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On the dose linearity of detectors for small field dosimetry

BACKGROUND AND OBJECTIVE The International Code of Practice (CoP) IAEA TRS-483 provides a formalism for dosimetry in small fields of MV photon beams. It includes specific guidance for suitable detectors, and summarizes their general characteristics, e.g., dose linearity, stability, dose rate linearity, and leakage. The guidance for dose linearity stipulates it to be “better than 0.1% over an absorbed dose range of at least three orders of magnitude”. The CoP did not provide any methodology for the measurement of dose linearity. The aim of our multicentre experimental study was to verify the guidance given in TRS-483 for dose linearity applying two alternative methods, using a large number of suitable detectors recommended in TRS-483 for small field dosimetry. METHODS All measurementsfor the determination of dose linearity were done following a strictstudy protocol, at each of the nine participating centres. Twenty different types of detectors (10 ionization chambers, 9 diodes, and 1 micro diamond detector) were used for testing the dose linearity, with 49 detectors in total. All measurements were performed using an isocentric set-up with SSD = 90 cm, a depth of 10 cm, gantry 0°, and field size of 4 x 4 cm2 in 6 and 10 MV photon beams with (WFF) and without (FFF) flattening filter on either Elekta Versa HDTM or Varian TrueBeamTM linear accelerators. To minimize fluctuations in ion chamber readings from linac output variation, an ionization chamber having sufficiently large cavity volume (PTW Semiflex or IBA CC13) was utilized as a reference detector. Detectors were irradiated with 5, 10, 20, 30, 50, 100, 200, 300, 500, and 1000 MUs, covering an approximate absorbed dose range of three orders of magnitude. We analyzed the results by two alternative methods, considering a pass criterion of 0.1% as stated in TRS-483. A. Dose Linearity A: Adapted formalism from “IEC 60731 Medical electrical equipment – Dosimeters with ionization chambers as used in radiotherapy” Dose linearity was calculated as shown in Eq. (1), where Mi = mi/mi, ref ; mi denotes a single, i-th measurement (data point) performed at a particular centre, for a selected detector, energy, and number of MUs, while mi, ref stands for the corresponding measurement with reference ionization chamber done at the same time. Mref was defined as shown in Eq. (2), where indices 50, 100, and 200 denote number of Mus. R2 indicates the proportion of the variation of the data explained by the best-fit linear function, where mi, ref and mi were considered as the independent and dependent variables, respectively. We also considered that the pass criteria of 0.1% is satisfied if R2 > 0.999. RESULTS Dose Linearity A: We found that the TRS-483 guidance on dose linearity (0.1%) was not met for the majority of the 1960 analyzed data points (Table 1). In particular, the dose linearity criterion was not satisfied for low number of MUs. Dose Linearity B: The coefficient of determination R2 was higher than 0.999 for all analyzed data sets (196) for the entire range of MUs investigated in this study. DISCUSSION AND CONCLUSIONS We tested dose linearity for 49 detectors (20 different types) using two methods. The present results show that the 0.1% tolerance for dose linearity cannot be met for the selected range of doses (MUs) if the first method is used (adapted methodology from IEC 60731) for the determination of dose linearity. A less stringent acceptability criterion is needed, especially for very small numbers of MUs. For instance, if the tolerance in linearity is set at 1.0%, then more than 90% of the data points with 20 or more MUs comply (Table 1). Alternatively, if we assume that R2 = 0.999 corresponds to 0.1% linearity criterion from TRS-483, the dose linearity acceptability criterion can be met with 100% of the data pointsfor the whole range of MUs investigated in this study. In our opinion, method B is an acceptable method for the determination of dose linearity of detectors for small field dosimetry. Therefore, we recommend reporting dose linearity in terms of the coefficient of determination R2. To avoid any potential ambiguity, the methodology for the determination of dose linearity should be specified along with the corresponding acceptability criterion in an eventual update of the TRS-483.

small field dosimetry, radiotherapy

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