were calculated at the phantom surface for full backscatter (t = 100 mm) and with either a lead or air interface at 2 mm or 8 mm depth. For each depth and beam quality lead fluorescent radiation at the surface was evident. The variation of DCF(t) for each beam and field size exhibits a minima at t ≈ 5 mm and in the range 1 mm ≤ t ≤ 40 mm surface dose reduction is larger for 100 kV than 150 kV. Monte Carlo simulated DCF(t) are consistent with corresponding measured DCF(t). From simulated spectra L-series fluorescent X-rays (≈ 15 keV) emanating from lead at t = 2 mm are evident for all beams and fluorescent K-series X-rays only occur with 100 kV and 150 kV beams.