Design and characterization of practical small-field collimators for radiotherapy calibration unit at a secondary standard dosimetry laboratory

We aimed to design small fields collimators made of lead blocks, with diameters of 40 mm, 30 mm, 20 mm, and 10 mm, for the calibration of small volume radiotherapy ionization chambers using the Algerian Secondary Standard Dosimetry Laboratory (SSDL) ELDORADO 78⁶⁰Co calibration unit. The radiation properties of the designed collimators were assessed using GAFChromic EBT3 dosimetry film (Ashland, USA) and two PTW small volume ionization chambers (PTW, Freiburg, Germany). Additionally, the photons spectra for the four different fields were also investigated using Monte Carlo (MC) method. The MC method was also utilized to define the dimensions of the collimators that achieve the desired field sizes at the reference distance of 80 cm. We simulated the photon beam of the⁶⁰Co source from the irradiation unit to estimate the influencing quantities including field size and beam attenuation. EBT3 film, PTW Pinpoint and PTW Semiflex ionization chambers were used to measure the beam profiles, the output factors (OF), and the percentage depth doses (PDD) for the designed collimators. These measurements were performed at a depth of 5 cm and a source-to-surface distance of 80 cm. The beam profiles simulated with MC method and experimentally measured were found to be agreed within 3%/1 mm gamma criteria. The ionization chambers underestimate the OFs in comparison to EBT3 film for the four field sizes. The PDD curves were measured by the Pinpoint chamber and calculated using MC simulation. Significant differences were noted between the curves of the 20 and 10 mm collimators compared to those of the 40 mm and 30 mm ones. The curves were in agreement with the relative simulated data. Custom small-field collimators were successfully produced and characterized, which will facilitate studies on the variation and stability of calibration coefficients for small volume detectors operating within small-fields at our SSDL.