This work investigated the suitability of passive dosimeters for reference dosimetry in small fields with acceptable accuracy. Absorbed dose to water rate was determined in nine small radiation fields with diameters between 4 and 35 mm in a Leksell Gamma Knife (LGK) and a modified linear accelerator (linac) for stereotactic radiosurgery treatments. Measurements were made using Gafchromic film (MD-V2-55), alanine and thermoluminescent (TLD-100) dosimeters and compared with conventional dosimetry systems. Detectors were calibrated in terms of absorbed dose to water in 60Co gamma-ray and 6 MV x-ray reference (10×10 cm2) fields using an ionization chamber calibrated at a standards laboratory. Absorbed dose to water rate computed with MD-V2-55 was higher than that obtained with the others dosimeters, possibly due to a smaller volume averaging effect. Ratio between the dose-rates determined with each dosimeter and those obtained with the film was evaluated for both treatment modalities. For the LGK, the ratio decreased as the dosimeter size increased and remained constant for collimator diameters larger than 8 mm. The same behaviour was observed for the linac and the ratio increased with field size, independent of the dosimeter used. These behaviours could be explained as an averaging volume effect due to dose gradient and lack of electronic equilibrium. Evaluation of the output factors for the LGK collimators indicated that, even when agreement was observed between Monte Carlo simulation and measurements with different dosimeters, this does not warrant that the absorbed dose to water rate in the field was properly known and thus, investigation of the reference dosimetry should be an important issue. These results indicated that alanine dosimeter provides a high degree of accuracy but cannot be used in fields smaller than 20 mm diameter. Gafchromic film can be considered as a suitable methodology for reference dosimetry. TLD dosimeters are not appropriate in fields smaller than 10 mm diameters.

High-accuracy in the absorbed-dose and small energy-dependence are important features of a dosimeter to be considered ideal for medical applications. This work investigated the relation between the optimum absorbed-dose in terms of combined standard-uncertainty and energyresponse of EBT2 and EBT3 films exposed to five x-ray beams and 60 Co. A systematic analysis was performed to control the fitting procedure in order to find the parameter-values that best describe the experimental data. Six EBT2 and three EBT3 film pieces per absorbed-dose were exposed to 0.05-100 Gy and 0.1-15 Gy, respectively. HP Scanjet-7650 and Epson Expression-11000XL document scanners were used to read the EBT2 and EBT3 films, respectively both in transmission mode, 48-bit at 300 dpi spatial-resolution. Images were analysed using ImageJ-software. The results indicate that, independent of the photon-energy beam, taking or not into account the uncertainty of the experimental data during the fitting process has a notable impact on the fit parameters for describing the measurements as well as on the uncertainty in the absorbed dose that can be measured with both films. Furthermore, the degree of energy dependence of both films depends on the absorbed-dose and the minimum absorbed-dose that can be measured with them is an intrinsic characteristic of each film model and depends on the photon energy as well as the colour channel. Lower the photon-energy, smaller the combined standard uncertainty, possibly associated to higher ionisation-density pattern of secondary-electrons released locally within the film active layer by low photon-energy comparing to high-energy. Thus, based on the results of this work, we suggest the inclusion of the uncertainty on the experimental data in the fitting procedure of the film calibration curve and an optimum absorbed-dose of 10 Gy for EBT3 in green-channel and 3 Gy for EBT2 in redchannel when use in modern radiotherapy techniques.

Minimum absorbed dose limit of Gafchromic EBT2 films were found to be energy dependent. The response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose This work is partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.