Dosimetric investigation of small fields in radiotherapy measurements using Monte Carlo simulations, CC04 ionization...

The motivation of this study is to check the dosimetry of small field sizes used in various treatment techniques using different methods (Monte Carlo simulations and detectors). We created two Monte Carlo models for Elekta Versa HD linear accelerators using EGSnrc (BEAMnrc-DOSXYZnrc) codes. Previous studies led us to define one model with an ideal symmetry full-width-half-maximum (FWHM) of 0.15 cm in the x and y directions for the Gaussian distribution of the primary electron source and redefine the other with a larger asymmetry FWHM of 0.35 cm in the X and 0.6 cm in the Y directions. We calculated the penumbra width using both models. We measured output factors using two different detectors including Razor Diode which is designed especially for small field size measurements and compared them with both models. Using these detectors aims to investigate different detector sensitivities for dose measurements. In addition, patient-specific planning quality assurance (PSQA) for four fictional cases using Elekta Versa HD with Nasopharyngeal, Astrocytoma, right cerebellum, and right breast cancers were done using an IBA-2D array and compared to the minimum segment width parameter in Monaco Treatment planning system (TPS) for (0.5 and 1) cm segment width. The results indicated that Monte Carlo simulation shows increasing in the values of penumbra width with increasing the size of FWHM for field size range 0.5 × 0.5 to 3 × 3 (in-plane: 0.33 to 0.45 for model 1 and 0.46 to 0.65 for model 2, cross-plane: 0.29 to 0.38 for model 1 and 0.44 to 0.62 for model 2). The results indicate that output factors decrease as FWHM increases. The Razor Diode and CC04 detectors show consistent results up until a field size of 1 × 1 cm². Additionally, plans with a minimum segment width of 0.5 cm demonstrate a lower gamma passing rate (GPR) compared to those with a 1 cm segment width. In conclusion, Inaccurate modeling of the FWHM of the primary source can lead to a significant error in the calculation when using a Monte Carlo model of the beam; Accordingly, this may lead to inaccurate delivery of treatment dose for cancer patients, in addition, this error increases as we go down field size 1 × 1 cm² to reach an unacceptable level in field size 0.5 × 0.5 cm². Thus, and as found, we can conclude that: to produce a more accurate radiotherapy treatment plan which in turn will lead to high-quality treatment for cancer patients, It is recommended that, during the beam-shaping process in IMRT or VMAT optimization, the minimum dimensions of any individual beamlet or segment within the treatment field should not be smaller than 1 × 1 cm².