Purpose: With the advent of state-of-the-art treatment technologies, the use of small fields has increased, and dosimetry in small fields is highly challenging. In this study, the potential use of Varian electronic portal imaging device (EPID) for small field measurements was explored for 6 and 15 MV photon beams. Materials and Methods: The output factors and profiles were measured for a range of jaw-collimated square field sizes starting from 0.8 cm×0.8 cm to 10 cm×10 cm using EPID. For evaluation purpose, reference data were acquired using Exradin A16 microionization chamber (0.007 cc) for output factors and stereotactic field diode for profile measurements in a radiation field analyzer. Results: The output factors of EPID were in agreement with the reference data for field sizes down to 2 cm×2 cm and for 2 cm×2 cm; the difference in output factors was +2.06% for 6 MV and +1.56% for 15 MV. For the lowest field size studied (0.8 cm×0.8 cm), the differences were maximum; +16% for 6 MV and +23% for 15 MV photon beam. EPID profiles of both energies were closely matching with reference profiles for field sizes down to 2 cm×2 cm; however, penumbra and measured field size of EPID profiles were slightly lower compared to its counterpart. Conclusions: EPID is a viable option for profile and output factor measurements for field sizes down to 2 cm×2 cm in the absence of appropriate small field dosimeters.

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The aim is to study the density, isodose depths, and doses at different points in slab-pinewood-slab (SPS) phantom, solid phantom SP34 (made up of polystyrene), and chest level of actual patient for developing heterogeneous chest phantom mimicking thoracic region of human body. A 6 MV photon beam of field size of 10 cm×10 cm was directed perpendicular to the surface of computed tomography (CT) images of chest level of patient, SPS phantom, and SP34 phantom. Dose was calculated using anisotropic analytical algorithm. Hounsfield units were used to calculate the density of each medium. Isodose depths in all the three sets of CT images were measured. Variations between planned doses on treatment planning system (TPS) and measured on linear accelerator (LA) were calculated for three points, namely, near slab-pinewood interfaces (6 and 18 cm depths) and 10 cm depth in SPS phantom and at the same depths in SP34 phantom. Density of pinewood, SP34 slabs, chest wall, lung, and soft tissue behind lung was measured as 0.329 ± 0.08, 0.999 ± 0.02, 0.898 ± 0.02, 0.291 ± 0.12, and 1.002 ± 0.03 g/cc, respectively. Depths of 100% and 90% isodose curves in all the three sets of CT images were found to be similar. Depths of 80%, 70%, 60%, 50%, and 40% isodose lines in SPS phantom images were found to be equivalent to that in chest images, while it was least in SP34 phantom images. Variations in doses calculated at 6, 10, and 18 cm depths on TPS and measured on LA were found to be 0.36%, 1.65%, and 2.23%, respectively, in case of SPS phantom, while 0.24%, 0.90%, and 0.93%, respectively, in case of SP34 slab phantom. SPS phantom seemed equivalent to the chest level of human body. Dosimetric results of this study indicate that patient-specific quality assurance can be done using chest phantom mimicking thoracic region of human body, which has been fabricated using polystyrene and pinewood.

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Aims: This study investigates the impact of cine acquisition mode on the dosimetric characteristics of a Varian aS500 amorphous silicon electronic portal imaging device (a-Si EPID). Materials and Methods: The performance of an a-Si EPID operated in cine mode was assessed and compared to its performance when operated in an integrated mode and dose measurements using an ionization chamber. This study was conducted at different photon energies and the EPID performance was assessed as function of the delivered dose, dose rate, multileaf collimator speed, field size, phantom thickness, and intensity-modulated radiation therapy fields. Results: The worst nonlinearity was observed at low monitor unit (MU) settings < 100 MU with the highest dose per frame. The nonlinearity of response at a low MU setting was attributed due to the loss of four cine images during each delivery. The EPID response with changing dose rate for 10 MU delivered had similar results to its performance in an integrated mode and ionization chamber. Despite the nonlinearity of response with low MU delivered, EPID performance operated in cine and integrated acquisition modes had comparable responses within 2%. Conclusions: For EPID dosimetry application using cine mode, this study recommends the calibration of the EPID images to be undertaken at a large MU. There were no additional corrections that were required when the EPID operated in cine acquisition mode as compared to calibration in integrated mode.

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Aim: This work investigated the dosimetric properties of a 10-MV photon beam emitted from a medical linear accelerator (linac) with no flattening filter (FF). The aim of this study is to analyze the radiation fluence and energy emitted from the flattening filter free (FFF) linac using Monte Carlo (MC) simulations. Materials and Methods: The FFF linac was created by removing the FF from a linac in clinical use. Measurements of the depth dose (DD) and the off-axis profile were performed using a three-dimensional water phantom with an ionization chamber. A MC simulation for a 10-MV photon beam from this FFF linac was performed using the BEAMnrc code. Results: The off-axis profiles for the FFF linac exhibited a chevron-like distribution, and the dose outside the irradiation field was found to be lower for the FFF linac than for a linac with an FF (FF linac). The DD curves for the FFF linac included many contaminant electrons in the build-up region. Conclusion: Therefore, for clinical use, a metal filter is additionally required to reduce the effects of the electron contamination. The mean energy of the FFF linac was found to be lower than that of the FF linac owing to the absence of beam hardening caused by the FF.

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Purpose: To investigate beam quality correction, K_QQ0 (r) and phantom scatter correction, K_phan (r) for lithium formate dosimeter as a function of distance r along the transverse axis of the high-energy brachytherapy sources ⁶⁰Co, ¹³⁷Cs, ¹⁹²Ir and ¹⁶⁹Yb using the Monte Carlo-based EGSnrc code system. Materials and Methods: The brachytherapy sources investigated in this study are BEBIG High Dose Rate (HDR) ⁶⁰Co (model Co0.A86), ¹³⁷Cs (model RTR), HDR ¹⁹²Ir (model Microselectron) and HDR ¹⁶⁹Yb (model 4140). The solid phantom materials investigated are PMMA, polystyrene, solid water, virtual water, plastic water, RW1, RW3, A150 and WE210. Result:K_QQ0 (r) is about unity and distance independent for ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources, whereas for the ¹⁶⁹Yb source, K_QQ0 (r) increases gradually to about 4 % larger than unity at a distance of 15 cm along the transverse axis of the source. For ⁶⁰Co source, phantoms such as polystyrene, plastic water, solid water, virtual water, RW1, RW3 and WE210 are water-equivalent but PMMA and A150 phantoms show distance-dependent K_phan (r) values. For ¹³⁷Cs and ¹⁹²Ir sources, phantoms such as solid water, virtual water, RW1, RW3 and WE210 are water-equivalent. However, phantoms such as PMMA, plastic water, polystyrene and A150 showed distance-dependent K_phan (r) values, for these sources. For ¹⁶⁹Yb source, all the investigated phantoms show distance-dependent K_phan (r) values. Conclusion: K_QQ0 (r) is about unity and distance independent for ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources. Phantoms such as solid water, virtual water, RW1, RW3 and WE210 are water-equivalent for ⁶⁰Co, ¹³⁷Cs and ¹⁹²Ir brachytherapy sources. For ¹⁶⁹Yb source, all the investigated phantoms show distance-dependent K_phan (r) values.

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Gafchromic films have been applied to X-ray dosimetry in diagnostic radiology. To correct nonuniformity errors in Gafchromic films, X-rays in the double-exposure technique can be replaced with ultraviolet (UV)-A rays. Intensities of the incident and transmitted UV-A rays were measured. However, it is unclear whether the chemical color change of Gafchromic films affects the UV-A transmission intensity. Gafchromic EBT3 films were suitable to be used in this study because non-UV protection layers are present on both sides of the film. The film is placed between UV-A ray light-emitting diodes and a probe of a UV meter. Gafchromic EBT3 films were irradiated by UV-A rays for up to 60 min. Data for analysis were obtained in the subsequent 60 min. Images from before and after UV-A irradiation were subtracted. When using 375 nm UV-A, the mean ± standard deviation (SD) of the pixel values in the subtracted image was remarkably high (11,194.15 ± 586.63). However, the UV-A transmissivity remained constant throughout the 60 min irradiation period. The mean ± SD UV-A transmission intensity was 184.48 ± 0.50 μm/cm². Our findings demonstrate that color density changes in Gafchromic EBT3 films do not affect their UV-A transmission. Therefore, Gafchromic films were irradiated by UV-A rays as a preexposure.

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