Background and Objective: In radiotherapy, measurement of the output factor in the air is one of the input parameters for Implementation of treatment planning systems and beam modeling. The aim of this study is to design a mini-phantom and select the appropriate detector for measuring the output factor.Subjects and Methods: In-air output factor was measured for Siemens linear accelerator 6 MV energy and field sizes of 4, 6, 8, 10, 15, 2 cm irradiated by 200MU. Measurements were performed using a Gafchromic film EBT2 and 0.6 cc Farmer ion chamber and then the results were compared with Report of AAPM Therapy Physics Committee Task Group 74. For lateral electron equilibrium and electron contamination removal, two top of the water and Cerrobend were designed for film dosimetry and plexiglass buildup cap for Farmer chamber was applied.Results: The output factor increases with field size, and this increase in Cerrobend top is less than water top and Farmer dosimeter. In comparison between the results reported by TG74 and this study shows that film dosimetry with water top and Farmer chamber have 0.35 and 0.68 mean percentage differences with TG74 respectively and meanwhile the film dosimetry with Cerrobend top presents 1.48 mean percentage differences.Conclusion: EBT2 film with mini phantom made of water as a good alternative to ion chamber dosimeters known as a standard dosimeter especially the film can be used for a wider range of field sizes

Background and Objective: The output factor is an important factor in calculating the dose in the treatment of cancer patients with Megavoltage photon beams. Since in the most cases, the treatment field is asymmetric, therefore the calculation of this factor is very important. Subjects and Methods: Measurements were done for Siemens Primus Plus accelerator, Golestan Hospital using Farmer ionization chamber 6cc. The output factors was measured for the symmetric fields and calibration curves were plotted for Collimator scatter factor. For each symmetric square field, many asymmetrical fields were defined. Using analysis and comparison with measured values for Siemens Linac, a tuning parameter was calculated for each symmetric square field. Then, the tuning parameter was applied to calculate Collimator scatter factor for the corresponding asymmetric field. Finally the calculation results were compared with experimental values.Results: The tuning parameter for the two symmetric fields 30×30 and 25×25 were 0.452±0.001 and 0.481±0.001 respectively. Taking into account this tuning parameter for asymmetric fields, the maximum percentage difference between calculated and measured results, was 0.7%.Conclusions: Results indicate a reduction in the asymmetric Collimator scatter factor relative to the symmetrical field. In addition, with increase in distance from center of symmetry, it becomes more significant decline. Also according to the calibration curve, the equivalent square field side for every asymmetric field can be obtained.

Background: The present study aims to estimate the effect of Collimator, phantom and Multi-Leaf Collimator (MLC) scatters on dose calculation in different breast cancer cases using acrylic, brass build-up caps and acrylic mini-phantom in the measurements. Materials and Methods: Collimator scatter factors (Sc), phantom scatter factors (Sp) and MLC transmission factors for different field sizes ranging from 1 × 1 cm2 to 40 × 40 cm2 for energies 6 MV and 15 MV were measured using acrylic mini-phantom (PTW and local mini-phantom), acrylic build up cap and brass build up cap where the farmer ionization chamber was used as detector in this study and semiflex detector was used but only with small field sizes from 1 × 1 cm2 to 4 × 4 cm2 and scatter effect on the dose calculation in different breast cancer cases was evaluated. Results: The results in this study show that there is no significant difference between MLC transmission factors using acrylic mini-phantom and brass build-up cap with energy 6 MV where the transmission factor value is 0. 007 and 0. 0071 with acrylic mini-phantom and brass build-up cap, respectively. Also it is clear that brass build-up cap gives the highest Collimator scatter factors results where Collimator scatter factors start at value 0. 963 at field size 4 × 4 cm2 then increase gradually to end at point value 1. 049 at field size 40 × 40 cm2. In breast cancer cases, there is sharp increase in organ at risk doses with brass build-up cap. Conclusion: From this study it is evident that almost there is large variation between the acrylic build-up cap, acrylic mini-phantom and brass build-up cap where brass build-up achieve higher results in most measurements.

Background: The Collimator scatter factor (Sc) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact Sc values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of Sc 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 Sc. Results: It was found in large field sizes, Sc 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 Sc values. The presence of wedge influenced Sc values significantly. Variation of Sc 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 Sc and increases with wedge angles. As such, it should be taken into account in manual MU calculations.