Abstract

Ileana Silvestre Patallo^1,2, David Maughan³, Rebecca Carter⁴, Andrew Nisbet⁵, Giuseppe Schettino³, Giuseppe Schettino⁶, Anna Subiel³

¹National Phyical Laboratory, Medical Radiation Physics, Teddington, United Kingdom; ²University College London , Medical Physics and Biomedical Engineering, London, United Kingdom; ³National Physical Laboratory, Medical Radiation Science, Teddington, United Kingdom; ⁴University College London (UCL), Cancer Institute, London, United Kingdom; ⁵University College London (UCL), Medical Physics and Biomedical Engineering, London, United Kingdom; ⁶University of Surrey, Physics, Guildford, United Kingdom

A number of translational radiation studies have been difficult to replicate [1]. Lack of harmonization in dosimetric methods and challenges implementing dosimetric protocols designed for clinical orthovoltage radiotherapy devices, undermines reproducibility of the results [2]. Image guided preclinical irradiation platforms (IGPIP) deliver small fields in the medium energy x-ray range. Commissioning of devices, followed by dosimetry verification of treatment planning system (TPS), is performed with gafchromic films, involving separate calibration and post-processing. This study characterized a small volume ionization chamber (IC), traceable to a primary standard, in medium energy x-rays for its use for reference dosimetry in small fields delivered by IGPIPs

A 3D PinPoint (PiP) IC type TW31022 was exposed to reference medium energy x-rays (0.5 to 4 mm Cu HVL). Leakage, reproducibility, linearity, direction dependent response and dose rate, as well as energy dependence were investigated. Stability of the chamber’s air kerma calibration factor (Nk) was assessed. Polarity and ion recombination in reference and user’s beams irradiation conditions were evaluated. The chamber’s correction factor (kch,31022) was experimentally determined. The PiP IC placed in small phantom, was used for an end-to-end test in an IGPIP, with a 10 mm x 10 mm field. Measured and TPS calculated were compared.

In the reference x-ray beams, corrections for ion recombination were smaller than 0.1%. Saturation curve (Figure 1) shows that, independently of dose rate, the chamber responds differently when collecting negative and positive current (charge). The largest polarity correction factor was 1.014±0.008 (4 mm Cu HVL). Table 1 shows experimentally determined kch for a TW31022 as well as those for a 2611 reference chamber. The implementation of the end-to-end test with the delivery of image targeted small field plans, resulted in differences smaller than 3% between measured and TPS calculated doses.

Figure 1: Saturation curve

Table 1 Experimentally determined chamber correction k(ch,31022)

The properties and response of the 3D PiP IC in medium energy x-rays demonstrated its suitability as reference detector for measurements in large and small fields in this energy range. Polarity and ion recombination effects were similar at different dose rates. The possibility to ionometrically validate small fields’ TPS calculations in IGPIPs will contribute to harmonization in dose reporting. This approach could lead to improvement of measurement uncertainties in reference and relative dosimetry of small fields delivered by IGPIP, while maintaining the traceability chain to primary standards.

[1] Draeger E, et al. Int J Radiat Oncol Biol Phys 2020

[2] Verhaegen F, et al. Radiother Oncol 2018