Response of TL dosimeters outside the field edge of a 6 MV Linac beam and its implication on patient dosimetry

Abstract

In dosimetry of organs at risk where accurate radiation dose measurements are needed, the variation of detector response with x-ray energy is of concern. Response of LiF:Mg,Ti and high-sensitivity LiF:Mg,Cu,P thermoluminescence dosemeters (TLDs) to a 6 MV Linac beam was measured outside the field edge near the phantom surface and the corresponding response on the central axis. LiF:Mg,Cu,P and LiF:Mg,Ti TLDs (Harshaw) were irradiated using 6 MV Linac beam (Siemens’Primus Linear Accelerator) at central axis and outside the field edge. Dose readings were compared with an ionization chamber to estimate variation in TL response. The response of LiF:Mg,Ti and LiF:Mg,Cu,P TLDs to a range of xray energies was analyzed for kilovoltage (spectrum) and megavoltage (Co60) therapy radiation beams with the aim of estimating variation in TL sensitivity. In this study the variation in response at off axis points as compared with central axis was on average 9 %(0–22%) for LiF:Mg,Ti and 8% (0–16%) for LiF:Mg,Cu,P TLDs, depending upon the position of measurement. In radiation therapy, the introduction of conformal techniques like IMRT has lead to dose escalation to the tumor and consequently increased risk of damage to sensitive organs around the tumor. For accurate estimation of dose to these organs at risk correction to detector response should be applied.