An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

Interpretation of In-air Output Ratio of Wedged Fields in Different Measurement Conditions

Background:

The collimator scatter factor (S_c) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact S_c values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of S_c with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam.

Methods:

In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm³ and Farmer-type ion chamber with a sensitive volume of 0.65 cm³. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on S_c.

Results:

It was found in large field sizes, S_c values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on S_c values. The presence of wedge influenced S_c values significantly. Variation of S_c in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively.

Conclusion:

It was found that the presence of wedges had a significant influence on S_c and increases with wedge angles. As such, it should be taken into account in manual MU calculations.

Total scatter factor for small fields in radiotherapy: a dosimetric comparison

Purpose

Small field dosimetry is complicated and accuracy in the measurement of total scatter factor (TSF) is crucial for dosimetric calculations, in making optimum intensity-modulated radiotherapy plans for treating small target volumes. In this study, we intended to determine the TSF measuring properties of CC01 and CC04 detectors for field sizes ranging from sub-centimetre to the centimetre fields.

Material and methods

CC01 and CC04 chamber detectors were used to measure TSF for 6 and 18 MV photon beam delivered from the linear accelerator, through small fields in a water phantom. Small fields were created by collimator jaws and multi-leaf collimators separately, with field sizes ranging from 0·6 to 10 cm2and 0·5 to 20 cm2, respectively.

Results

CC01 measured TSF at all the given field sizes created by jaws and multi-leaf collimators for both 6 and 18 MV beams whereas CC04 could not measure TSF for field sizes <1 cm2due to volume averaging and perturbation effects.

Conclusion

CC01 was shown to be effective for measurement of TSF in sub-centimetre field sizes. CC01 can be employed to measure other dosimetric quantities in small fields using different energy beams.