Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation

Abstract

Purpose: This study aims to evaluate dosimetric parameters like percentage depth dose, dosimetric field size, depth of maximum dose surface dose, penumbra and output factors measured using IBA CC01 pinpoint chamber, IBA stereotactic field diode (SFD), PTW microDiamond against Monte Carlo (MC) simulation for 6 MV flattening filter-free small fields. Materials and Methods: The linear accelerator used in the study was a Varian TrueBeam® STx. All field sizes were defined by jaws. The required shift to effective point of measurement was given for CC01, SFD and microdiamond for depth dose measurements. The output factor of a given field size was taken as the ratio of meter readings normalised to 10 × 10 cm2 reference field size without applying any correction to account for changes in detector response. MC simulation was performed using PRIMO (PENELOPE-based program). The phase space files for MC simulation were adopted from the MyVarian Website. Results and Discussion: Variations were seen between the detectors and MC, especially for fields smaller than 2 × 2 cm2 where the lateral charge particle equilibrium was not satisfied. Diamond detector was seen as most suitable for all measurements above 1 × 1 cm2. SFD was seen very close to MC results except for under-response in output factor measurements. CC01 was observed to be suitable for field sizes above 2 × 2 cm2. Volume averaging effect for penumbra measurements in CC01 was observed. No detector was found suitable for surface dose measurement as surface ionisation was different from surface dose due to the effect of perturbation of fluence. Some discrepancies in measurements and MC values were observed which may suggest effects of source occlusion, shift in focal point or mismatch between real accelerator geometry and simulation geometry. Conclusion: For output factor measurement, TRS483 suggested correction factor needs to be applied to account for the difference in detector response. CC01 can be used for field sizes above 2 × 2 cm2 and microdiamond detector is suitable for above 1 × 1 cm2. Below these field sizes, perturbation corrections and volume averaging corrections need to be applied.

Point dose verification of Cranial Stereotactic Radiosurgery using micro Ionization Chamber and EBT3 film for 6MV FF and FFF beams in Varian TrueBeam® LINAC

“…The output factor of CC01 has been published and has been shown to under-respond steeply below 2x2 cm 2 . 30 This has been clearly observed in the results of this study, where nearly 50% of PSQA results for CC01 underresponded in comparison with Film PSQA. As the lesion size increased the agreement between the two detectors was seen to improve.…”

“…Furthermore, the absence of charged particle equilibrium in small fields and beamlet based IMRT also makes CC01 ionization not necessarily proportional to the dose distributed in the area due to perturbations. 30 About 15-25% of the CC01 PSQA results showed over-response against Film irrespective of lesion size which may have been caused due to perturbations in electronic fluence. A clear indicator of the increasing agreement between the two detectors is seen in the decreasing standard deviation with volume size (5.19% for <1cc, 2.91% for 1cc-2cc, 2.15% for 2cc-3cc).…”

Monte Carlo simulation of linac using PRIMO

“…There were many studies about linac simulation using PRIMO [ 25 – 39 ]. Different types of linac machines were simulated using PRIMO, including Varian TrueBeam [ 25 , 29 – 31 , 35 , 37 , 39 ], Varian 2100 C/D [ 27 ], Clinac 2300 [ 28 ], Clinac® iX [ 33 , 36 ], Trilogy [ 32 ], Novalis [ 32 ], EDGE [ 34 ]. Some of the previous studies adopted the phase space files (PSF) from MyVarian website to model the linac head [ 25 , 29 , 35 , 39 ].…”

“…Different types of linac machines were simulated using PRIMO, including Varian TrueBeam [ 25 , 29 – 31 , 35 , 37 , 39 ], Varian 2100 C/D [ 27 ], Clinac 2300 [ 28 ], Clinac® iX [ 33 , 36 ], Trilogy [ 32 ], Novalis [ 32 ], EDGE [ 34 ]. Some of the previous studies adopted the phase space files (PSF) from MyVarian website to model the linac head [ 25 , 29 , 35 , 39 ]. Some researchers simulated linacs using the default beam parameters given by PRIMO [ 31 , 34 ], while other studies modified the beam parameters based on the default recommendations in PRIMO [ 27 , 28 , 30 , 32 , 33 , 36 ].…”

“…Previous studies using PRIMO focused on the evaluation of linac head model [ 25 – 27 , 30 , 31 , 35 , 36 , 38 ]. However, the evaluation of multileaf collimator (MLC) model was critical in clinical use [ 40 ].…”