Ion Chamber Readings Research Articles

SU‐FF‐T‐183: Dosimetric Verification of a Commercial Research Irradiator

Purpose: To present a collection of simple tests that can be used to verify the dosimetric characteristics of the MDS Nordion Gammacell 40 Extractor Low Dose‐Rate Research Irradiator. Method and Materials: The manufacturer specifies that the irradiator is provided with two 61.1 TBq Cesium 137 sources that together produce a central dose rate of 1.10 Gy/minute ± 15% in its sample tray. In the central 260 mm diameter by 100 mm height region, the specified dose uniformity is ± 7%.Custom measurement jigs were made to fit inside the irradiator's sample trays and assemblies. Ionization chamber readings were made to determine exposure rate constancy and timer errors. Radiochromic film was used to measure dose and dose rate constancy, the attenuation of manufacturer‐provided attenuators, depth dose characteristics in Solid Water™, and uniformity of dose across the sample tray. Thermoluminescent devices (TLD's) were assessed as an alternate means of measuring dose and dose rate constancy. Results: The exposure rate was constant down to two seconds and timer error was about 1.4 seconds. Film measurements showed that dose reduction for the attenuation filters (33%, 60%, 80%) is within 3% of specifications. Film measurements in a central 10×10 cm area showed dose uniformity within about 2%. Depth doses in Solid Water™ showed a variation of about 10%. Film and TLD's yielded similarly accurate dose measurements and constancy estimates. Conclusion: Our facility's irradiator operates within the manufacturer's specifications except for dose variance with depth which may be somewhat greater than 7%. Although the manufacturer provides basic dosimetric information about each irradiator, it is advantageous to verify and expand this information. As with linacs, irradiators must certainly differ from machine to machine and change over time. It is prudent when doing research using these devices to validate their performance under their conditions of usage.

SU-FF-T-281: Leakage Radiation of Elekta Synergy-S LINAC Machine

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy-S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy-S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x-ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulator treatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy-S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.

Calibration of the borated ion chamber at NIST reactor thermal column

In boron neutron capture therapy and boron neutron capture enhanced fast neutron therapy, the absorbed dose of tissue due to the boron neutron capture reaction is difficult to measure directly. This dose can be computed from the measured thermal neutron fluence rate and the (10)B concentration at the site of interest. A borated tissue-equivalent (TE) ion chamber can be used to directly measure the boron dose in a phantom under irradiation by a neutron beam. Fermilab has two Exradin 0.5 cm(3) Spokas thimble TE ion chambers, one loaded with boron, available for such measurements. At the Fermilab Neutron Therapy Facility, these ion chambers are generally used with air as the filling gas. Since alpha particles and lithium ions from the (10)B(n,alpha)(7)Li reactions have very short ranges in air, the Bragg-Gray principle may not be satisfied for the borated TE ion chamber. A calibration method is described in this paper for the determination of boron capture dose using paired ion chambers. The two TE ion chambers were calibrated in the thermal column of the National Institute of Standards and Technology (NIST) research reactor. The borated TE ion chamber is loaded with 1,000 ppm of natural boron (184 ppm of (10)B). The NIST thermal column has a cadmium ratio of greater than 400 as determined by gold activation. The thermal neutron fluence rate during the calibration was determined using a NIST fission chamber to an accuracy of 5.1%. The chambers were calibrated at two different thermal neutron fluence rates: 5.11 x 10(6) and 4.46 x 10(7)n cm(-2) s(-1). The non-borated ion chamber reading was used to subtract collected charge not due to boron neutron capture reactions. An optically thick lithium slab was used to attenuate the thermal neutrons from the neutron beam port so the responses of the chambers could be corrected for fast neutrons and gamma rays in the beam. The calibration factor of the borated ion chamber was determined to be 1.83 x 10(9) +/- 5.5% (+/- 1sigma) n cm(-2) per nC at standard temperature and pressure condition.

Characterization of the ADII‐33 diamond detector

Dosimetry characteristics of the ADII-33 diamond detector were investigated. Sensitivity, stability, bias voltage, hardening, dose rate, energy dependence, and spatial resolution were examined. Current generated in the detector was found to be proportional to the bias voltage applied to the detector with stability degrading as a function of the bias voltage. The average current increased with increasing bias voltage. The statistical fluctuation in current was less than 0.3% regardless of the applied bias voltage. The optimal bias voltage, at which the current uncertainty is negligible, was found to be 125+/-25 V. The detector was hardened up to 85 kGy without significant degradation in output signal. A sub-linearity in the current as a function of dose rate was observed when the dose rate varied from 600.0 to 11.1 cGy/min. A fitting parameter of delta=0.978 was observed in the power relationship of IinfinityDdelta. When comparing the diamond detector readings to corrected ion chamber readings over a wide energy range of electron beams, differences of only 0.2% were observed suggesting no energy dependence for electron beam. PDD curves for a 10 x 10 cm2 field for 6 and 20 MV photon beams measured with the diamond detector and the farmer type ion chamber were also compared and they closely agreed up to a depth of 14 cm. Beyond a depth of 14 cm, diamond detector starts to overestimate the PDD curve reaching difference of 1.90% and 1.0% at 18 cm depth for 6 and 20 MV, respectively, to those values measured with the ion chamber. The diamond detector presents slightly better spatial resolution than the Exradin A16 microchamber. We conclude that at the optimal bias voltage, this new diamond detector is stable and the uncertainties in the current will not affect its suitability for clinical use. If compared against a calibrated ion chamber to correct any energy and dose rate dependence and considering any radiation damage effect, this diamond detector can be used to measure absolute and relative dose.

WE-D-224C-07: A Comprehensive Patient-Specific IMRT Quality Assurance Procedure On Hi-Art Tomotherapy® Unit

Purpose: To present a comprehensive patient-specific IMRT QA procedure on Tomotherapy® unit using ion chamber and radiographic film for those facilities with RIT® film dosimetry system. Method and Materials: A cylindrical Virtual Water® phantom was scanned in CT with an ion chamber in the hole 1.5 cm below the middle coronal plane. Three-dimensional dose calculation in fine grid using treatment beam was performed to the phantom which is appropriately positioned so that the measuring point and plane are within the volume of interest. A sheet of Kokak® EDR-2 Readypack was sandwiched between two halves of the phantom and pricked on green lasers. Film exposure and ion chamber measurement were made simultaneously. To verify planar absolute dose distribution, the film was calibrated in a separate sheet of the same batch on the same day IMRT QA was performed. To facilitate the film calibration and film dosimetry analysis on Tomotherapy planning station, an in-house software was developed to perform step-valley film calibration and film image data conversion from RIT® format to DICOM format which is acceptable by the current version of Tomotherapy planning system. Results: 213 patient-specific IMRT QA procedures have been performed on Tomotherapy unit since it was installed about two years ago. The average discrepancy between ion chamber measurement and dose calculation in phantom is 1.74%. Film dosimetry results agree with ion chamber measurements in an average discrepancy 1%. Three cases have failed the QA procedure based on 5% discrepancy criteria. Conclusion: Three-dimensional dose calculation algorithm for helical IMRT works very well in homogeneous phantom. This QA procedure is clinically practical and necessary when independent dose calculation tools are not available. It is feasible to verify two-dimensional absolute dose distribution using radiographic film if it is calibrated against ion chamber reading which is traceable to ADCL or NIST.

SU‐FF‐T‐84: An Alternative Calibration Method for Solid Modulator IMRT

Purpose: Solid compensators for intensity modulation have become very popular. The purpose of this presentation is to investigate the differences, for three separate measuring tools, in calibration using open fields and calibration with an attenuating material in the beam. Doses for both open fields and attenuated fields were confirmed using farmer chamber measurements. It is crucial to understand the differences in detector response between open and attenuated beams in order to make measurements of modulator based IMRT more accurate. Method and Materials: Three different types of measuring devices were investigated for this analysis: GafChromic EBT film, Kodak EDR2 film, and the SunNuclear Mapcheck system, each compared against ion chamber readings. Each system was irradiated with a 4 × 4 cm2 field size for various doses. The same dose measurements were made for each system with a 3cm brass modulator in the wedge slot. After the films were scanned, light transmission value readings were obtained. For the Sun Nuclear Mapcheck system, direct readings of the central axis diode were obtained, again using the open field and the brass modulator. Measurements were made with a Farmer chamber to confirm that the doses from open and attenuated fields were equal. Results: The percent error differences in measurements using open versus compensated fields for EBT, EDR2, and MapCheck are 0.63%, 2.88%, and 1.01% respectively, while the Farmer chamber was only 0.07%. Conclusion: The small value in the Farmer chamber indicated that our calculation was accurate. The EDR2 film showed a high difference in readings compared to that for the EBT film and the MapCheck system. Due to these differences we conclude that one should use a modulator in the beam while calibrating a system for the use of modulator based IMRT. Conflict of Interest: Chris Warner works for .decimal.

SU‐FF‐T‐176: Dose Verification in Moving Targets During Helical Tomotherapy Beam Delivery

Purpose: To compare dose distributions from stationary and moving targets subject to simulated intra‐fraction motion during treatment deliveries on a helical tomotherapy unit. Method and Materials: We have constructed an in house platform that moves in the superior‐inferior direction in a controlled manner. Using a stationary CT scan of a thorax phantom, a fictitious target volume was defined and a helical tomotherapy plan was generated. The phantom was placed on top of the moving platform and set in motion at a frequency of 16.7 cycles/min and 20 mm peak‐to‐trough excursion. An IMRT plan was delivered while the phantom was in motion and when it was still. Ion chamber readings were recorded at the center of the target. Different phases were artificially introduced by starting the platform motion at different times before the start of irradiation. In each delivery a film was placed in the coronal plane inside the phantom to register 2D dose. Results: Profiles along the direction of motion from the co‐registered images of moving and stationary deliveries have shown dose differences as large as 70% in the penumbra region. Gamma function maps for different dose and distance to agreement criteria revealed the regions at which the gamma criteria fails and was shown to be a strong function of initial motion phase. Ion chamber point measurements recorded insignificant differences between still and moving deliveries. Conclusions: We have shown that there is a significant difference in 2D dose distributions in the penumbra region both in the inferior‐superior and transverse directions between stationary and moving deliveries that simulate breathing motion. Our results indicate that the differences are highly dependent on the breathing motion initial phase. The phantom motion had little effect on the delivered dose at the central target region.

TU‐E‐330D‐03: Evaluation of An OSL Dosimetry System for CT Quality Assurance and Dose Optimization

Purpose: Recent development of wide‐beam multidetector CT and conebeam CT demands alternative methodology to CTDI. In this study, the Optically Stimulated Luminescence (OSL) technique was evaluated for CT quality assurance and dose optimization. Method and Materials: CT scans were performed on 38‐mm2 thin Luxel™ dosimeters oriented in axial plane of a GE LightSpeed Ultra and at the center of a CT body dosimetry phantom. A 5‐mm beam collimation was selected to determine the energy response of the OSL detector at 80, 120, and 140 kVp stations. mA response of the OSL detector was evaluated from 60 to 350 mA. Helical scans of varying length coverage were also performed with the detector placed at the isocenter. The exposed OSL discs were measured using a Riso TL/OSL‐DA‐15 reader. Each OSL measurement was followed by a standard beta source irradiation and subsequent OSL measurement to normalize for the differing mass of the disks. The normalized OSL signal reading was used in the data analysis. Results: Good mA linearity was observed at all 3 kVp stations. There are discontinuities seen at ∼ 250 mA and it is known to be caused by tube focal spot change. The 120 and 140 kVp data show good correlation with ionization chamber reading. For 80 kVp, OSL signals to ion chamber readings are all higher, particularly at low mA range, indicating higher sensitivity of the OSL detector over the ion chamber. The helical scans show an increased OSL signal with scan length and a leveling of the signal at large scan coverage. Conclusion: The OSL detectors are of small size and responded well to CT exposure. It provides a practical technique for quantifying the dose at any location of a phantom for quality assurance and hence exhibits a potential for estimating patient organ dose.

Monte Carlo calculations for absolute dosimetry to determine machine outputs for proton therapy fields

The prescribed dose in radiation therapy has to be converted into machine monitor units for patient treatment. This is done routinely for each spread-out Bragg peak (SOBP) field either by calibration measurements, by using analytical algorithms or by relying on empirical data. At the Northeast Proton Therapy Center, a monitor unit corresponds to a fixed amount of charge collected in a segmented transmission ionization chamber inside the treatment head. The goal of this work was to use a detailed Monte Carlo model of the treatment head to calculate the dose delivered to the patient as a function of ionization chamber reading, i.e. to yield absolute dose in patients in terms of machine monitor units. The results show excellent agreement with measurements. For 50 SOBP fields considered in this study, the mean absolute difference between the experimental and the calculated value is 1.5%, where ∼50% of the fields agree within 1%. This is within the uncertainties of the data. The Monte Carlo method has advantages over analytical algorithms because it takes into account scattered and secondary radiation, does not rely on empirical parameters, and provides a tool to study the influence of parts of the treatment head on the ionization chamber reading. Compared to experimental methods the Monte Carlo method has the advantage of being able to verify the dose in the patient geometry.

Monte Carlo correction factors for a Farmer 0.6 cm3 ion chamber dose measurement in the build-up region of the 6 MV clinical beam

Reference dosimetry of photon fields is a well-established subject and currently available protocols (such as the IAEA TRS-398 and AAPM TG-51) provide methods for converting the ionization chamber (IC) reading into dose to water, provided reference conditions of charged particle equilibrium (CPE) are fulfilled. But these protocols cannot deal with the build-up region, where the lack of CPE limits the applicability of the cavity theorems and so the chamber correction factors become depth dependent. By explicitly including the IC geometry in the Monte Carlo simulations, depth-dependent dose correction factors are calculated for a PTW 30001 0.6 cm3 ion chamber in the build-up region of the 6 MV photon beam. The corrected percentage depth dose (PDD) agrees within 2% with that measured using the NACP 02 plane-parallel ion chamber in the build-up region at depths greater than 0.4 cm, where the Farmer chamber wall reaches the phantom surface.

Evaluation of a diode array for QA measurements on a helical tomotherapy unit

A helical tomotherapy system is used in our clinic to deliver intensity-modulated radiation therapy (IMRT) treatments. Since this machine is designed to deliver IMRT treatments, the traditional field flatness requirements are no longer applicable. This allows the unit to operate without a field flatness filter and consequently the 400 mm wide fan beam is highly inhomogeneous in intensity. The shape of this beam profile is mapped during machine commissioning and for quality assurance purposes the shape of the beam profile needs to be monitored. The use of a commercial diode array for quality assurance measurements is investigated. Central axis beam profiles were acquired at different depths using solid water built-up material. These profiles were compared with ion chamber scans taken in a water tank to test the accuracy of the diode array measurements. The sensitivity of the diode array to variations in the beam profile was checked. Over a seven week period, beam profiles were repeatedly measured. The observed variations are compared with those observed with an on-board beam profile monitor. The diode measurements were in agreement with the ion chamber scans. In the high dose, low gradient region the average ratio between the diode and ion chamber readings was 1.000 +/- 0.005 (+/- 1 standard deviation). In the penumbra region the agreement was poorer but all diodes passed the distance to agreement (DTA) requirement of 2 mm. The trend in the beam profile variations that was measured with the diode array device was in agreement with the on-board monitor. While the calculated amount of variation differs between the devices, both were sensitive to subtle variations in the beam profile. The diode array is a valuable tool to quickly and accurately monitor the beam profile on a helical tomotherapy unit.

Gated Radiotherapy for Lung Tumors Using Infrared Detectors, Stereoscopic X-rays, and Implanted Markers

Gated Radiotherapy for Lung Tumors Using Infrared Detectors, Stereoscopic X-rays, and Implanted Markers

SU-FF-T-240: Evaluation of a New Calibration Method for In-Vivo Diodes with Sliding Window IMRT Delivery

Purpose: To evaluate the response of in-vivo diodes for IMRT delivery under new calibration condition. Materials and Method: In-vivo diodes (QED p-type, 6–12 MV and 15–25 MV, Sun Nuclear Inc) with model 1131 dosimeter were calibrated with actual patient IMRT (sliding window technique) treatment fields (hereafter referred to as IMRT diodes). A 6–12 MV diode was placed on top of a solid water phantom at 100 cm SSD with an ion chamber (Capintec PR-06) at dmax. First a 10×10 static open field was delivered to obtain the ion chamber reference reading. Then an IMRT breast patient's medial field was delivered, and the ion chamber reading was converted to dose based on the reference reading. This dose number was used to calibrate the diode. A 15–25 MV diode was calibrated the same way with an IMRT prostate's PA field delivered. Regular diodes were calibrated to dmax dose under standard conditions (100 cm SSD, 10×10 FS). The response of both regular and IMRT diodes under IMRT delivery were measured with a farmer chamber at dmax used as reference. Results: Diodes calibrated with the new method, i.e. IMRT diodes over-responded by 2.0% for 18 MV photons and 1.0% for 6 MV photons respectively for IMRT delivery when compared with their corresponding regular diodes, indicating IMRT diodes sensitivity increased by about 2.0% for 18 MV photons and about 1.0% for 6 MV photons respectively. Conclusion: A new method to calibrate diode for IMRT delivery was presented. The responses were compared with regular diodes. IMRT diodes over-responded when compared with regular diodes by 2.0% for 18 MV photons and 1.0% for 6 MV photons with IMRT delivery. The evaluation of field size dependence, gap dependence and SSD dependence of the diodes for IMRT delivery have also been submitted to this conference.

TU‐E‐I‐611‐07: Characterization of Two Optically Stimulated Luminescence (OSL) Dosimeter Systems for Monitoring Entrance Skin Dose During Interventional Procedures

Purpose: There is a great concern of patient skin injury and a demand for an effective technique for direct measurement of patient dose in real‐time. We have characterized two dosimeter systems for possible future development of a clinically feasible dosimetry system. Success of this investigation may lead to significant reduction of patient radiation dose and skin injury. Method and Materials: High‐sensitivity, fiber‐optic dosimetry systems based on OSL from and KBr:Eu single crystals (detectors) were developed at Oklahoma State University. The physical characteristics of the detector systems were investigated using clinical fluoroscopic machines. The detector's dependence on x‐ray energy, exposure rate, and the angle of x‐ray incidence were evaluated. The data from the dosimetry systems are normalized to ion chamber readings under the same conditions. The detectors were attached to an anthropomorphic chest phantom for physicians to evaluate the visibility of the detectors in the fluoroscopic image. A mathematical model was developed to simulate data sampling of the KBr:Eu system when used with pulsed fluoroscopic equipment. Results: Both types of detectors were nearly invisible under various kVp conditions for typical fluoroscopic procedures. Across the energy range of 60 to 120 kVp, energy dependence was less than 23% for system and less than 8% for the KBr:Eu system. Dose rate dependence was less than 2.1% for the KBr:Eu system, but the system responded too slowly. Angular dependence was less than 6% for the system and 3.5% for the KBr:Eu system. The mathematical model shows that the KBr:Eu system should function consistently under various pulsed fluoroscopic operations. Conclusion: The OSL detectors are of small size, yet sensitive to low exposure, and exhibit practically invisible in fluoroscopic images. Response to variation of fluoroscopic x‐ray parameters is acceptable. Both systems show promise for a clinically feasible skin dose dosimetry system.

Influence of field size on a PTW type 23342 plane‐parallel ionization chamber's response

The response of a PTW type 23342 plane-parallel ionization chamber, both in air and in phantom, was evaluated for x-ray tube potentials between 30 and 100 kV and radiation field diameters ranging from 30 to 70 mm. The experiments were performed with a calibrated Pantak x-ray machine and made use of the same set of x-ray qualities adopted by the PTB primary laboratory for the calibration of such chambers. A Plexiglas phantom (1.18 g cm(-3)) 110 mm long, 110 mm wide, and 80 mm deep was used for phantom measurements. X-ray qualities were characterized by using 99.99% pure aluminum filters. On the basis of the IAEA's TRS 398, the article discusses the dependence of the plane-parallel ionization chamber readings with field size in air and in phantom, its implication with regard to clinical dosimetry, cross-calibration, and dissemination of calibration factors.

Matchline dosimetry in step and shoot IMRT fields: a film study

The Varian millennium 120 multileaf collimator has curved leaf ends. Transmission through the leaf ends generates a small asymmetric penumbral dose effect. This design can lead to hot spots between neighbouring beam segments during step and shoot IMRT dose delivery. We have observed some matchlines with film for clinical beams optimized using the pinnacle radiotherapy treatment planning system; hence we sought to verify the optimum leaf offset required to minimize the matchline effect. An in-house program was created to control the MLC leaf banks in 2 cm steps with a 2 cm gap. The gap was varied by the following offset values from 0.0 to 0.1 cm. Two types of radiographic films (Kodak EDR and XV films) and a radiochromic film (Gafchromic MD-55-2) were used to measure the optical density maps. The films were positioned in a solid water phantom perpendicular to the beam axis and irradiated at dmax using a 6 MV photon beam. An ion chamber (IC4) was used to measure point doses for normalization in a beam umbral minima position. The relative mean peak to valley dose ratios measured with no leaf offset were 1.31, 1.30 and 1.31 for the XV, EDR2 and Gafchromic films, respectively. For a 0.07 cm gap per leaf and a performance of end leaf repeatability of 0.01 cm, the central matchline was reduced to about 1.0 for all dosimeters, with two mini-peaks measured as 1.05, 1.05 and 1.08 each side of the matchline, for XV, EDR2 and Gafchromic, respectively. The average relative dose across the umbra for this offset was XO-mat V = 1.01, EDR = 1.01 and radiochromic film = 1.02, respectively.While we expected the beam penumbral tails from segment neighbours to cause overprediction of the dose in the central valley regions due to the energy response of radiographic films, by normalizing all dosimeters to an ion chamber reading in the minimum we could not observe any major shape distortion between the radiographic film and radiochromic film results. In conclusion, relative doses measured by radiographic and radiochromic films agree well with IC4 within ±2%.

Interpolation and extrapolation of dose measurements with different detector sizes to improve the spatial resolution of radiotherapy dosimetry as demonstrated for helical tomotherapy

A new technique for intensity modulated radiation therapy (IMRT) delivery is helical tomotherapy (HT). Like most IMRT delivery methods, HT utilizes many small fields as part of the treatment plan, which can be difficult to characterize. A novel technique for small field characterization, based on inter- and extrapolation of ion chamber readings, is presented in the context of HT. As a fan beam is characterized by its thickness and output factor, plane parallel chambers with different active volumes were used to scan the fan beam profiles. The fan beam thickness (FBT) can be determined from the thickness measured with the chamber by extrapolating to an infinitesimally small chamber size. The effective output was derived from the integral under the dose profile divided by the FBT. This was done for five FBTs and demonstrated a sharp fall off in dose when the FBT decreased below 8 mm. Similar techniques can be applied to other IMRT techniques to improve the characterization of various beam parameters.

In-phantom spectra and dose distributions from a high-energy neutron therapy beam

In radiotherapy with external beams, healthy tissues surrounding the target volumes are inevitably irradiated. In the case of neutron therapy, the estimation of dose to the organs surrounding the target volume is particularly challenging, because of the varying contributions from primary and secondary neutrons and photons of different energies. The neutron doses to tissues surrounding the target volume at the Louvain-la-Neuve (LLN) facility were investigated in this work. At LLN, primary neutrons have a broad spectrum with a mean energy of about 30 MeV. The transport of a 10×10 cm 2 beam through a water phantom was simulated by means of the Monte Carlo code MCNPX. Distributions of energy-differential values of neutron fluence, kerma and kerma equivalent were estimated at different locations in a water phantom. The evolution of neutron dose and dose equivalent inside the phantom was deduced. Measurements of absorbed dose and of dose equivalent were then carried out in a water phantom using an ionization chamber and superheated drop detectors (SDDs). On the beam axis, the calculations agreed well with the ionization chamber data, but disagreed significantly from the SDD data due to the detector's under-response to neutrons above 20 MeV. Off the beam axis, the calculated absorbed doses were significantly lower than the ionization chamber readings, since gamma fields were not accounted for. The calculated data are doses from neutron-induced charge particles, and these agreed with the values measured by the photon-insensitive SDDs. When exposed to the degraded spectra off the beam axis, the SDD offered reliable estimates of the neutron dose equivalent.

Measurement and validation of benchmark-quality thick-target tungsten X-ray spectra below 150 kVp.

Pulse-height distributions of two constant potential X-ray tubes with fixed anode tungsten targets were measured and unfolded. The measurements employed quantitative alignment of the beam, the use of two different semiconductor detectors (high-purity germanium and cadmium-zinc-telluride), two different ion chamber systems with beam-specific calibration factors, and various filter and tube potential combinations. Monte Carlo response matrices were generated for each detector for unfolding the pulse-height distributions into spectra incident on the detectors. These response matrices were validated for the low error bars assigned to the data. A significant aspect of the validation of spectra, and a detailed characterization of the X-ray tubes, involved measuring filtered and unfiltered beams at multiple tube potentials (30-150 kVp). Full corrections to ion chamber readings were employed to convert normalized fluence spectra into absolute fluence spectra. The characterization of fixed anode pitting and its dominance over exit window plating and/or detector dead layer was determined. An Appendix of tabulated benchmark spectra with assigned error ranges was developed for future reference.

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams.

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.