An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry

Abstract

Purpose: An experimental method using the linear portion of the relative film dose-response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.

Materials and methods: The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.

Results: The maximum percentage deviation of PDD_FS with PDD_IC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDD_FS with PDD_IC for electron beams was within 3% for energy range studied of 8-15 MeV. The gamma passing rates of PDD_FS with PDD_IC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDD_FS with PDD_IC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROF_FS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDD_FS and PDD_MC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROF_FS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.

Conclusions: The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

Keywords: CyberKnife; EBT3 film; EDR2 film; Percentage depth dose; film slope method; relative dosimetry; relative output factor; small-field dosimetry.

Figure 1

(a) and (b) Phantom measurement of PDD at points A and B at depths ‘d_max’ and ‘d’ with dose rates ‘D_max’ and ‘D_d’, respectively

Figure 2

The different representations of characteristic curves used in dosimetry for EDR2 Ready-Pack film using (a) net optical density versus Log₁₀ (dose) or (b) net optical density versus dose for 6 MV photon beam

Figure 3

Hurter and Driffield curve using Gafchromic EBT3 film for 6 and 15 MV photon beams showing the energy dependence

Figure 4

The linear portion of the relative film dose–response plots at two different depths A and B with the slopes, m_A and m_B

Figure 5

The relative film dose response at their respective depths for determining percentage depth dose; (a) EDR2 Ready-Pack films, (b) Gafchromic EBT3 films, Photon energy: 6 MV photon, field size: 10 cm × 10 cm, source-to-surface distance: 100 cm

Figure 6

The relative film dose response at their respective linear ranges plotted at various depths using EDR2 Ready-Pack film to determine percentage depth dose of 6 MV photon beam with (a) 5-mm and (b) 10-mm diameter stereotactic radiosurgery cones

Figure 7

Calculated slopes from the linear curve fit of the relative film dose–response curves for radiosurgery fields of 5, 7.5, 10, 12.5, and 15 mm diameter at ‘d_max’ using EDR2 film. The output factors for these circular fields were calculated by normalizing the slopes from the linear portion of the relative film dose–response curve to the slope of reference field, 10 cm × 10 cm

Figure 8

Percentage depth dose comparison for different energy beams; using EDR2 film; (a) 6 MV photon, (b) 4 MV photon, (c) 15 MV photon, (d) 8 MeV electron, (e) 12 MeV electron, and (f) 15 MeV electron; using EBT3 film (g) 6 MV photon; percentage depth dose: Film slope method (dotted black line), PTW 31010 (solid red line); 1D gamma evaluation between film slope method and PTW 31010 (solid orange line)

Figure 9

Percentage depth dose comparison for different radiosurgery cones. (a) 5 mm diameter, (b) 10 mm diameter, and (c) 15 mm diameter; percentage depth dose: Film slope method (dotted black line), Monte Carlo (solid red line), diode (N60008) (solid blue line), and 1D gamma evaluation between film slope method and Monte Carlo (solid orange line). Percentage depth dose in the buildup region for (d) 5 mm diameter, (e) 10 mm diameter, and (f) 15 mm diameter