Dosimetry Of High-energy Photon Beams Research Articles

Comparison of derived correction factors for effects of ion recombination and photon beam quality index following TG-51 and TRS-398 dosimetry protocols

In this study, ion recombination correction factor (kS) and beam quality conversion factor (kQ,Q0) values were extracted following the recommendations of the TRS-398 and TG-51 dosimetry protocols for widely used cylindrical ionization chambers for high energy photon beam dosimetry to quantify the agreement between the instructions for these two protocols for absolute dosimetry inside water. Four different types of cylindrical ionization chambers comprising Farmer (TM30013), Semiflex 0.125 cm3 (TM31010), Semiflex 0.3 cm3 (TM31013), and PinPoint (TM31016) were considered, and kS and kQ,Q0 values were determined at photon energies of 6 MV and 15 MV. The maximum difference between the measured kS values according to the instructions in the TRS-398 and TG-51 protocols was 0.03%. The kS data measured with both protocols agreed well with those measured by using the Jaffe-plot approach, where the maximum difference was about 0.33%. The observed differences between the kQ,Q0 factors measured by using the TRS-398 and TG-51 dosimetry protocols at photon energies of 6 MV and 15 MV were 0.37% and 0.55%, respectively. The kQ,Q0 values measured using the TG-51 dosimetry protocols were slightly closer to those measured by a reference ionization chamber dosimeter. We conclude that the maximum differences were about 0.4% and 0.6% in the absorbed dose measurements according to the TRS-398 and TG-51 instructions at photon energies of 6 MV and 15 MV, respectively. The type of ionization chamber employed also affected the differences, where the maximum and minimum dose differences were found using the Farmer and PinPoint chambers, respectively.

Effect of ICRU report 90 recommendations on Monte Carlo calculated kQ for ionization chambers listed in the Addendum to AAPM’s TG‐51 protocol

The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (sw, air and sg, air ) and the individual perturbation correction factors Pi was calculated. The effect on the beam quality conversion factors kQ for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol. The sw, air , sg, air , individual Pi, and kQ were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the Pi and kQ were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one 60 Co beam and ten photon beams with nominal energy between 4MV and 25MV. Then, the change in kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of 60 Co beam and 24MV photon beam were used. The significant changes (p<0.05) were observed for sw, air , sg, air , the wall correction factor Pwall , and the waterproofing sleeve correction factor Psleeve . The decrease in sw, air varied from -0.57% for a 60 Co beam to -0.36% for the highest beam quality. The decrease in sg, air varied from -0.72% to -1.12% in the same range. The changes in Pwall and Psleeve were up to 0.41% and 0.14% and those maximum changes were observed for the 60 Co beam. All changes in the central electrode correction factor Pcel , the stem correction factor Pstem , and the replacement correction factor Prepl were from -0.02% to 0.12%. Those changes were statistically insignificant (p=0.07 or more) and were independent of photon energy. The change in kQ was mainly characterized by the change in sw, air , Pwall , and Psleeve . The relationship between the change in kQ and the beam quality index was linear approximately. The changes in kQ of the simple-models were agreed with those of the precise-models within 0.08%. The effects of ICRU-90 recommendations on kQ for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the kQ based on ICRU-37 should be updated to the kQ based on ICRU-90.

Effective Point of Measurement (EPOM) of Some Ionization Chambers for High Energy Photon Beam Dosimetry used in Radiotherapy for the Treatment of Cancer Patient

Cancer is a serious health issue and be a principle determinant of the human and economic wealth in a country.

On the impact of ICRU report 90 recommendations on kQ factors for high-energy photon beams.

To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, kQ , used for clinical reference dosimetry of megavoltage linac photon beams. The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth-dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt-60 beam and MV photon beams with nominal energy between 6MV and 25MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping-power data for graphite and water from the original ICRU-37 report and the new proposed values from the recently published ICRU-90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors. Less than about 0.1% effects from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10cm, %dd(10)x , and the tissue-phantom ratio at 20cm and 10cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water-to-air stopping-power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5% in high-energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU-90 recommendations. Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.

Uncertainty in positioning ion chamber at reference depth for various water phantoms.

Uncertainty in the calibration of high-energy radiation sources is dependent on user and equipment type. We evaluated the uncertainty in the positioning of a cylindrical chamber at a reference depth for reference dosimetry of high-energy photon beams and the resulting uncertainty in the chamber readings for 6- and 10-MV photon beams. The aim was to investigate major contributions to the positioning uncertainty to reduce the uncertainty in calibration for external photon beam radiotherapy. The following phantoms were used: DoseView 1D, WP1D, 1D SCANNER, and QWP-07 as one-dimensional (1D) phantoms for a vertical-beam geometry; GRI-7632 as a phantom for a fixed waterproofing sleeve; and PTW type 41023 and QWP-04 as 1D phantoms for a horizontal-beam geometry. The uncertainties were analyzed as per the Guide to the Expression of Uncertainty in Measurement. The positioning and resultant uncertainties in chamber readings ranged from 0.22 to 0.35mm and 0.12-0.25%, respectively, among the phantoms (using a coverage factor k=1 in both cases). The major contributions to positioning uncertainty are: definition of the origin for phantoms among users for the 1D phantoms for a vertical-beam geometry, water level adjustment among users for the phantom for a fixed waterproofing sleeve, phantom window deformation, and non-water material of the window for the 1D phantoms for a horizontal-beam geometry. The positioning and resultant uncertainties in chamber readings exhibited minor differences among the seven phantoms. The major components of these uncertainties differed among the phantom types investigated.

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams.

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

L’activité du système limbique corrèle avec l’indice de masse corporelle à l’éveil et pendant le sommeil lent

The dosimetry of high-energy photon beams in the treatment of superficial lesions occurring in extremities was examined. Large parallel-opposed fields with different photon beam energies were used. The extremity was immersed in water contained in a commercially available plastic wastebasket. The water bolus serves to even out the surface irregularities of the extremities and to remove the skin sparing effect. A polystyrene block was placed at the floor of the wastebasket to ensure that the extremity was encompassed in the radiation fields. The photon beam energies considered were 4 MV, 6 MV, 10 MV, and 24 MV. The results show that the dose distributions are more homogeneous with higher photon beam energies. The isodose lines are more constricted at mid-plane for low energy photon beams. Higher energy photon beams, 10 MV and up would be preferable for the treatment of superficial lesions of the extremities immersed in water bolus contained in a typical wastebasket size.

Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams

To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations. Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k(Q) factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k(Q) factors. Systematic uncertainties in Monte Carlo calculated k(Q) factors are investigated by varying material properties and chamber dimensions. Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although k(Q) factors were consistent on average within 0.17%. Chamber-to-chamber variations in k(Q) factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated k(Q) factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated k(Q) factors were - 0.02%, 0.18%, and - 0.16% for 6, 10, and 25 MV beams, respectively. Excellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated k(Q) factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.

Calibration of helical tomotherapy machine using EPR/alanine dosimetry

Current codes of practice for clinical reference dosimetry of high-energy photon beams in conventional radiotherapy recommend using a 10 x 10 cm2 square field, with the detector at a reference depth of 10 cm in water and 100 cm source to surface distance (SSD) (AAPM TG-51) or 100 cm source-to-axis distance (SAD) (IAEA TRS-398). However, the maximum field size of a helical tomotherapy (HT) machine is 40 x 5 cm2 defined at 85 cm SAD. These nonstandard conditions prevent a direct implementation of these protocols. The purpose of this study is twofold: To check the absorbed dose in water and dose rate calibration of a tomotherapy unit as well as the accuracy of the tomotherapy treatment planning system (TPS) calculations for a specific test case. Both topics are based on the use of electron paramagnetic resonance (EPR) using alanine as transfer dosimeter between the Laboratoire National Henri Becquerel (LNHB) 60Co-gamma-ray reference beam and the Institut Curie's HT beam. Irradiations performed in the LNHB reference 60Co-gamma-ray beam allowed setting up the calibration method, which was then implemented and tested at the LNHB 6 MV linac x-ray beam, resulting in a deviation of 1.6% (at a 1% standard uncertainty) relative to the reference value determined with the standard IAEA TRS-398 protocol. HT beam dose rate estimation shows a difference of 2% with the value stated by the manufacturer at a 2% standard uncertainty. A 4% deviation between measured dose and the calculation from the tomotherapy TPS was found. The latter was originated by an inadequate representation of the phantom CT-scan values and, consequently, mass densities within the phantom. This difference has been explained by the mass density values given by the CT-scan and used by the TPS which were not the true ones. Once corrected using Monte Carlo N-Particle simulations to validate the accuracy of this process, the difference between corrected TPS calculations and alanine measured dose values was then found to be around 2% (with 2% standard uncertainty on TPS doses and 1.5% standard uncertainty on EPR measurements). Beam dose rate estimation results were found to be in good agreement with the reference value given by the manufacturer at 2% standard uncertainty. Moreover, the dose determination method was set up with a deviation around 2% (at a 2% standard uncertainty).

Calorimetric determination of kQ factors for NE 2561 and NE 2571 ionization chambers in 5 cm × 5 cm and 10 cm × 10 cm radiotherapy beams of 8 MV and 16 MV photons

The relative uncertainty of the ionometric determination of the absorbed dose to water, Dw, in the reference dosimetry of high-energy photon beams is in the order of 1.5% and is dominated by the uncertainty of the calculated chamber- and energy-dependent correction factors kQ. In the present investigation, kQ values were determined experimentally in 5 cm × 5 cm and 10 cm × 10 cm radiotherapy beams of 8 MV and 16 MV bremsstrahlung by means of a water calorimeter operated at 4 °C. Ionization chambers of the types NE 2561 and NE 2571 were calibrated directly in the water phantom of the calorimeter. The measurements were carried out at the linear accelerator of the Physikalisch-Technische Bundesanstalt. It is shown that the kQ factor of a single ionization chamber can be measured with a standard uncertainty of less than 0.3%. No significant variations of kQ were found for the different lateral sizes of the radiation fields used in this investigation.

ICRU Report 64. Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water J. ICRU vol. 1, 1 (2001 ).

ICRU Report 64. Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water J. ICRU vol. 1, 1 (2001 ).

Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water

Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water

Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water

Mail Stop K3-53 Pacific Northwest National Laboratory PO Box 999 Richland, WA 99352 [email protected]

Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water: Abstract

AbstractThis report examines the methods by which absorbed dose to water can be determined for photon radiations with maximum energies from approximately 1 MeV to 50 MeV, the beam qualities most co...

Dosimetry of High-Energy Photon Beams Based on Standards of Absorbed Dose to Water: Abstract

This report examines the methods by which absorbed dose to water can be determined for photon radiations with maximum energies from approximately 1 MeV to 50 MeV, the beam qualities most commonly used for radiation therapy. The report is primarily concerned with methods of measurement for photon radiation, but many aspects are also relevant to the dosimetry of other therapeutic beams (high-energy electrons, protons, etc.). It deals with methods that are sufficiently precise and well established to be incorporated into the dosimetric measurement chain as primary standards (i.e., methods based on ionisation, radiation-induced chemical changes, and calorimetry using either graphite or water). The report discusses the primary dose standards used in several national standards laboratories and reviews the international comparisons that have been made. The report also describes the reference conditions that are suitable for establishing primary standards and provides a formalism for determining absorbed dose, including a discussion of correction factors needed under conditions other than those used to calibrate an instrument at the standards laboratory.

Dosimetric considerations of water-based bolus for irradiation of extremities

The dosimetry of high-energy photon beams in the treatment of superficial lesions occurring in extremities was examined. Large parallel-opposed fields with different photon beam energies were used. The extremity was immersed in water contained in a commercially available plastic wastebasket. The water bolus serves to even out the surface irregularities of the extremities and to remove the skin sparing effect. A polystyrene block was placed at the floor of the wastebasket to ensure that the extremity was encompassed in the radiation fields. The photon beam energies considered were 4 MV, 6 MV, 10 MV, and 24 MV. The results show that the dose distributions are more homogeneous with higher photon beam energies. The isodose lines are more constricted at mid-plane for low energy photon beams. Higher energy photon beams, 10 MV and up would be preferable for the treatment of superficial lesions of the extremities immersed in water bolus contained in a typical wastebasket size.

A Belarusian protocol for the dosimetry of high-energy photon beams

A Belarusian protocol for the dosimetry of high-energy photon beams

Absorbed dose beam quality factors for the dosimetry of high-energy photon beams

The parallelism before existing air kerma formalisms based on the 'absorbed dose to air' chamber factor, ND (or Ngas), and the more simple approach based on calibrations in terms of absorbed dose to water, Nw, is described. The importance of avoiding steps performed by the user that introduce avoidable uncertainties in the dosimetric procedure, is emphasized. Radiation beam quality factors normalized to 60 Co gamma -rays have been calculated from a comparison between the two formalisms, and sets of tables produced for the different ionization chambers included in the 1987 IAEA Code of Practice. The calculated set of data is compared with existing experimental determinations at Primary Standard Dosimetry Laboratories, showing agreement within estimated uncertainties.

Experimental verification of the air kerma to absorbed doseconversion factor Cw,u

In a recently published code of practice for the dosimetry of high-energy photon beams, the absorbed dose to water is determined using an ionization chamber having an air kerma calibration factor and applying the air kerma to absorbed dose conversion factor Cw,u. The consistency of these Cw,u values has been determined for four commonly employed types of ionization chambers in photon beams with quality varying between 60Co gamma-rays and 25 MV X-rays. Using a graphite calorimeter, Cw,u has been determined for a graphite-walled ionization chamber (NE 2561) for the same qualities. The values of Cw,u determined with the calorimeter are within the experimental uncertainty equal to Cw,u values determined according to any of the recent dosimetry protocols.

Stopping power data for high-energy photon beams

Spencer-Attix stopping power ratios for the dosimetry of high-energy photon beams used in radiation therapy have been calculated using the Monte Carlo method. The stopping power ratios are calculated in a more consistent way than previously and are given as a function of the attenuation properties of the beam. The dependence of the stopping power ratio on the electron contamination of the beam as well as on depth and field size has also been investigated. Results are compared with stopping power ratios recommended in different dosimetry protocols and to experimental results. The agreement with most dosimetry protocols is within about one per cent and with recent experimental data is better than half a per cent.