Fluence Perturbation Correction Factor Research Articles

Ion chamber and film-based quality assurance of mixed electron-photon radiation therapy.

In previous work, we demonstrated that mixed electron-photon radiation therapy (MBRT) produces treatment plans with improved normal tissue sparing and similar target coverage, when compared to photon-only plans. The purpose of this work was to validate the MBRT delivery process on a Varian TrueBeam accelerator and laying the groundwork for a patient-specific quality assurance (QA) protocol based on ion chamber point measurements and 2D film measurements. MC beam models used to calculate the MBRT dose distributions of each modality (photons/electrons) were validated with a single-angle beam MBRT treatment plan delivered on a slab of Solid Water phantom with a film positioned at a depth of 2cm. The measured film absorbed dose was compared to the calculated dose. To validate clinical deliveries, a polymethyl methacrylate (PMMA) cylinder was machined and holes were made to fit an ionization chamber. A complex MBRT plan involving a photon arc and three electron delivery angles was created with the aim of reproducing a clinically realistic dose distribution in typical soft tissue sarcoma tumours of the extremities. The treatment plan was delivered on the PMMA cylinder. Point measurements were taken with an Exradin A1SL chamber at two nominal depths: 1.4cm and 2.1cm. The plan was also delivered on a second identical phantom with an insert at 2cm depth, where a film was placed. An existing EGSnrc user-code, SPRRZnrc, was modified to calculate the stopping power ratios between any materials in the same voxelized geometry used for dose calculation purposes. This modified code, called SPRXYZnrc, was used to calculate a correction factor, , accounting for the differences in electron fluence spectrum at the measurement point compared to that at reference conditions. The uncertainty associated with neglecting potential ionization chamber fluence perturbation correction factors using this approach was estimated. The film measurement from the Solid Water phantom treatment plan was in good agreement with the simulated dose distribution, with a gamma pass rate of 96.1% for a 3%/2mm criteria. For the PMMA phantom delivery, for the same gamma criteria, the pass rate was 97.3%. The ion chamber measurements of the total delivered dose agreed with the MC-simulated dose within 2.1%. The beam quality correction factors amounted to, at most, a 4% correction on the ion chamber measurement. However, individual contribution of low electron energies proved difficult to precisely measure due to their steep dose gradients, with disagreements of up to 28%±15% at 2.1cm depth (6MeV). Ion chamber measurement procedure of electron beams was achieved in less than 5min, and the entire validation process including phantom setup was performed in less than 30min. The agreement between measured and simulated MBRT doses indicates that the dose distributions obtained from the MBRT treatment planning algorithm are realistically achievable. The SPRXYZnrc MC code allowed for convenient calculations of simultaneously with the dose distributions, laying the groundwork for patient-specific QA protocol practical for clinical use. Further investigation is needed to establish the accuracy of our ionization chamber correction factors calculations at low electron energies.

SU‐F‐T‐73: Experimental Determination of the Effective Point of Measurement in Electron Beams Using a Commercial Scintillation Detector

Purpose:The aim of this work was to determine experimentally the effective point of measurement (EPOM) in clinical electron beams for three cylindrical ionization chambers using a commercial scintillation detector as a reference detector.Methods:Percent depth dose (PDD) curves were measured using an Exradin W1 scintillation detector and were used as a representative PDD to water. Depth dose curves were measured with the Exradin A18, A1SL, and A28 ionization chambers. The raw ionization chamber curve data were corrected by the chamber fluence perturbation correction factor and restricted mass collisional stopping power ratio at each depth to obtain a percent depth dose curve to the gas volume (PDDGV) of the detector. Ratios of the W1 PDD to the ion chamber PDDGV were calculated for each measurement depth. The W1 PDD curve was shifted by small depth increments, Δz, until the ratio of the W1 PDD to the ion chamber PDDGV was depth‐independent (optimal Δz). A MATLAB routine was developed to determine the optimal Δz value.Results:The optimal Δz shift was used as an estimate of the EPOM for each chamber. The average calculated EPOM shifts (expressed as a fraction of the chamber cavity radius) for the A18, A1SL, and A28 ionization chambers were 0.21 ± 0.04, 0.10 ± 0.05, and 0.22 ± 0.03, respectively.Conclusion:The experimentally determined EPOM values for the A18 and A1SL in this work agreed with the simulated values of Muir and Rogers (MedPhys 2014). The results also indicate that the Exradin W1 scintillator is water equivalent for electron energies of 6 MeV, 9 MeV, 12 MeV, and 16 MeV. In addition, we confirmed that the AAPM TG51 recommended EPOM shift of 0.5 times the cavity radius is not accurate for the A18 and A1SL chambers.

SU-E-T-30: Monte Carlo Calculated Detector Corrections K(Q, F-Clin) for Determination of Output Factors for the Leksell Gamma Knife

Purpose: Conventional determination of output factors for photon beams relies on the assumption that the product of stopping-power ratio and fluence perturbation correction factors is constant; thus output factors are determined as ratios of detector readings, rather than by ratios of absorbed doses. For small fields this assumption may be wrong, which combined with possible volume averaging effects, may introduce dosimetry errors for detector sizes comparable to the dose profile FWHM of the beam to be calibrated. The aim of the work was to determine corrections to measured detector readings, in order to derive dose-based output factors for detectors used in the Co-60 beams of the Leksell Gamma Knife® (LGK) models 4C and Perfexion. Methods: The k(Q, f-clin) correction factor has been defined as the quotient of two MC calculated absorbed dose ratios, water to detector, for a clinical field relative to the machine-specific-reference field. Absorbed doses were calculated with the MC system Penelope for spherical phantoms, where the inserted detectors of various types were simulated in great detail. Correction factors were determined for all field sizes of the LGK. Results: Detectors simulated were alanine, TLD, a PTW diode and liquid chamber. Calculated corrections for 14 mm and 8 mm circular fields were smaller than 1% for all detectors. For 4 mm fields the resulting corrections were 0.96 for the diode, 0.98 for the TLD, less than 1.01 for the liquid ion chamber, and 1.16 for alanine. Conclusions: Most of the detectors investigated are generally well suited for output factor measurements on the Leksell GK fields provided k(Q, f-clin) correction factors are available. The exception is alanine for the smallest field size, as its relative volume becomes too large. Additional results for other detector types are in progress. One of the authors (J. Johansson) is employed by Elekta, the manufacturers of Leksell Gamma Knife.

Comparing calibration methods of electron beams using plane-parallel chambers with absorbed-dose to water based protocols.

Recent absorbed-dose-based protocols allow for two methods of calibrating electron beams using plane-parallel chambers, one using the N(Co)D,w for a plane-parallel chamber, and the other relying on cross-calibration of the plane-parallel chamber in a high-energy electron beam against a cylindrical chamber which has an N(Co)D,w factor. The second method is recommended as it avoids problems associated with the Pwall correction factors at 60Co for plane-parallel chambers which are used in the determination of the beam quality conversion factors. In this article we investigate the consistency of these two methods for the PTW Roos, Scanditronics NACP02, and PTW Markus chambers. We processed our data using both the AAPM TG-51 and the IAEA TRS-398 protocols. Wall correction factors in 60Co beams and absorbed-dose beam quality conversion factors for 20 MeV electrons were derived for these chambers by cross-calibration against a cylindrical ionization chamber. Systematic differences of up to 1.6% were found between our values of Pwall and those from the Monte Carlo calculations underlying AAPM TG-51, and up to 0.6% when comparing with the IAEA TRS-398 protocol. The differences in Pwall translate directly into differences in the beam quality conversion factors in the respective protocols. The relatively large spread in the experimental data of Pwall, and consequently the absorbed-dose beam quality conversion factor, confirms the importance of the cross-calibration technique when using plane-parallel chambers for calibrating clinical electron beams. We confirmed that for well-guarded plane-parallel chambers, the fluence perturbation correction factor at d(max) is not significantly different from the value at d(ref). For the PTW Markus chamber the variation in the latter factor is consistent with published fits relating it to average energy at depth.

Electron fluence correction factors for various materials in clinical electron beams.

Relative to solid water, electron fluence correction factors at the depth of dose maximum in bone, lung, aluminum, and copper for nominal electron beam energies of 9 MeV and 15 MeV of the Clinac 18 accelerator have been determined experimentally and by Monte Carlo calculation. Thermoluminescent dosimeters were used to measure depth doses in these materials. The measured relative dose at dmax in the various materials versus that of solid water, when irradiated with the same number of monitor units, has been used to calculate the ratio of electron fluence for the various materials to that of solid water. The beams of the Clinac 18 were fully characterized using the EGS4/BEAM system. EGSnrc with the relativistic spin option turned on was used to optimize the primary electron energy at the exit window, and to calculate depth doses in the five phantom materials using the optimized phase-space data. Normalizing all depth doses to the dose maximum in solid water stopping power ratio corrected, measured depth doses and calculated depth doses differ by less than +/- 1% at the depth of dose maximum and by less than 4% elsewhere. Monte Carlo calculated ratios of doses in each material to dose in LiF were used to convert the TLD measurements at the dose maximum into dose at the center of the TLD in the phantom material. Fluence perturbation correction factors for a LiF TLD at the depth of dose maximum deduced from these calculations amount to less than 1% for 0.15 mm thick TLDs in low Z materials and are between 1% and 3% for TLDs in Al and Cu phantoms. Electron fluence ratios of the studied materials relative to solid water vary between 0.83+/-0.01 and 1.55+/-0.02 for materials varying in density from 0.27 g/cm3 (lung) to 8.96 g/cm3 (Cu). The difference in electron fluence ratios derived from measurements and calculations ranges from -1.6% to +0.2% at 9 MeV and from -1.9% to +0.2% at 15 MeV and is not significant at the 1sigma level. Excluding the data for Cu, electron fluence correction factors for open electron beams are approximately proportional to the electron density of the phantom material and only weakly dependent on electron beam energy.

Comparative dosimetry of diode and diamond detectors in electron beams for intraoperative radiation therapy

The aim of the present study is to examine the validity of using silicon semiconductor detectors in degraded electron beams with a broad energy spectrum and a wide angular distribution. A comparison is made with diamond detector measurements, which is the dosimeter considered to give the best results provided that dose rate effects are corrected for. Two-dimensional relative absorbed dose distributions in electron beams (6-20 MeV) for intraoperative radiation therapy (IORT) are measured in a water phantom. To quantify deviations between the detectors, a dose comparison tool that simultaneously examines the dose difference and distance to agreement (DTA) is used to evaluate the results in low- and high-dose gradient regions, respectively. Uncertainties of the experimental measurement setup (+/- 1% and +/- 0.5 mm) are taken into account by calculating a composite distribution that fails this dose-difference and DTA acceptance limit. Thus, the resulting area of disagreement should be related to differences in detector performance. The dose distributions obtained with the diode are generally in very good agreement with diamond detector measurements. The buildup region and the dose falloff region show good agreement with increasing electron energy, while the region outside the radiation field close to the water surface shows an increased difference with energy. The small discrepancies in the composite distributions are due to several factors: (a) variation of the silicon-to-water collision stopping-power ratio with electron energy, (b) a more pronounced directional dependence for diodes than for diamonds, and (c) variation of the electron fluence perturbation correction factor with depth. For all investigated treatment cones and energies, the deviation is within dose-difference and DTA acceptance criteria of +/- 3% and +/- 1 mm, respectively. Therefore, p-type silicon diodes are well suited, in the sense that they give results in close agreement with diamond detectors, for practical measurements of relative absorbed dose distributions in degraded electron beams used for IORT.

Electron fluence perturbation correction factors for solid state detectors irradiated in megavoltage electron beams

The perturbation correction factor (p) is defined as the deviation of the absorbed dose in the medium from that predicted by the Spencer-Attix extension of the Bragg-Gray cavity theory where the medium occupies exactly the same volume as the solid state cavity and the electron fluence energy spectrum in the cavity is identical in shape, but not necessarily in magnitude, to that in the medium. The value of (p) has been examined for TL detectors irradiated in megavoltage electron beams (5-20 MeV) using the EGS4 Monte Carlo code. LiF and CaF2 solid state detectors simulated were standard size discs of thickness 1 mm and diameter 3.61 mm irradiated in a water phantom with their centres at dmax, or close to it. Values of (p) for LiF ranged from 0.998±0.005 to 0.994±0.005 for electron beams with initial energies of 5 and 20 MeV respectively. For CaF2 the corresponding values were 0.956±0.006 to 0.989±0.006 for the same size cavities irradiated at the same depth. EGS4 Monte Carlo simulations demonstrate that the total electron fluence (primary electrons and -rays) in these solid state detector materials is significantly different from that in water for the same incident electron energy and depth of irradiation. Thus the Spencer-Attix assumption that the electron fluence energy spectrum in the cavity is identical in shape to that in the medium is violated. Differences in the total electron fluence give rise to electron fluence perturbation correction factors which were up to 5% less than unity for CaF2, indicating a strong violation in this case, but were generally less than 1% for LiF. It is the density of the cavity which perturbs the electron fluence, but it is actually the atomic number differences between the medium and cavity that are responsible for the large electron fluence perturbation correction factors for detectors irradiated close to dmax because the atomic number affects the change in stopping power with energy. When correction is made for the difference between the electron fluence spectrum in the uniform water phantom and the solid state cavity, the Spencer-Attix cavity equation predicts the dose to water within 0.3% in both clinical and monoenergetic electron beams. Harder's formulation for computing the average mass collision stopping power of water to calcium fluoride, surprisingly, requires perturbation correction factors that are closer to unity than those determined using the Spencer-Attix integrals at depths close to dmax.

Photon fluence perturbation correction factors for solid state detectors irradiated in kilovoltage photon beams

Dose perturbation correction factors, (p), for LiF, CaF2 and Li2B4O7 solid state detectors have been determined using the EGS4 Monte Carlo code. Each detector was simulated in the form of a disc of diameter 3.61 mm and thickness 1 mm irradiated in a clinical kilovoltage photon beam at a depth of 1 cm in a water phantom. The perturbation correction factor (p) is defined as the deviation of the absorbed dose ratio from the average mass energy absorption coefficient ratio of water to the detector material, (en/)med,det, which is evaluated assuming that the photon fluence spectrum in the medium and in the detector material are identical. We define another mass energy absorption coefficient ratio, (en/)med,det, which is evaluated using the actual photon fluence spectrum in the medium and detector for LiF and CaF2 rather than assuming they are identical. (en/)med,det predicts the average absorbed dose ratio of the medium to the detector material within 0.3%. When the difference in atomic number between the cavity and the phantom material is large then their photon fluence spectra will differ substantially resulting in a difference between (en/)med,det and (en/)med,det. The value of (p) calculated using (en/)med,det is up to 27% greater than unity for a cavity of CaF2 in 50 kV x-rays. When the atomic number of the medium and detector are similar, their photon fluence spectra are similar, and the difference between (en/)med,det and (en/)med,det is small. For instance their difference for LiF is less than 2%. The average mass energy absorption coefficient ratio, (en()/)w,LiF, evaluated using the mean or representative energy, , is up to 8% different from (en/)w,LiF. For calcium fluoride the difference between (en/)w,CaF2 and (en()/)w,CaF2 is up to 42% in the energy range studied.

Verification of absorbed doses determined with thimble and parallel-plate ionization chambers in clinical electron beams using ferrous sulphate dosimetry.

Absorbed dose values determined with the commonly applied NACP and PTW/Markus parallel-plate chambers and the cylindrical NE2571 Farmer chamber were compared to values obtained with ferrous sulphate dosimetry in a number of electron beams. For the ionometry with the parallel-plate chambers the dose-to-air chamber factor ND (or Ngas) was derived from a 60Co beam calibration free in air with an additional buildup layer of 0.54 g cm-2 graphite as proposed by the protocol for electron dosimetry published by the Netherlands Commission on Radiation Dosimetry. For the product kattkm in this calibration geometry values of 0.980 +/- 0.003 [1 standard deviation (s.d.)] and 0.993 +/- 0.004 (1 s.d.) were obtained for the parallel-plate NACP and PTW/Markus chambers, respectively. The behavior of the fluence perturbation correction factor pf versus the mean electron energy at depth was deduced for the flat PTW/Markus and cylindrical NE2571 chamber by comparison with the NACP chamber, for which pf was assumed unity. Our results show a small but significant energy dependence of pf for the PTW/Markus chamber. The absorbed dose values, determined ionometrically with the different chambers considered in the study using the experimentally determined kattkm and pf values, are systematically 0.5% higher than those obtained with ferrous sulphate dosimetry adopting 352 x 10(-6) m-2 kg-1 Gy-1 for epsilon mG. The performed comparative study confirms also that for the NACP chamber pf is unity independent of the electron energy down to a mean energy at depth of 2 MeV.

Perturbation correction factors for some ionization chambers commonly applied in electron beams

Values for the fluence perturbation correction factor were determined in a number of electron beams for the commonly applied PTW/Markus plane-parallel chamber and the cylindrical NE 2571 Farmer chamber. These data were determined relative to the NACP plane-parallel chamber as a function of the mean energy at depth, Ez, calculated according to the method recommended in modern dosimetry protocols. For the cylindrical chamber the results are in agreement with the data recommended in these protocols. For the PTW/Markus chamber a small but significant fluence perturbation correction is necessary: up to about 3% for a value of Ez=2 MeV. In order to check the value of these fluence perturbation correction values, dose measurements were performed ionometrically and compared with values determined with the Fricke dosimetry system.

Total perturbation correction factor for PTW and NACP plane parallel chambers in electron beams

The international protocols of electron dosimetry published by the Nordic Association of Clinical Physics and the Sociedad Española de Fisica Médica recommend the use of a total perturbation correction factor p u, pp which is constant for all plane parallel chambers. In this work we have compared the dose measured with the PTW-Markus and NACP plane parallel chambers in respect to a cylindrical one. The obtained results indicate that for the ionization chamber NACP the value of p u, pp = 1.000 ± 0.005 is adequate while for the ionization chamber PTW the observed difference is explained by considering an individual total perturbation correction factor variable with the mean energy at depth z, E z , as the fluence perturbation correction factor p f is usually given for cylindrical chambers.

Ngas determination for a parallel-plate ion chamber.

A parallel-plate ion chamber has been investigated with a view to determining Ngas, the cavity-gas calibration factor, introduced by the Task Group 21 protocol for dose determination. In-air and in-phantom irradiation of a Memorial Hospital parallel-plate ion chamber in a 60Co beam led to the determination of ion collecting volume and a calibration factor for the chamber. When these quantities were used to deduce Ngas factors for the chamber, an agreement within 1% was found. An average of the cavity-gas factors obtained from 60Co data agreed to within 0.5% with the factor determined through force fitting the doses received by the parallel-plate chamber and a cylindrical chamber in a 22-MeV (nominal) electron beam. Additionally, the parallel-plate chamber was utilized to study the effectiveness, in electron beam dosimetry, of the electron fluence perturbation correction factors introduced when the phantom material is replaced by a cylindrical ion chamber.