Setup and operation of a thermoluminescent dosimetry system for patient organ dosimetry

Abstract

Background: Direct measurement of absorbed dose to patient organs requires dosimeters, e.g. thermoluminescent dosimeters (TLDs), placed in-vivo in the patient, or in anthropomorphic phantoms. Malta currently lacks an established process for patient dosimetry, which is essential under EU directive 2013/59. This study provides the necessary groundwork for future patient dosimetry in Malta. Objectives: To understand the characteristics of TLD crystals under various conditions and to prepare the first set of standard operating procedures (SOPs) for a patient thermoluminescent dosimetry system in Malta. Research Methodology: The response of two TLD materials, Lif:Mg,Cu,P (MCP) and LiF:Mg,Ti (MTS), was compared under the following conditions and exposure parameters: 3 readout temperature-time profiles, use and otherwise of nitrogen during readout, and, using a general radiography system, exposures at 5-100 mAs and 50-120 kVp. MCP calibration to a semiconductor dosimeter enabled the conversion of photon counts to absorbed dose (μGy). Results: Readout heating rates did not significantly impact TLD response (MCP p = 0.105, MTS p = 0.119). Nitrogen use improved the linearity response and relative sensitivity of MTS over no nitrogen use with mAs at constant kVp, particularly at low mAs (linearity over the investigated range without nitrogen R2 = 0.8908, with nitrogen R2 = 0.9998; relative sensitivity 1.53). In contrast, nitrogen had minimal impact on MCP (linearity without nitrogen R2 = 0.9997, with nitrogen R2 = 0.9999; relative sensitivity 0.98). MCP response with kVp and X-ray tube output at constant mAs was linear over the investigated range (R2 = 0.9905 and R2 = 0.9961, respectively). There was no statistically significant difference between TLD and semiconductor dosimeter doses (p = 0.137). SOPs were drawn up to standardise the TLD measurement process. Conclusions and Recommendations: Following the characterisation of TLDs and preparation of SOPs, the groundwork for patient dosimetry in Malta has been laid which can be applied either directly to future TLD measurements or to validate Monte Carlo techniques.