Build-up Cap Research Articles

Experimental Investigation of X-Ray Radiation Shielding and Radiological Properties for Various Natural Composites.

Shielding from radiation and plan dose verification is vital during the potential applications in industrial and medical applications. A number of natural composites have been investigated for protecting against high-energy X-ray shielding. The aim is to learn about how natural composites behave under various X-ray energies at STP. The radiological parameters of wood samples were determined using computed tomography imaging, specifically relative electron density (RED), Hounsfield units (HUs), and mass density (MD). Percentage attenuation was measured using a semiflux ionization chamber incorporated with a brass build-up cap and an ionization chamber placed at the beam Isocenter for a different type of natural composite. Measurements are being carried out on a Linear accelerator at an SSD of 110 cm with different collimator sizes. Measured values of HUs, RED, and MD were -232 ± 40, 0.738 ± 0.039, 0.768 ± 0.024 g/cc,-368 ± 41, 0.662 ± 0.047, 0.632 ± 0.024 g/cc, -334 ± 44, 0.639 ± 0.042, 0.666 ± 0.026 g/cc, -370±61, 0.604±0.059, 0.63± 0.036 g/cc, -433±39, 0.543±0.038, 0.608 ± 0.035 g/cc, -382±54, 0.5±0.052, 0.618 ± 0.0316 g/cc, -292±68, 0.680±0.066, 0.708 ± 0.039 g/cc, -298±27, 0.680±0.0229, 0.702± 0.131 g/cc, for Acacia Nilotica, Mangifera Indica, Azadirachta Indica, Tectona Grandis L, Ficus Religiosa, Tecomella Undulata, Sesamum Indicum, Pinus respectively. Measurements show that attenuation is affected by the energy of incident photons, collimator opening, and the type of density of the wood. Various radiological parameters were determined for wood samples that can be utilized to create inhomogeneous phantoms in dosimetry. The largest attenuation is found in Acacia Nilotica and Sesamum Indicum, while the lowest attenuation is found in Ficus religiosa.

Erratum to: A metallic buildup cap for glass rod dosimeters in radiotherapeutic in vivo dosimetry

Erratum to: A metallic buildup cap for glass rod dosimeters in radiotherapeutic in vivo dosimetry

A MOSFET-Based Method for Measuring Peak Kilovoltage (kVp) in Diagnostic X-Ray Beams

Peak kilovoltage (kVp) is an X-ray tube parameter that can influence the radiograph image quality and therefore, if the kVp is different from the nominal value, it can provide an unnecessary radiation dose to the patient if the examination has to be repeated. Therefore, some radiation protection institutions and health organizations (ICRP, IAEA) make recommendations to monitor kVp in medical diagnostic X-ray equipment in clinics and hospitals. There are some methods for measuring kVp in diagnostic X-ray beams, and the most common is the well-known two-sensor method, which uses two filters to attenuate the radiation beam, and it is based on the measurement of each identical sensor. The purpose of this article is to present a method for non-invasive measurement of kVp in medical diagnostic X-ray beams using the MOSFET device. Actually, MOSFET is well known as a detector for ionizing radiation dosimetry mainly in radiotherapy beams. Recently, MOSFET has also been used as dosimeter in diagnostic X-ray beams. In this work it will be shown a kVp measurement technique with MOSFET, which does not use the attenuation factor of the radiation filters, but instead takes advantage of the effect of the device package itself, known as the buildup cap effect. The results show that even with damage to the device exposed to radiation, MOSFET can be used as sensor for kVp measurement.

A metallic buildup cap for glass rod dosimeters in radiotherapeutic in vivo dosimetry

In vivo measurement of radiotherapeutic doses is an important procedure for ensuring the accuracy of radiation treatments. The glass rod dosimeter is a known dosimeter for in vivo dosimetry, which has high reproducibility and low dependencies with respect to major radiotherapeutic conditions. A hemispherical buildup cap made of brass was fabricated to avoid a large measurement error owing to the partial lack of buildup effect during the measurement on the patient's skin. The correction factors (CFs) were experimentally obtained and verified using a Monte Carlo simulation. A clinical trial was performed with 117 patients. As a result, the measured CF was consistent with the simulation by a 2.3% error, and the mean errors of each site were within 2%. The total mean error was 1.84%, with a standard deviation of 1.39%. To conclude, this methodology can provide high accuracy and convenience in radiotherapeutic in vivo measurements.

Dosimetric impact and modeling of prone breast positioning device and couch structures

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 200, −100, and −900 for the insert, the board “base shell” and “base inside,” respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.

Techniques for and uncertainties of MRI-based reconstruction of titanium tandem and ring brachytherapy applicators

PurposeEliminating patient computed tomography (CT) scans for tandem and ring (T&R) brachytherapy can reduce overall procedure time and eliminates imaging dose. However, reconstructing titanium applicators in magnetic resonance imaging (MRI) is challenging. We evaluated the uncertainty of different applicator reconstruction workflows in MR-guided brachytherapy, and assessed the clinical impact of reconstruction uncertainties. Methods and materialsTitanium MRI-compatible T&Rs with aqueous gel in the buildup cap were reconstructed on CTs and MRIs to assess the uncertainties of four different workflows. Reconstruction was performed using (1) proton density–weighted MRIs with solid applicator from a library, (2) applicator-only reference CT fused with MRIs, (3) T2-weighted (T2W) MRIs following GEC-ESTRO guidelines, and (4) patient CTs fused with patient MRIs with in situ applicators. We evaluated dwell positions and plan quality differences using high-risk clinical target volume coverage, and EQD2 D2cc of rectum, sigmoid, bladder, and small bowel. ResultsThe 2σ uncertainty for dwell positions for each workflow were (1) 2.7 mm for both ring and tandem, (2) 1.4 mm ring and 0.8 mm tandem, (3) 0.2 mm ring and 0.8 mm tandem, and (4) 1.9 mm ring and 0.4 mm tandem. Reconstruction uncertainties resulted in dose variations within acceptable levels (below 10%) except for (1) which resulted in larger dose to the rectum (20%). Dose uncertainties were similar between reference CT and patient CT. ConclusionsReconstruction with a reference CT results in similar uncertainty to a patient CT. T2W MRI plans have acceptable uncertainty levels for the applicator reconstruction and resulting dose distributions.

In-Vivo Dosimetry for Small Animal Irradiation: A Preliminary Study

This study assessed applications for in-vivo dosimetry using a 3D printer-based, self-manufactured mouse immobilization device in a small field. The mouse immobilization device was created using a 3D printer (Makerbot Replicator, MakerBot Industries, Brooklyn, NY, USA) and consisted of the support flat, fixing units, sensor areas, and a build-up cap. Radio-photoluminescence glass dosimeters (RPLGD, GD-302M) were inserted at the upper left (UL), upper right (UR), lower left (LL), and lower right (LR) of the device at the center of the target. EBT3 film was inserted into the device at the top of the mouse’s head. Irradiation planning was performed using the ECLIPSE system after a CT simulation of mice with an immobilization device. The mice were irradiated 5 times at a dose 180 cGy with 6 MV X-rays. The dose measurements from the RPLGDs and films were compared with the doses calculated in the Eclipse system. The percentage differences between the Radiation Treatment Planning (RTP) System and RPLGD measurements were 5.56 ± 3.90%, 6.52 ± 5.32%, 10.0 ± 8.97%, and 15.9 ± 17.5% at the UL, UR, LL, and LR positions, respectively. The gamma passing rate of all film measurements exceeded 90% in the 2%/2 mm range. The error values of the 3rd measurements were outliers due to the set up. With the exception of the 3rd measurements, the percentage difference decreased to 4–7%. In this study, we evaluated the applicability of a mouse immobilization device in a small field. We performed in-vivo dosimetry using RPLGD and EBT3 film; this approach may be helpful for using radiation to accurately analyze results in animal studies.

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

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.

Measurement and comparison of head scatter factor for 9MV photon beam using the build-up cap and a columnar mini-phantom

Introduction: The Sc is an important parameter to monitor unit (MU) calculation in radiation therapy procedure. The aim of this study is to measurement and comparison of head scatter factor (Sc) for 9MV photon beam using the build-up cap and a columnar mini-phantom. Materials and Methods: The measurements performed using a calibrated ion chamber (Scanditronix / Wellhofer Farmer Type Chamber FC65-G), and electrometer (Scanditronix / Wellhofer, Dose 1 Reference class electrometer). Sc’s values were measured on Neptun linear accelerator. The Sc measured in the source to surface distance (SSD) of 100cm in different square field sizes (5×5 to 38×38 cm) using two available devices: the PMMA build- up cap and dmax dose water equivalent columnar mini-phantom. Results: Sc’s values for 5×5, 10×10, 15×15, 20×20, 25×25, 30×30, 35×35 and 38×38 cm2 field sized with columnar mini-phantom were 0.97, 1, 1.02, 1.04, 1.06, 1.07, 1.08, and 1.09, respectively. Sc’s values for 5×5, 10×10, 15×15, 20×20, 25×25, 30×30, 35×35 and 38×38 cm2 field sized with build-up cap were 0.96, 1, 1.03, 1.06, 1.08, 1.09, 1.10, and 1.11, respectively. Conclusion: Though measured Sc value using build-up cap in 5×5 cm2 field size was smaller than of those Sc measured using columnar mini-phantom, higher values result in larger field sizes than 10×10 cm2.

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

Introduction: The head scatter factor (Sc) is one of the important parameters for monitor unit (MU) calculation. There are multiple factors that impact the Sc values, such as, head structures, back scattering in to dose monitoring chambers, wedges and so on. This study aimed to investigate the variations of SC with different build-up cap materials, wall thickness, Source to Skin Distance (SSDs), ionization chambers and variation of the SC with wedge angles in 6MV photon beam. Materials and Methods: In this study, Copper and Perspex build-up caps were designed and fabricated with two different thicknesses for each build-up cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK with sensitive volume of 0.12 cc and Farmer-type ion chamber with sensitive volume of 0.65 cc. In all measurements, build-up caps and ionization chambers were positioned in a stand vertical to the beam central axis. It is also investigated the effect of different SSDs on SC. Results: The presence of wedge influences the SC values significantly. Variation of SC in open and wedged fields had maximum deviation of 0.9% and 6.8% and minimum of 0.4% and 2.7% for 30° and 60° angles of wedge, respectively. different SSD and type of ion chamber had no significant influence on the SC. In small field sizes, the SC values in copper build-up cap is higher than Perspex and in large field sizes, measured SC in perspex build-up cap is higher than copper. Conclusion: It was found that the presence of internal wedge has significant contribution to head scatter factor and it increases with increasing wedge angle. Therefore, it should be takes into account in manually MU calculations.

Effect of Material and Wall Thickness Buildup Caps on the Head ScatterFactor Measurements in Irregular Fields Shielded by Cerrobend

Introduction: The head scatter factor (Sc) is important to measurements radiation beam and beam modeling of treatment planning systems used for advanced radiation therapy techniques. This study aimed to investigate the design of a mini-phantom to measurement variations in collimator Sc in the presence of shielding blocks for shaping the beam using different field sizes. Martials and Methods: Copper, Brass, and Perspex buildup caps were designed and fabricated locally as material with three different thicknesses for buildup caps (mini- phantoms). Measurements were performed on an Elekta Compact medical linear accelerator (6MV) in Shafa Kerman Hospital, Iran. The Farmer-type ion chamber FG65-P (Scanditronix, Wellhofer) was used for all measurements. To measure the Sc, mini-phantom was positioned in a stand vertical to the beam central axis. Results: The data indicated that the Sc measurements using different buildup cap materials and thicknesses in 5 × 10, 7.5 × 7.5, and two 10 × 10 cm Cerrobend shield blocks ranged 0.98 to 1.00, 1.04 to 1.05, and 1.04 to 1.06, respectively. Also, it was observed that by increasing the block shield area from 50 cm2 to both 56.25 and 100 cm2, the Sc increased in all situations. Conclusion: We concluded using Brass compared to Perspex and Copper has less uncertainty due to its simple preparation and cutting which is useful to measurement of variations in collimator Sc.

Measuring midline dose without build-up cap for patients with brain tumor undergoing 15MV external radiotherapy by using EBT3 Gafchromic film

Introduction: The Purpose of this study has been measuring midline dose for the patients undergoing brain radiotherapy without any need to build-up cap. Materials and Methods: The study was performed on 28 patients with brain tumor undergoing 15 MV radiotherapy by a Siemens LINAC. To estimate the midline dose, the entrance and exit doses were calculated by a conversion ratio. The conversion ratio was determined as the ratio of the measured dose by EBT3 Gafchromic film at the phantom surface to that measured by an ionization chamber at the depth of 3 and 12 cm. Then, the midline and exit transmission curves were obtained in a solid water phantom for various irradiation conditions with and without a 30-degree wedge for various field sizes of 5×5, 10×10 and 15×15 cm2 and at various SSDs including 80, 90 and 100 cm at various depths ranged from 6 to 22 cm by a 0.6 CC ionization chamber. Eventually, to measure the midline dose, we used both of the midline transmission and arithmetic mean algorithms. Results: The mean and standard deviation of the whole sample for midline transmission algorithm were -3% and 4.52% and for that of arithmetic mean algorithm -2.97% and 4.3%, respectively. However, for 9 patients, more than 5% error was noted in estimated midline doses. Conclusion: A new method was developed for measuring the midline dose without any need to use the build-up cap. The main advantage of the developed method is that there is no disruption in the dose reached to the treatment volume.

Collimator scatter factor: Monte Carlo and in-air measurements approaches

BackgroundLinac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.MethodsBeams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm2 were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions.Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.ResultsThe depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.ConclusionA better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.

Beam characterisation of the 1.5 T MRI-linac

As a prerequisite for clinical treatments it was necessary to characterize the Elekta 1.5 T MRI-linac 7 MV FFF radiation beam. Following acceptance testing, beam characterization data were acquired with Semiflex 3D (PTW 31021), microDiamond (PTW 60019), and Farmer-type (PTW 30013 and IBA FC65-G) detectors in an Elekta 3D scanning water phantom and a PTW 1D water phantom. EBT3 Gafchromic film and ion chamber measurements in a buildup cap were also used. Special consideration was given to scan offsets, detector effective points of measurement and avoiding air gaps.Machine performance has been verified and the system satisfied the relevant beam requirements of IEC60976.Beam data were acquired for field sizes between 1 × 1 and 57 × 22 cm2. New techniques were developed to measure percentage depth dose (PDD) curves including the electron return effect at beam exit, which exhibits an electron-type practical range of cm. The Lorentz force acting on the secondary charged particles creates an asymmetry in the crossline profiles with an average shift of +0.24 cm. For a 10 × 10 cm2 beam, scatter from the cryostat contributes 1% of the dose at isocentre. This affects the relative output factors, scatter factors and beam profiles, both in-field and out-of-field. The average 20%–80% penumbral width measured for small fields with a microDiamond detector at 10 cm depth is 0.50 cm. MRI-linac penumbral widths are very similar to that of the Elekta Agility linac MLC, as is the near-surface dose PDD(0.2 cm) = 57%. The entrance surface dose is ∼36% of . Cryostat transmission is quantified for inclusion within the treatment planning system.As a result, the 1.5 T MRI-linac 7 MV FFF beam has been characterised for the first time and is suitable for clinical use. This was a key step towards the first clinical treatments with the MRI-linac, which were delivered at University Medical Center Utrecht in May 2017 (Raaymakers et al 2017 Phys. Med. Biol. 62 L41–50).

Spatial and Dosimetric Comparison of Tandem/Ring Applicator Against Adjustable Tandem/Ovoid and Tandem/Split-Ring for Intracavitary Brachytherapy Treatment of Cervical Cancer

There is limited clinical guidance for the selection of a tandem and ring (TR) applicator versus the tandem and ovoid (TO), and tandem and split-ring (TSR) applicators during intracavitary brachytherapy treatment of cervical cancer. We evaluated the geometric configuration and dosimetric coverage for cases treated with tandem and ring, a rigid applicator system, as compared with TSR and TO, which utilize adjustable colpostats. 12 patients with FIGO stage IB-IIIB cervical cancer between 2010 and 2016 received external beam therapy to pelvis of 45 Gy followed by two intraoperative insertions, totaling 4 fractions of 7 Gy each. All patients received an HDR brachytherapy boost using the TR for one insertion and either the TSR or TO for the second insertion. All patient data was de-identified in accordance with institutional IRB requirements to evaluate the geometric and dosimetric impact of applicator type. Geometric analysis of the applicator included measurement of the horizontal max dimension in the transverse plane (X) and colpostat build-up cap dimension (c). Dosimetric analysis of ICRU points A, bladder, rectum and vagina and coverage of the CTV were evaluated for both applicator insertions. Standard two-tailed T-Test was used to assess the relationship between applicator types regarding geometric parameters and dosimetric values. Significance was assessed at p<0.05. We included 12 patients with 24 insertions in our analysis, 9 had TR and TSR insertions and 3 received TR and TO insertions. The fixed applicator (TR) had significantly smaller measured X and c dimensions than the adjustable applicators (TO and TSR) (p<0.001). The absolute dose per fraction to the left and right vagina was significantly greater in plans utilizing the fixed applicators versus the adjustable applicators (p<0.01). There were no other statistically significant differences in ICRU reference points or dosimetric volumes between plans utilizing fixed versus adjustable applicators (p=NS).Abstract 2743; Table 1Mean metrics (N=12)ApplicatorX (cm)c (mm)CTV Vol (cm3)CTV V95 (%)ICRUA point R (Gy)A point L (Gy)Bladder (Gy)Rectum (Gy)Vagina R (Gy)Vagina L (Gy)TR (N=12)3.24.946.792.86.36.35.84.217.317.7TSR (N=9)5.06.751.893.46.56.76.34.511.912.6TO (N=3)3.514.037.297.35.95.97.14.66.56.9TR (Fixed) vs TSR and TO Combined (Adjustable)Paired T-test (P value)0.00010.0050.570.850.830.630.320.480.0030.001 Open table in a new tab Geometric design of the TSR and TO applicator allow for greater transverse displacement of the colpostasts as compared to the fixed TR applicator, however, this did not translate into differences in the dosimetric coverage of the CTV or organs at risk. However the TR applicator is limited to a small buildup cap size which is associated with increased vaginal doses as compared to TSR and TO applicator systems.

Gamma Putty shielding effect in megavoltage photon beam

Purpose: Traditionally, lead and Cerrobend have been employed for field shaping in radiation therapy. Lately, another shielding material called Gamma Putty has emerged. The objective of this report is to examine its dosimetric and shielding characteristics in megavoltage photon beam. Methods: All measurements were carried out in a dual energy linac. Data were collected using a calibrated ionization chamber. Percent transmission, linear attenuation, and field size dependence were evaluated for open square fields (4 × 4 cm 2 to 10 × 10 cm 2 ) defined by collimator jaws and for different Gamma Putty thicknesses (t = 0, 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm) at 6 and 18 MV photon beams. The measurements were performed both in air using appropriate acrylic buildup cap and in solid water. Results: The Gamma Putty tray factor (GPTF) increased steadily with field size for both 6 and 18 MV. It was characterized by a half value thickness (HVT) of 2.513 ± 0.101 and 2.855 ± 0.024 cm for 6 and 18 MV, respectively. The reduction in surface dose was about 6%, 14.5%, 22%, 36.37%, and 54% for 6 MV and 2.75 %, 9.36 %, 16.25 %, 28.95 %, and 44.47 % for 18 MV for Gamma Putty thicknesses of 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm. Conclusion: The result of Gamma Putty shielding on the photon beam output increases with thickness, beam energy, and field size. Therefore, clinical use of Gamma Putty tray factors should be tailored for all thicknesses, beam energies, and field sizes.

Effect of Material and Wall Thickness Buildup Caps on the Head Scatter Factor Measurements in Irregular Fields Shielded by Cerrobend

The head scatter factor (Sc ) is important to measurements radiation beam and beam modeling of treatment planning systems used for advanced radiation therapy techniques. This study aimed to investigate the design of a miniphantom to measurement variations in collimator Sc in the presence of shielding blocks for shaping the beam using different field sizes. Copper, Brass, and Perspex buildup caps were designed and fabricated locally as material with three different thicknesses for buildup caps (miniphantoms). Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) in Shafa Kerman Hospital, Iran. The Farmer-type ion chamber FG65-P (Scanditronix, Wellhofer) was used for all measurements. To measure the Sc , miniphantom was positioned in a stand vertical to the beam central axis. The data indicate that the Sc measurements using different buildup cap materials and thicknesses in 5 × 10, 7.5 × 7.5, and two 10 × 10 cm Cerrobend shield blocks ranged 0.98 to 1.00, 1.04 to 1.05, and 1.04 to 1.06, respectively. Also, it was observed that by increasing the block shield area from 50 cm2 to both 56.25 and 100 cm2, the Sc increased in all situations. Results showed that using Brass compared to Perspex and Copper has less uncertainty due to its simple preparation and cutting which is useful to measurement of variations in collimator Sc and shaping the photon beam.

Determination of contributions of scatter and distance error to the source strength of 192Ir HDR brachytherapy source

Abstract High dose rate (HDR) brachytherapy commonly employs a 192Ir encapsulated source to deliver high dose to the malignant tissues. Calibrations of brachytherapy sources are performed by the manufacturer using a well-type chamber or by in-air measurement using a cylindrical ionization chamber. Calibration using the latter involves measurements to be carried out at several distances and room scatter can also be determined. The aim of the present study is to estimate the scatter contribution from the walls, floor and various materials in the room in order to determine the reference air kerma rate of an 192Ir HDR brachytherapy source by in-air measurements and also to evaluate the error in the setup distance between the source centre and chamber centre. Air kerma measurements were performed at multiple distances from 10 cm to 40 cm between the source and chamber. The room scatter correction factor was determined using the iterative technique. The distance error of −0.094 cm and −0.112 cm was observed for chamber with and without buildup cap respectively. The scatter component ranges from 0.3% to 5.4% for the chamber with buildup cap and 0.3% to 4.6% without buildup cap for distances between 10 to 40 cm respectively. Since the average of the results at multiple distances is considered to obtain the actual air kerma rate of the HDR source, the seven distance method and iterative technique are very effective in determining the scatter contribution and the error in the distance measurements.

SU-G-201-13: Investigation of Dose Variation Induced by HDR Ir-192 Source Global Shift Within the Varian Ring Applicator Using Monte Carlo Methods

Purpose: The Varian tandem and ring applicators are used to deliver HDR Ir-192 brachytherapy for cervical cancer. The source path within the ring is hard to predict due to the larger interior ring lumen. Some studies showed the source could be several millimeters different from planned positions, while other studies demonstrated minimal dosimetric impact. A global shift can be applied to limit the effect of positioning offsets. The purpose of this study was to assess the necessities of implementing a global source shift using Monte Carlo (MC) simulations. Methods: The MCNP5 radiation transport code was used for all MC simulations. To accommodate TG-186 guidelines and eliminate inter-source attenuation, a BrachyVision plan with 10 dwell positions (0.5cm step sizes) was simulated as the summation of 10 individual sources with equal dwell times for simplification. To simplify the study, the tandem was also excluded from the MC model. Global shifts of ±0.1, ±0.3, ±0.5 cm were then simulated as distal and proximal from the reference positions. Dose was scored in water for all MC simulations and was normalized to 100% at the normalization point 0.5 cm from the cap in the ring plane. For dose comparison, Point A was 2 cm caudal from the buildup cap and 2 cm lateral on either side of the ring axis. With seventy simulations, 108 photon histories gave a statistical uncertainties (k=1) <2% for (0.1 cm)3 voxels. Results: Compared to no global shift, average Point A doses were 0.0%, 0.4%, and 2.2% higher for distal global shifts, and 0.4%, 2.8%, and 5.1% higher for proximal global shifts, respectively. The MC Point A doses differed by < 1% when compared to BrachyVision. Conclusion: Dose variations were not substantial for ±0.3 cm global shifts, which is common in clinical practice.

On-line estimations of delivered radiation doses in three-dimensional conformal radiotherapy treatments of carcinoma uterine cervix patients in linear accelerator.

Transmission of radiation fluence through patient's body has a correlation to the planned target dose. A method to estimate the delivered dose to target volumes was standardized using a beam level 0.6 cc ionization chamber (IC) positioned at electronic portal imaging device (EPID) plane from the measured transit signal (St) in patients with cancer of uterine cervix treated with three-dimensional conformal radiotherapy (3DCRT). The IC with buildup cap was mounted on linear accelerator EPID frame with fixed source to chamber distance of 146.3 cm, using a locally fabricated mount. Sts were obtained for different water phantom thicknesses and radiation field sizes which were then used to generate a calibration table against calculated midplane doses at isocenter (Diso,TPS), derived from the treatment planning system. A code was developed using MATLAB software which was used to estimate the in vivo dose at isocenter (Diso,Transit) from the measured Sts. A locally fabricated pelvic phantom validated the estimations of Diso,Transit before implementing this method on actual patients. On-line dose estimations were made (3 times during treatment for each patient) in 24 patients. The Diso,Transit agreement with Diso,TPS in phantom was within 1.7% and the mean percentage deviation with standard deviation is −1.37% ±2.03% (n = 72) observed in patients. Estimated in vivo dose at isocenter with this method provides a good agreement with planned ones which can be implemented as part of quality assurance in pelvic sites treated with simple techniques, for example, 3DCRT where there is a need for documentation of planned dose delivery.