Dosimetry for Small Fields in Stereotactic Radiosurgery Using Gafchromic MD-V2-55 Film, TLD-100 and Alanine Dosimeters

Abstract

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 dose‐per‐pulse electron beam dosimetry: Usability and dose‐rate independence of EBT3 Gafchromic films

“…The use of Gafchromic films in combination with a flatbed scanner is today considered a reliable dosimetry system, which can be used, for example, for reference small field dosimetry, as a replacement of ionization chambers . The detection principle of Gafchromic films relies on the polymerization upon irradiation of components of the active layer, resulting in film coloration.…”

Absorbed Dose to Water Rate in a Cyberknife VSI System Reference Field Using Ionization Chambers and Gafchromic Films

“…were irradiated simultaneously in liquid water at each absorbed dose, at values from 0 to 60 Gy, using the absorbed dose rate obtained with the IC-A12 dosimetry system. All film pieces were vacuum-sealed and statistic agreement within 0.65% (less than measurement uncertainties) has been reported between the absorbed dose to water inside the waterproof package and that measured directly in the water phantom [6]. The irradiation of the films in liquid water ( Figure 1) was performed according to our protocol [6].…”

“…All film pieces were vacuum-sealed and statistic agreement within 0.65% (less than measurement uncertainties) has been reported between the absorbed dose to water inside the waterproof package and that measured directly in the water phantom [6]. The irradiation of the films in liquid water ( Figure 1) was performed according to our protocol [6]. The measurements were carried out under the same conditions of the reference calibration; i.e.…”

“…Due to a technical problem, the dosimetric system IC-2258 was not used in this case. For comparison, the dose rate was measured directly in liquid water with radiochromic films EBT3 and MD-V3 under the same conditions as the ionization chambers [6], that is, 70 cm SSD, 10 cm water depth and 6 cm diameter field. The films were exposed to the same MU as mentioned in section 2.3.2 above.…”

Absorbed dose distribution in liquid water for a CyberKnife VSI using radiochromic EBT3 film

“…10 cm x 10 cm field, 90 cm source to surface distance (SSD) and 10 cm depth. For the calibration, the films were irradiated perpendicular to the beam axis (see Figure 1a) following the same procedure reported previously [11] while in the Cyberknife, the films were collocated at 70 cm SSD in the parallel orientation (see Figure 1b) in order to obtain all the parameters in one measurement using the irradiation method proposed previously [12]. To do the irradiation in parallel orientation, new acrylic holders were fabricated depending on the size of each beam as shown in Figure 1c and the width of each holder was equal to or greater than twice the beam diameter.…”