Session Item

It is common practice to use generic data published in BJR supplement 25 [BJR 1996] to derive kV radiotherapy treatment output factors (MU per cGy). Due to difficulties in measuring dose for small fields at low energies this is an acceptable data sourse. However, the data is based on HVL and takes no account of the beam characteristics of specific kV treatment units which can lead to uncertainty. The objective of this work was to devise a novel method using small silica bead TL detectors to accurately measure the output factors for kV radiotherapy treatments, and compare results to a traditional ionisation chamber technique. 

Two independent dosimetry methods (silica TL beads, 1.1mm thick, and CCO4 ionisation chamber, 0.04cm³ volume, 4 mm cavity diameter) were used to measure the dose delivered for a range of treatment applicator field sizes (2-20cm diameter) and lead cut-outs, for the Xstrahl D3300 treatment unit, over a range of energies (70-250kV).

The novel aspect of this work was to experimentally determine output factors using silica beads, which have only had limited use in kV energies [Palmer et al 2017] and [Jafari et al 2014]. Due to their small size, nine beads were used for each dose measurement to reduce uncertainly. Beads were placed on the surface of a block of solid water with a  stand-off of 6 mm, see Figure 1.

Calibration of the glass beads was performed for each energy against a traceably calibrated ionisation chamber to generate an energy specific calibration factor. This was in addition to bead-specific individual sensitivity calibration using 6 MV photons in a reference condition factors, completed due to small variation measured across a batch of beads. The glass bead TL signal was read-out using a TLD reader (Toledo 654). 

Differences were observed between the experimentally determined output factors using the two detectors and existing plan data obtained from BJR 25, as illustrated in Figure 2 for a 4cm applicator with various lead cut-outs The experimental output factors measured using the CC04 ionization chamber and silica beads TLDs were in agreement, within experimental uncertainty. These discrepancies were higher for the smallest and largest field sizes, for all energies. The beads and CC04 results differed by up to 1% (uncertainty +/- 1.1%), the mean value of which was up to 3.1% different to BJR data.

The output factors for a kV treatment unit have been successfully measured using silica bead TL dosimeters, verified against an ionisation chamber. Statistically significant difference was measured from the current clinically used BJR 25 data and should be adopted clinically. This work also shows that silica beads can be successfully used to verify the dose delivery of small field sizes on a kV unit,  important as they are only 1.1mm thick (compared to 4mm diameter for the CC04) so can be used to measure very small field sizes, down to 1cm diameter cut-outs in this work.