Small field dosimetry using three solid state dosimeters for advanced dose audit in radiotherapy

Purpose/Objective: This study aims at identifying suitable dosimeters for the postal dosimetry audit of small beams used for more advanced radiation treatments such as stereotactic radiation therapy (SRT) and intensity-modulated radiotherapy (IMRT). As the first step, the characteristics of three types of solid state dosimeters were investigated using high energy photon beams and small field sizes. The preliminary results are presented below.\nMaterials/Methods:Dosimeters used in this study were thermoluminescent dosimeters (TLD chips, TLD-100, Harshaw), radiophotoluminescent dosimeters (RPLD; glass rods, GD-302M, Asahi Glass Co.) and optically stimulated luminescent dosimeters (OSLD; nanodots, Al2O3, Landauer). Measurements were performed at 50 mm depth, using 6 MV photon beam generated by an Elekta Synergy linear accelerator. The field sizes varied from 10×10 mm2 to 50×50 mm2. In addition, reference measurements with a pinpoint ionisation chamber (pinpoint IC) were performed in order to validate the response of solid state dosimeters for larger field sizes used within the study. The sensitive areas from the beam’s eye view of each dosimeter were 3.2×3.2 mm2 for TLD chips, 0.6 mm diameter for RPLD used in a high dose mode, 5 mm diameter for OSLD and approximately 5×2 mm2 for the pinpoint IC. Several dosimeters of each type were used per measurement point. All dosimeter measurements were corrected for the volume averaging effects using 2D dose distributions obtained from gafchromic film measurements. The pinpoint IC measurements were additionally corrected for the detector-to-alanine response derived from a previous study [1]. The latter correction accounted for non-water equivalence of the air cavity of the pinpoint IC.Results:The summary of each detector’s relative response vs. filed size, normalised to a 30×30 mm2 field size, is given in Figure 1. The agreement between the results of TLD, OSLD and RPLD is within 1.0% (1 std dev.) for the fields of 20×20 mm2 and above, and about 2.0% (1 std dev.) for 10×10 mm2 field. These results also compared favourably with those of the pinpoint IC used as reference for the field sizes of 20×20 mm2 and above, resulting in differences of ≤ 1.2%. However, for the field size of 10×10 mm2 the response by the pinpoint IC was about 5% lower, although the relevant corrections were applied. This indicates that the pinpoint IC cannot be reliably used as the reference dosimeter for the smallest fields when aiming at high precision measurements required for dosimetry audit purposes.Conclusion:These results demonstrate that all solid dosimeters used in this study can be of interest for audits of small radiotherapy beams provided the appropriate corrections are applied. \nReference:[1] Azangwe G, Grochowska P, Georg D, et al. Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams, submitted to Medical Physics.ESTRO 3