Farmer-type Chamber Research Articles

A finite element method for the determination of the relative response of ionization chambers in MR-linacs: simulation and experimental validation up to 1.5 T

In magnetic resonance (MR) guided radiotherapy, the magnetic field-dependent change in the dose response of ionization chambers is typically included by means of a correction factor . This factor can be determined experimentally or calculated by means of Monte Carlo (MC) simulations. To date, a small number of experimental values for at magnetic flux densities above 1.2 T have been available to benchmark these simulations. Furthermore, MC simulations of the dose response of ionization chambers in magnetic fields (where such simulations are based on manufacturer blueprints) have been shown to converge with results that deviate considerably from experimental values for orientations where the magnetic field is perpendicular to the axis of the ionization chamber and the influence of the magnetic field is largest.In this work, was simulated for a PTW 30013 Farmer ionization chamber using an approach based on finite element simulations. First, the electrical field inside the ionization chamber was simulated using finite element methods. The collecting volume of the ionization was not defined in terms of the physical dimensions of the detector but in terms of the simulated electrical field lines inside the chamber. Then, an MC simulation of the dose response of a Farmer type chamber (PTW 30013) was performed using EGSnrc with a dedicated package to consider the effect of the magnetic field. In the second part, was determined experimentally for two different PTW 30013 ionization chambers for a range of magnetic flux densities between B = 0 and 1.5 T, covering the range of commercially available MR-linacs.In the perpendicular orientation, the maximum difference between the simulated values for and the experimental values for was 0.31(30)% and the minimum difference was 0.02(24)%. For the PTW 30013 ionization chambers, the experimental values for were 0.9679(1) and 0.9681(1) for a magnetic flux density of 1.5 T. The value resulting from the simulation was 0.967(3).The comparison of the correction factors simulated using this new approach with the experimental values determined in this study shows excellent agreement for all magnetic flux densities up to 1.5 T. Integrating the explicit simulation of the collection volume inside the ionization chambers into the MC simulation model significantly improves simulations of the chamber response in magnetic fields. The results presented suggest that intra-type variations for may be neglectable for ionization chambers of the PTW 30013 type.

Technical Note: Consistency of PTW30013 and FC65-G ion chamber magnetic field correction factors.

PurposeReference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65‐G detectors are waterproof Farmer‐type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber‐to‐chamber variation and determine whether generic chamber type‐specific magnetic field correction factors can be applied for each of the PTW30013 and FC65‐G type ion chambers when they are oriented anti‐parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field.MethodsThe experiment was conducted with 12 PTW30013 and 13 FC65‐G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross‐calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI‐linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR‐compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter‐clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber‐type determined.ResultsThe magnetic field correction factors within both PTW30013 and FC65‐G chamber‐types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65‐G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%.ConclusionThe consistency of the results for the PTW30013 and FC65‐G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65‐G chamber. Values for each chamber‐type (with careful attention to beam energy, magnetic field strength and beam‐field‐chamber orientations) can be applied from the literature.

PENELOPE/PRIMO-calculated photon and electron spectra from clinical accelerators

BackgroundThe availability of photon and electron spectra in digital form from current accelerators and Monte Carlo (MC) systems is scarce, and one of the packages widely used refers to linacs with a reduced clinical use nowadays. Such spectra are mainly intended for the MC calculation of detector-related quantities in conventional broad beams, where the use of detailed phase-space files (PSFs) is less critical than for MC-based treatment planning applications, but unlike PSFs, spectra can easily be transferred to other computer systems and users.MethodsA set of spectra for a range of Varian linacs has been calculated using the PENELOPE/PRIMO MC system. They have been extracted from PSFs tallied for field sizes of 10 cm × 10 cm and 15 cm × 15 cm for photon and electron beams, respectively. The influence of the spectral bin width and of the beam central axis region used to extract the spectra have been analyzed.ResultsSpectra have been compared to those by other authors showing good agreement with those obtained using the, now superseded, EGS4/BEAM MC code, but significant differences with the most widely used photon data set. Other spectra, particularly for electron beams, have not been published previously for the machines simulated in this work. The influence of the bin width on the spectrum mean energy for 6 and 10 MV beams has been found to be negligible. The size of the region used to extract the spectra yields differences of up to 40% for the mean energies in 10 MV beams, but the maximum difference for TPR 20,10 values derived from depth-dose distributions does not exceed 2% relative to those obtained using the PSFs. This corresponds to kQ differences below 0.2% for a typical Farmer-type chamber, considered to be negligible for reference dosimetry. Different configurations for using electron spectra have been compared for 6 MeV beams, concluding that the geometry used for tallying the PSFs used to extract the spectra must be accounted for in subsequent calculations using the spectra as a source.ConclusionsAn up-to-date set of consistent spectra for Varian accelerators suitable for the calculation of detector-related quantities in conventional broad beams has been developed and made available in digital form.

Evaluation of dosimetry a performance parameter for a homemade farmer-type ionization chamber according to IEC60731

Introduction: Due to its simplicity, fast response and high reliability, ion chamber detectors are one of the most suitable dosimetry instruments for X and gamma rays. The dosimeter of photon beams, which used in radiotherapy centers, requires high accuracy and should pass requirements of IEC 60731 standards. About 80 percent of the IEC 60731 standard tests such as stability, leakage current, the dose-response linearity, repeatability and resolution were assessed and the results were compared with the values reported by the standard. All tests were carried out on the PTW-30001 Farmer-type chamber and the response was compared with the manufactured chamber. Materials and Methods: In this study, according to the IEC 60731, IAEA guidelines and standards, using Cobalt-60 gamma radiation, the dosimetric and functional parameters of an ionization cavity detector of a cylindrical Farmer-type with sensitive volume of 0.6 cc, (homemade 0.6 cc Cylindrical Ionization Chamber), compared and calibrated against the calibrated reference Farmer chamber response (PTW-30001). Results: By applying minimum dose, leakage current was ±0.35 % and It was reduced by less than 1 % after radiation exposure. The percentage change in stability was ±0.3 %. Angular dependency in the chamber response was +0.2 %. Temperature effect on chamber reading was +0.2 % and the repeatability error was ±0.2 %. Conclusion: Unlike a simple look, Farmer-type chambers have their own technical complexity and cannot easily be used as a dosimeter by doing a few simple tests. Therefore, the IEC 60731 has been developed to address these complexities and introduce them as requirements for the application of an ionization chamber. The present work was a part of the project of manufacturing and commercializing a Farmer-type ionization chamber at the Nuclear Science and Technology Research Institute (NSTRI) in order to face the ahead challenges and try to resolve them. Most of the challenges that come from the technical knowledge and experience can be solved. All of the tests performed well with the standard requirements, but at the same time, compared to the reference ion chamber performance. There was negligible difference between the PTW reference chamber and the criteria required in the standard which demonstrates well-designed PTW chamber.

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.

188 Orthovoltage rotational breast radiotherapy: A Monte Carlo and experimental study

Purpose To investigate depth-dose characteristics of a new technique for orthovoltage rotational breast radiotherapy (RT) via Monte Carlo (MC) simulations and experimental phantom study, following the initial work by Boone [1] . Methods Geant4 MC simulations were developed to calculate dose distributions generated by an orthovoltage (320 kVp, 4 mm Cu filtration) or kilovoltage (150 kV, 1 mm Cu) X-ray beams rotating 360° around a cylindrical phantom (polyethylene or PMMA) placed at isocentre to simulate prone patient’s breast. Horizontal beam collimation was 1–14 cm. Radial and longitudinal depth-dose distributions were performed for homogeneous phantoms (with or without lesions simulated for different depths in phantom). Experimental validation was performed on the same geometry with the 150 kV beam. Farmer-type air ionization chamber and radiochromic films are used to measure radial dose profiles. Results MC simulations at 320 kVp show that rotational kilovoltage radiotherapy assures sufficient skin sparing during breast RT. Centre to periphery dose ratio is less than 20:1 (1 cm collimation in the horizontal plane at 320 kVp). The vertical dose spread profile shows limited influence of the scatter dose at distance of 10 mm from the irradiated slice. MC simulations will be compared to laboratory measurements at 150 kV. Conclusions We showed the potential of orthovoltage external beam rotational RT for breast cancer with satisfactory skin sparing, though using a kilovoltage beam.

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.

Оценка индекса дозы при использовании протоколов сканирования компьютерной томографии в коническом пучке для пациентов детского возраста

Purpose: Cone-beam computed tomography (CBCT) is an indispensable procedure for accurate patient positioning during radiation therapy (RT) in many clinical cases. However, the patients get an additional dose using CBCT. This dose is neither therapeutic nor diagnostic. It is very difficult to obtain the reliable information about the dose distribution within the patient using the CBCT. Despite this, there is a need to control the additional dose for the pediatric patients and reduce it. There are different approaches of imaging dose evaluation. Most accurate methods are based on the Monte-Carlo calculation and thermoluminescent dosimeters-based measurements. However, the implementation of these methods is complex and cumbersome, that makes impossible their application in routine clinical practice. The evaluation of dose indexes is an accessible and convenient alternative. The purpose of this study is evaluation of the cone beam computed tomography dose indexes for different imaging protocols and object sizes. Material and methods: The technique based on absolute and relative dose measurements for CBCT was used in this study. Absolute dose measurements were performed at the periphery and center of the FREEPOINT (CIRS) phantom using the Farmer type chamber FC65-P for each CBCT protocols. FREEPOINT (20 cm height, 30 cm width, 30 cm length) was used for imitation big chest and pelvis. Inner insert (16 cm diameter) of the phantom was used for imitation head, small chest and pelvis. The dose profiles were measured using I’mRT MatriXX (IBA) and analyzed by OmniPro-I’mRT software, dose indexes DLP (dose–length product) were calculated. Results: The dose indexes were identified for five protocols corresponding three scanning areas (Head and Neck, Chest and Pelvis). The dose indexes were 51.82 and 90.25 mGy×cm using Head and Neck S20 and Head and Neck M20 protocols respectively. The lowest dose index was obtained 13.28 mGy×cm for Fast Head and Neck S20. It was established that the scanning object size strongly affects on the dose index values and, as result, on the absorbed dose within the patient. The dose indexes were 305.42 and 187.53 mGy×cm using scanning protocol Chest M20 for small and big phantoms respectively. The similar results were obtained for scanning protocol Pelvis M15. The highest dose index was obtained 846.93 mGy×cm for the small phantom, while the dose index was 563.79 mGy×cm for the big phantom. The necessity of several clinical protocols to scan different areas was shown. Using of the Pelvis M15 protocol for head scanning may increase the additional point dose 96 times in comparison with Fast Head and Neck S20 protocol. Conclusion: The dose indexes were evaluated taking into account the size of the scanning object for different imaging protocols. Routine use of CBCT in clinical practice requires a sensible choice of the scanning protocol based on the results of the dose index estimation.

Empirical study of the spatial variation of recombination, polarity and polarization effects in ionization chambers

We present measurements of the spatial response of two ionization chambers obtained by scanning them through a 0.1 mm diameter beam of synchrotron radiation (weighted-average energy 95 keV). The technique was used to investigate the spatial variation of effects associated with recombination, bias voltage and beam polarization on the signal. The chamber types investigated were the PTW model 30013 0.6 cm3 Farmer-type chamber and the Exradin model A5 100 cm3 spherical chamber. Recombination was found to vary according to the local electric field strength inside the chamber and the volume of air presented to the beam. The bias polarity effect was found to be small and relatively uniform across the chamber. Within the accuracy of the measurements, no difference could be determined between response scans with the electric field vector of the incident beam aligned parallel and orthogonal to the chamber axis.

Profile- and output-determined Prp output corrections for a Varian TrueBeam and Elekta Versa HD

For calibration of flattening filter-free (FFF) linac beams, one needs to either use a chamber small enough to avoid volume averaging that occurs with the conventional Farmer-type chamber, or apply a gradient correction (Prp) to the Farmer chamber. Continued use of the Farmer chamber is attractive due to the desirable characteristics of the chamber. We measured Prp using two methods, the recommended profile gradient method, and an output method with a small chamber having negligible volume averaging, for both a Varian TrueBeam and Elekta Versa 6MV and 10MV FFF beams for 90 cm source-to-surface distance (SSD), 100 cm source-to-axis (SAD) calibrations. For profile-determined Prp, profiles were measured along the calibration plane (100 cm SAD, 90 cm SSD, 10 cm depth) using a diode having a sensitive diameter of 0.8mm. The profiles were integrated numerically along both the length and width of both chamber types, with the deviation from unity giving the magnitude of Prp. The Farmer chamber was 23 mm in length, 3.05 mm in radius, and had a volume of 0.6cc. The smaller chamber was 6.5 mm in length, 2.75 mm in radius, and a volume of 0.125cc. For the output-determined Prp, beam outputs were measured for 6MV conventional flattening filter (cFF), 6FFF, 10cFF, and 10FFF beams using the TG51 protocol for six Farmer and six 0.125cc chambers. Use of six chambers was thought to improve statistics. The 0.125cc chamber was not reference grade, and required cross calibration with the Farmer chamber using cFF beams. To validate the lack of a gradient for the cFF beams, profiles of the cFF beams were measured as well, with a corresponding profile-determined Prp. A ratio of the FFF beam output between Farmer and 0.125cc chambers gives the output-determined Prp. Values of profile-determined Prp for the TrueBeam were 1.003 and 1.007 for 6FFF and 10FFF beams, respectively. Values of profile-determined Prp for the Versa were 1.005 and 1.007 for 6FFF and 10FFF beams, respectively. Surprisingly, the Versa 10cFF beam used was not “flat”, having a central axis divot, giving a Prp 0.995. This beam was not included in the cross-calibration of the 0.125cc chambers. Values of the output-determined Prp were within 0.1% of the profile-determined Prp for the TrueBeam, and within 0.4% for the Versa. Repeated Versa measurements gave similar results. One reference for the 0.125cc chamber used here indicates an anomalous dose-per-pulse issue for cFF beams, which may be exacerbated for FFF beams. Values of Prp from other linacs available to us and from the literature indicate good agreement (within 0.1%, one 0.2% outlier) for the TrueBeam and profile-determined Prp for the Versa. Profile- and output-determined Prp values for the TrueBeam are consistent. A Prp correction for the Versa 10cFF may be warranted, but appears linac or energy dependent. The profile-determined Prp appears to be more reproducible.

Simple index for validity of the evaluation point for dosimetric verification results of intensity-modulated radiation therapy using a Farmer-type ionization chamber.

Based on a retrospective analysis, this study aims to develop a simple index for validity of the evaluation point for the dosimetric verification of intensity-modulated radiation therapy (IMRT). The results for the dosimetric verifications of a total of 69 IMRT plans were analyzed in this study. A Farmer-type ion chamber was used as a dose detector, and a solid water-equivalent phantom was used. Index values were obtained by dividing the difference between the maximum and minimum dosages by the mean dosage of the 69 plans, and the values were classified into five groups with index value <4, 4-8, 8-12, 12-16, and >16. A t-test was used to assess the statistical significance of the mean differences of the absolute values of the relative errors among these groups. We found that there was no significant difference between the groups with index value <4 and 4-8 (p = 0.152); however, there were significant differences between the other groups (p < 0.01). In addition, when the index values were smaller than 8, the pass ratio of 3% tolerance was 96.2% and the pass ratio of 5% tolerance was 99.9%. We observed that the smaller the index value, the smaller the uncertainty of the dose measurement. The results obtained in this study may prove to be useful for accurate dosimetric verifications of IMRTs when ion chambers are used.

Comparison of kQ factors measured with a water calorimeter in flattening filter free (FFF) and conventional flattening filter (cFF) photon beams

Recently flattening filter free (FFF) beams became available for application in modern radiotherapy. There are several advantages of FFF beams over conventional flattening filtered (cFF) beams, however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. Beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger. Despite several studies describing FFF beam characteristics, only a limited number of studies investigated their effect on kQ factors. Some of those studies predicted significant discrepancies in kQ factors (0.4% up to 1.0%) if TPR20,10 based codes of practice (CoPs) were to be used. This study addresses the question to which extent kQ factors, based on a TPR20,10 CoP, can be applied in clinical reference dosimetry. It is the first study that compares kQ factors measured directly with an absorbed dose to water primary standard in FFF-cFF pairs of clinical photon beams. This was done with a transportable water calorimeter described elsewhere. The measurements corrected for recombination and beam radial non-uniformity were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers (eight serial numbers). The ratio of measured kQ factors of the FFF-cFF beam pairs were compared with the TPR20,10 CoPs of the NCS and IAEA and the %dd(10)x CoP of the AAPM. For the TPR20,10 based CoPs differences less than 0.23% were found in kQ factors between the corresponding FFF-cFF beams with standard uncertainties smaller than 0.35%, while for the %dd(10)x these differences were smaller than 0.46% and within the expanded uncertainty of the measurements. Based on the measurements made with the equipment described in this study the authors conclude that the kQ factors provided by the NCS-18 and IAEA TRS-398 codes of practice can be applied for flattening filter free beams without additional correction. However, existing codes of practice cannot be applied ignoring the significant volume averaging effect of the FFF beams over the ion chamber cavity. For this a corresponding volume averaging correction must be applied.

Dosimetric Analysis of Physical and Enhanced Dynamic Wedge and Its Implication in 3D-Conformal Radiotherapy Planning

Background: Across the history of radiotherapy, wedges are generally used to modify the shape of isodose lines to achieve desired clinical dose coverage to the target and to reduce the hot spot as well. Aim: The main aim of this study was to analyse and compare the dosimetric properties of Varian’s physical and enhanced dynamic wedges, and their dosimetric impact on radiotherapy plan (3D-Conformal). Materials and Methods: All plans were generated and evaluated in Varian’s eclipse planning system. For comparing the isodose line alteration, the plans were prepared in water phantom with field of size 10 cmx 10 cm. Wedges of angle 15°, 30°, 45° and 60° were used for generating isodose lines in the current study. All wedge factors were measured in water phantom using FC65 farmer type chamber. Discussion: In the current study, the dosimetric characteristics of EDW and PW were analyzed and compared. For analyzing the impact of EDW and PW, 22 patients were taken into consideration, and 50 Gy dose was prescribed to PTV in 25 fractions. Conclusion: The analysis of EDW and PW shows slightly different dosimetric features. EDW gives better target’s coverage, less hyper dose and comparatively less MUs as compare to PW. Hence the use EDW in 3D-conformal radiotherapy plan is a prudent practice.

Evaluation and comparison of dose distributions for nasopharyngeal carcinoma patients treated by Jaws-Only IMRT technique and by 3D-CRT technique at Dong Nai General Hospital

The goal of radiation therapy is twofold: maximize the possibility of destroy malignant cells while minimizing the damage to healthy tissue. The introduction of intensity modulated radiation therapy (IMRT) technique has brought improvements in this goal. Multi-leaf collimator (MLC) is a useful tool for IMRT. However, the use of MLC is not necessarily mandatory. The Panther Treatment Planning System version 4.6, Prowess Inc., enables the implementation of this technique for accelerator without MLC (the socalled Jaws-Only IMRT technique). This study aims to evaluate the results of application of Jaws-only IMRT technique for nasopharyngeal carcinoma patients at Dong Nai general hospital. Twenty five patients were randomly selected for this study. For each patient, two plans were generated: 3D-CRT (Three-Dimensional Radiation Treatment) and JO-IMRT. The dose distributions, dose-volume histograms (DVH), conformity indexes (COIN), homogeneity indexes (HI) were used to compare between these two plans and find out the best plan. Pretreatment verifications were performed for all patients' plans using ion chamber (Farmer Type Chamber FC65-P, IBA), detector array (MapCHECK2, Sun Nuclear Corporation and Octavius 4D 1500, PTW). The average deviation between measurement and calculation for point dose was 2.3±1.1 %, within limit dose constraint. For detector array measurements, the gamma index with 3 % dose difference and 3 mm was higher than 95 %. The results showed that the JO-IMRT technique had generated better dose distribution in the target volume and reduced dose to healthy tissues compared to 3D-CRT.

Technical Note: On the use of cylindrical ionization chambers for electron beam reference dosimetry.

To investigate the use of cylindrical chambers for electron beam dosimetry independent of energy by studying the variability of relative ion chamber perturbation corrections, one of the main concerns for electron beam dosimetry with cylindrical chambers. Measurements are made with sets of cylindrical and plane-parallel reference-class chambers as a function of depth in water in 8MeV and 18MeV electron beams. The ratio of chamber readings for similar chambers is normalized in a high-energy electron beam and can be thought of as relative perturbation corrections. Data are plotted as a function of mean electron energy at depth for a range of depths close to the phantom surface to R80 , the depth at which the ionization falls to 80% of its maximum value. Additional, similar measurements are made in a Virtual Water® phantom with cylindrical chambers at the reference depth in a 4MeV electron beam. The variability of relative ion chamber perturbation corrections for nominally identical cylindrical Farmer-type chambers is found to be less than 0.4%, no worse than plane-parallel chambers with similar specifications. This work discusses several issues related to the use of plane-parallel ion chambers and suggests that reference-class cylindrical chambers may be appropriate for reference dosimetry of all electron beams. This would simplify the reference dosimetry procedure and improve accuracy of beam calibration.

A novel 3D-printed phantom insert for 4D PET/CT imaging and simultaneous integrated boost radiotherapy.

To construct a 3D-printed phantom insert designed to mimic the variable PET tracer uptake seen in lung tumor volumes and a matching dosimetric insert to be used in simultaneous integrated boost (SIB) phantom studies, and to evaluate the design through end-to-end tests. A set of phantom inserts was designed and manufactured for a realistic representation of gated radiotherapy steps from 4D PET/CT scanning to dose delivery. A cylindrical phantom (φ80 × 120 mm) holds inserts for PET/CT scanning. The novel 3D printed insert dedicated to 4D PET/CT mimics high PET tracer uptake in the core and low uptake in the periphery. This insert is a variable density porous cylinder (φ44.5 × 70.0 mm), ABS-P430 thermoplastic, 3D printed by fused deposition modeling an inner (φ11 × 42 mm) cylindrical void. The square pores (1.8 × 1.8 mm2 each) fill 50% of outer volume, resulting in a 2:1 PET tracer concentration ratio in the void volume with respect to porous volume. A matching cylindrical phantom insert is dedicated to validate gated radiotherapy. It contains eight peripheral holes and one central hole, matching the location of the porous part and the void part of the 3D printed insert, respectively. These holes accommodate adaptors for Farmer-type ion chamber and cells vials. End-to-end tests were designed for imaging, planning, and dose measurements. End-to-end test were performed from 4D PET/CT scanning to transferring data to the planning system, target volume delineation, and dose measurements. 4D PET/CT scans were acquired of the phantom at different respiratory motion patterns and gating windows. A measured 2:1 18F-FDG concentration ratio between inner void and outer porous volume matched the 3D printed design. Measured dose in the dosimetric insert agreed well with planned dose on the imaging insert, within 3% for the static phantom and within 5% for most breathing patterns. The novel 3D printed phantom insert mimics variable PET tracer uptake typical of tumors. Obtained 4D PET/CT scans are suitable for segmentation and treatment planning and delivery in SIB gated treatments. Our experiments demonstrate the feasibility of this set of phantom inserts serving as end-to-end quality-assurance phantoms of SIB radiotherapy.

Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, , and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T.A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.

The impact of water temperature on absolute dose calibration of photon beams

The Task Group (TG) 51 protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre-defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. The temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall, and the density of water. In this work the overall effect of water temperature on absolute dosimetry has been investigated. Dose measurements were made using a Farmer-type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 °C to 40 °C. A reference chamber was used to account for fluctuations in beam output between successive measurements. For the same beam output, the dose at the depth of calibration, determined using TG 51, was found to be dependent on the temperature of the water in the tank. The dose for both photon energies studied followed a similar trend reaching a maximum around 26 °C. For the 6 MV beam, the dose was found to increase 0.7% from 10 °C to 26 °C and then decrease 0.18% from 26 °C to 40 °C. Similarly, for the 23 MV beam the dose increased 0.95% from 10 °C to 26 °C and then decreased 1.25% from 26 °C to 40 °C. There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.

Ion recombination and polarity corrections for small-volume ionization chambers in high-dose-rate, flattening-filter-free pulsed photon beams.

To investigate ion recombination and polarity effects in scanning and microionization chambers when used with digital electrometers and high-dose-rate linac beams such as flattening-filter-free (FFF) fields, and to compare results against conventional pulsed and continuous photon beams. Saturation curves were obtained for one Farmer-type ionization chamber and eight small-volume chamber models with volumes ranging from 0.01 to 0.13 cm3 using a Varian TrueBeam™ STx with FFF capability. Three beam modes (6 MV, 6 MV FFF, and 10 MV FFF) were investigated, with nominal dose-per-pulse values of 0.0278, 0.0648, and 0.111 cGy/pulse, respectively, at dmax . Saturation curves obtained using the Theratronics T1000 60 Co unit at the UWADCL and a conventional linear accelerator (Varian Clinac iX) were used to establish baseline behavior. Jaffé plots were fitted to obtain Pion , accounting for exponential effects such as charge multiplication. These values were compared with the two-voltage technique recommended in TG-51, and were plotted as a function of dose-per-pulse to assess the ability of small-volume chambers to meet reference-class criteria in FFF beams. Jaffé- and two-voltage-determined Pion values measured for high-dose-rate beams agreed within 0.1% for the Farmer-type chamber and 1% for scanning and microionization chambers, with the exception of the CC01 which agreed within 2%. With respect to ion recombination and polarity effects, the Farmer-type chamber, scanning chambers and the Exradin A26 microchamber exhibited reference-class behavior in all beams investigated, with the exception of the IBA CC04 scanning chamber, which had an initial recombination correction that varied by 0.2% with polarity. All microchambers investigated, with the exception of the A26, exhibited anomalous polarity and ion recombination behaviors that make them unsuitable for reference dosimetry in conventional and high-dose-rate photon beams. The results of this work demonstrate that recombination and polarity behaviors seen in conventional pulsed and continuous photon beams trend accordingly in high-dose-rate FFF linac beams. Several models of small-volume ionization chambers used with a digital electrometer have been shown to meet reference-class requirements with respect to ion recombination and polarity, even in the high-dose-rate environment. For such chambers, a two-voltage technique agreed well with more rigorous methods of determining Pion . However, the results emphasize the need for careful reference detector selection, and indicate that ionization chambers ought to be extensively tested in each beam of interest prior to their use for reference dosimetry.

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.