Thimble Chamber Research Articles

Thimble ionization chambers in medium-energy x-ray beams and the role of constructive details of the central electrode: Monte Carlo simulations and measurements

This paper presents investigations of thimble ionization chamber response in medium-energy kilovoltage x-ray beams (70–280 kVp, 0.09–3.40 mm Cu HVL). Two thimble ionization chambers (PTW30015 and PTW30016) were investigated, regarding the influence of the central electrode dimensions made of aluminum. Measurements were carried out in photon fields of different beam quality. Corresponding Monte Carlo simulations employing the EGSnrc Monte Carlo code system were performed. The simulations included the modelling of the x-ray tube and measurement setup for generation of x-ray spectra. These spectra were subsequently used to calculate the absorbed energy in the air cavity of the two thimble ionization chamber models and the air kerma at the reference point of the chambers. Measurements and simulations revealed an optimal diameter of the central electrode, concerning an almost energy-independent response of the ionizaton chamber. The Monte Carlo simulations are in good agreement with the measured values, expressed in beam quality correction factors kQ. The agreement was generally within 0.6% but could only be achieved with an accurate model of the central electrode including its exact shape. Otherwise, deviations up to 8.5% resulted, decreasing with higher photon energies, which can be addressed to the high yield of the photoelectric effect in the electrode material aluminum at low photon energies.

SU‐GG‐T‐179: Analysis of Electron Cutout Factor Measurements

Purpose: To compare electron cutout factor (COF) measured with various dosimeters for different cone sizes. Method and Materials: The COFs for various electron fields were measured on a Varian‐2300EX linear accelerator at 6, 9, 12, 16 and 20 MeV. A thimble ion chamber, plane parallel chamber, thermoluminescent dosimeters (TLDs), EDR film, X Omat‐V (XV) film, GafChromic film and Thomson‐Nielsen mosfets were used to measure COFs. Each detector was placed along the beam's central axis at 100 cm from the source. Measurements were made in solid water at dmax for 6×6, 10×10 and 20×∼20 cone sizes with circular cutouts. The thimble chamber was used as the standard to compare COF measurements. The raw measurements made with the two chambers and the diodes were used to determine the COFs. Films were analyzed with RIT software and TLDs were read the next day to determine COFs. Results: Measurements for the 10×10 and 20×20 cone sizes agreed within 3.5% between all the dosimeters and the thimble chamber. Measurements made for the 6×6 cone and cutout were more than 3.5% from those made with the thimble chamber. There was better agreement for COF measurements between the mosfets, TLDs, EDR film and XV film. Conclusion: This study takes a look at the various dosimeters available at the clinic that can be used for determining electron cutout factors. For most cutout sizes, ionization chambers, mosfets, TLDs, EDR, XV and GafChromic film can be used to measure the COFs for all electron energies. For small field sizes, the use of TLDs, mosfets, and film can give more accurate COFs.

TH-D-352-02: New KQ Factors for Reference Dosimetry in High-Energy Photon Beams

Purpose: The AAPM is currently working to provide an update/addendum to the TG‐51 dosimetry protocol. As part of this work measured and calculated kQ factors for a range of thimble‐type ionization chambers not listed in TG‐51 were obtained. The aim was to investigate the accuracy of the calculations and obtain objective evidence on the performance of these chambers for reference dosimetry.Method and Materials: The same formalism and computer programs as employed in TG‐51 were used to calculate kQ factors. As well as 0.6 cm3 reference chambers, factors were calculated for scanning and micro chambers from the major manufacturers. Measurements were made using the 60 Co and Elekta Preciselinac facilities at the National Research Council of Canada. The aim was to characterize the chambers over the range of energies applicable to TG‐51 and determine whether the chamber met the requirements of a reference class instrument. Chamber settling, ion recombination and polarity were investigated and absorbed dose calibration coefficients were obtained for 60 Co and 6, 10 and 25 MV photon beams. Results: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental kQ factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and micro chambers was somewhat variable. Some chambers showed very good behaviour and gave reasonable agreement between measured and calculated kQ factors but others showed anomalous polarity and recombination corrections that require further investigation. For the well‐behaved chambers, agreement between measured and calculated kQ factors was within 0.4%. However, for some chambers differences of nearly 1% were seen that may be related to the recombination/polarity issues. Conclusion: Experimental and calculated kQ data have been obtained for a wide range of thimble chambers that can be used by the AAPM and clinical users in choosing suitable detectors for reference dosimetry.

TU‐C‐AUD B‐09: Comparison of Two Photon Dosimetry Protocols: AAPM TG‐51 & IPSM 1990

Purpose: A comparison of two dosimetry codes of practice for photon therapy beams was performed. These codes of practice are: AAPM TG‐51 and IPSM 1990. Both protocols are based on standards of absorbed dose to water. This comparison was carried out for five beam qualities with nominal energies: 4 MV, 6 MV, 8 MV, 10 MV and 18 MV. Method and Materials: The photon beams were generated by two Elekta Synergy and two Elekta Precise linear accelerators. A PTW MP3 plotting tank was used for relative and absolute measurements. A PTW 31002 thimble chamber was utilized for relative measurements. A NE 2571 Farmer graphite chamber was chosen for absolute dose measurements. A Keithley Therapy 35040 electrometer was used for absolute measurements and a PTW Tandem for relative ones. All measurements are traceable to National Physical Laboratory (UK) dose standards. Absolute dose was measured at 5 cm deep and 7 cm deep (for 18 MV) according to IPSM 1990 and at 10 cm deep for AAPM TG‐51. All correction factors (polarity and recombination) were computed using measurements performed at the recommended depths and following each protocol. Results: Values for all correction and calibration factors were obtained by measurement or computation. ND,w calibration factors provided by NPL for each quality were used for IPSM 1990; in the case of AAPM TG‐51, its method using ND,w for Co‐60 and tabulated kq factors was applied. AAPM TG‐51 and IPSM 1990 absolute dose results lie within 0.6%, which falls within uncertainty limits. Conclusion: A comprehensive absolute dose study was performed according to two protocols: beam quality indexes, correction factors and dose measurements. A complete uncertainty analysis was carried out for all experimental results, according to ISO GUM standards of practice. Results show that absorbed dose to water according to both protocols agrees within uncertainty.

Dosimetric audits of photon beams in radiation therapy centres in Rio de Janeiro, Brazil

Data related to 11 y of high-energy photon radiotherapy beam dosimetry are presented and analysed. Dosimetric evaluations were carried out using water phantoms and thimble ionisation chambers and are part of the radiation protection regulatory licensing process for medicine facilities of Brazilian government. Measurements were done at reference conditions for a standard absorbed dose of 100 cGy [cGy (=1 rad)]. The absolute per cent deviation between the measured and presumed delivered doses should not exceed the tolerance level of +/-3%. The first dosimetry survey from 1996 to 1998 showed a situation that was an object of concern. Deviations of 22 and 18.7% could be measured, although small deviations were also obtained. After 1998, the improvement in dosimetry quality control by the radiotherapy centres became clear, with most of the deviations situated within the +/-3% range. The decrease in the measured deviations presents the effective success of the Institute of Radiation Protection and Dosimetry audit programme for the improvement in the control of radiotherapy photon beams in Rio de Janeiro. Also, it is possible to recommend to Brazilian regulatory organisation a decrease in the tolerance level for dosimetric deviations in order to achieve a more precise dose delivered to patients in radiotherapy centres.

Monte-Carlo-based perturbation and beam quality correction factors for thimble ionization chambers in high-energy photon beams

This paper presents a detailed investigation into the calculation of perturbation and beam quality correction factors for ionization chambers in high-energy photon beams with the use of Monte Carlo simulations. For a model of the NE2571 Farmer-type chamber, all separate perturbation factors as found in the current dosimetry protocols were calculated in a fixed order and compared to the currently available data. Furthermore, the NE2571 Farmer-type and a model of the PTW31010 thimble chamber were used to calculate the beam quality correction factor kQ. The calculations of kQ showed good agreement with the published values in the current dosimetry protocols AAPM TG-51 and IAEA TRS-398 and a large set of published measurements. Still, some of the single calculated perturbation factors deviate from the commonly used ones; especially prepl deviates more than 0.5%. The influence of various sources of uncertainties in the simulations is investigated for the NE2571 model. The influence of constructive details of the chamber stem shows a negligible dependence on calculated values. A comparison between a full linear accelerator source and a simple collimated point source with linear accelerator photon spectra yields comparable results. As expected, the calculation of the overall beam quality correction factor is sensitive to the mean ionization energy of graphite used. The measurement setup (source–surface distance versus source–axis distance) had no influence on the calculated values.

Design and implementation of a head‐and‐neck phantom for system audit and verification of intensity‐modulated radiation therapy

The head and neck is a challenging anatomic site for intensity‐modulated radiation therapy (IMRT), requiring thorough testing of planning and treatment delivery systems. Ideally, the phantoms used should be anatomically realistic, have radiologic properties identical to those of the tissues concerned, and allow for the use of a variety of devices to verify dose and dose distribution in any target or normal‐tissue structure.A phantom that approaches the foregoing characteristics has been designed and built; its specific purpose is verification for IMRT treatments in the head‐and‐neck region. This semi‐anatomic phantom, HANK, is constructed of Perspex (Imperial Chemical Industries, London, U.K.) and provides for the insertion of heterogeneities simulating air cavities in a range of fixed positions. Chamber inserts are manufactured to incorporate either a standard thimble ionization chamber (0.125 cm3: PTW, Freiburg, Germany) or a smaller PinPoint chamber (0.015 cm3: PTW), and measurements can be made with either chamber in a range of positions throughout the phantom. Coronal films can also be acquired within the phantom, and additional solid blocks of Perspex allow for transverse films to be acquired within the head region.Initial studies using simple conventional head‐and‐neck plans established the reproducibility of the phantom and the measurement devices to within the setup uncertainty of ±0.5 mm. Subsequent verification of 9 clinical head‐and‐neck IMRT plans demonstrated the efficacy of the phantom in making a range of patient‐specific dose measurements in regions of dosimetric and clinical interest. Agreement between measured values and those predicted by the Pinnacle3 treatment planning system (Philips Medical Systems, Andover, MA) was found to be generally good, with a mean error on the calculated dose to each point of +0.2% (range: −4.3% to +2.2%;n=9) for the primary planning target volume (PTV), −0.1% (range: −1.5% to +2.0%;n=8) for the nodal PTV, and +0.0% (range: −1.8% to +4.3%;n=9) for the spinal cord. The suitability of the phantom for measuring combined dose distributions using radiographic film was also evaluated.The phantom has proved to be a valuable tool in the development and implementation of clinical head‐and‐neck IMRT, allowing for accurate verification of absolute dose and dose distributions in regions of clinical and dosimetric interest.PACS numbers: 87.53.‐j, 87.53.Xd, 87.56.Fc

Comparison of linear accelerator photon outputs from the IAEA TRS-398 and TRS-277 codes of practice

Megavoltage linac photon outputs are determined using the IAEA TRS-398 and TRS-277 codes of practice and the results compared. It is found that for two types of thimble chambers the contributions to absorbed dose to water due to the two codes of practice are comparable and it is suggested that observed differences are mainly due to the different standards used to determine ND,W and NK calibration coefficients at a standards laboratory. A method is proposed to determine a priori the expected difference between absorbed dose to water from the two codes of practice. The contribution to the difference that depends on the respective codes is expressed by a code dependent factor fC. It is shown that for Wellhöfer FC65-G and NE 2571 ionisation chambers this factor is largely independent of beam quality. To calculate the expected shift in absorbed dose to water when moving from TRS-277 to TRS-398, an fC. value which is representative of the range of beam qualities in the clinic is calculated and multiplied by the ratio ND,W/NK.

SU-EE-A2-02: Evaluation of Radiation Dosimeters for Kilovoltage X-Ray Beam Dosimetry

Purpose: To evaluate radiation detectors for the dosimetry of kilovoltage x-ray beams. Method and Materials: The study evaluated cylindrical and parallel plate ionization chambers provided with the PTW MP3 water tank system. Kilovoltage x-ray beams were generated by a Pantak DXT300 x-ray unit with energies ranging from 50 to 280 kVp. The following data was measured: percentage depth dose curves in water, relative detector response both in air and in solid water with the 6 cm diameter applicator. The reference detector was an NE 2571 ionization chamber that had been calibrated in a primary standards laboratory. The EGSnrc Monte Carlo code V4.2.2 was used to calculate depth dose curves in water for comparison with measured data. The x-ray spectrum of primary beam was determined using an analytical program, XRAYBEAM, and subsequently verified by calculation of the half value layer. Results: For the measured depth doses, the PTW 0.3 cc thimble chamber, NACP, Markus and Roos chamber were in good agreement with the Monte Carlo calculated depth dose data. The agreement was better than 3% in most cases, and a maximum deviation of 4% for the 50 kVp x-ray beam. The M31002 pinpoint chamber gave a greater deviation, up to 9% for the 50 kVp beam. The relative detector response, as compared to the NE 2571, indicated that thimble chambers had a small variation of up to 5% over the energy range. However the relative detector response for the parallel plate chambers was large, with the deviation increasing as the x-ray beam energy decreased, up to 70% for the Markus chamber. Similar large response changes also occurred for the pinpoint chamber. Conclusion: The parallel plate chambers (Markus, Roos, NACP) were found to be suitable for depth dose measurements in water for kilovoltage x-ray beams.

SU‐FF‐T‐97: Characterization of Amorphous Silicon Flat Panel Imager for Dose Measurements

Purpose: The major objective of the work was to categorize the dosimetric response of a Varian aS500 type Electronic Portal Imaging Device (EPID) for dose measurements. Method and Materials: For a 6MV beam from a Varian 21EX linear accelerator, reproducibility of EPID pixel response, dose rate effect, and cumulative dose linearity were studied. In addition, scatter characteristics and depth doses were also determined using the EPID. A Capintec ion chamber in a polystyrene miniphantom and a Wellhöfer water phantom were used for comparative measurements. Results: The EPID response was found to be independent of dose rates from 100 MU/min to 600 MU/min at larger source to detector distances. However, pixel saturation was observed for dose rates beyond 300 MU/min with the EPID placed at the isocentre. Raw pixel response was reproducible to within 2 % when measured over a period of 6 weeks. Changes in scatter conditions with field size were also measured for the EPID. The EPID scatter was found to be in between 5 to 10 cm of a full water phantom. The gray scale response is linear with the dose to the portal imager. The validity of inverse square relationship was also tested to be within 0.5%, for the EPID at various distances from the source. Conclusions: The depth dose data measured with EPID showed good correspondence with that measured with a thimble type ionization chamber in a water phantom. Compared to ionization chamber data, open beam profiles measured with EPID show flat response in high dose region, rapid fall‐off in high dose gradients (> 30%/cm), and higher penumbra tail. The agreement with the ion chamber measurements could be improved if the flood field correction is performed to restore off‐axis variation of beam profile. In general the EPID qualifies as a useful tool for planar dose measurements.

An EGSnrc investigation of the PTP correction factor for ion chambers in kilovoltage x rays

As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature-pressure correction factor (P(TP)). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P(TP) correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P(TP)-corrected chamber response deviated by over 2% relative to expected values for a 40 kV spectrum incident on a graphite thimble chamber at an air density typical of Mexico City. The relative deviation from the expected response was much worse for a large spherical graphite chamber, exceeding 16% at an air density of 0.6 kg/m3 (approximately 0.5 atm at 22 degrees C) for the same beam energy. The breakdown of the P(TP) correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co beams, the P(TP) correction factor behaved as expected. For day-to-day variations in pressure, only a negligible of the P(TP) correction factor was observed with low x-ray energies. Factors contributing to the breakdown of the P(TP) correction factor at low x-ray energies and large pressure variations, such as the range of electrons, the material of the wall, the chamber dimensions and air-photon interactions, are discussed in depth.

TH-C-230A-01: A Monte Carlo Investigation of the Temperature-Pressure Correction Factor for Kilovoltage X-Rays

Purpose: To investigate the validity of the standard temperature-pressure correction factor (PTP) for kilovoltage x-rays incident on various ionization chambers using Monte Carlo simulations of radiation transport. Method and Materials: The EGSnrc Monte Carlo computer code was used to calculate the response due to 20 kV, 40 kV and 60 kV beams as a function of chamber air density for thimble and spherical ionization chambers. The chambers studied had both graphite and C-552 plastic walls to investigate the effect of the wall material in addition to the dimensions of the cavity. In principle, the PTP-corrected response is independent of air density. Thus, a breakdown of the PTP correction factor is identified by any variation in the calculated response as the air density is varied. The air density associated with the reference temperature and pressure conditions in North America (22 °C, 101.325 kPa) is 1.205 kg/m3. Results: At an air density of 1.0 kg/m3, typical of Denver Colorado, the normalized PTP-corrected response of a graphite-walled thimble chamber due to the 20 kV and 40 kV spectra is as much as 1.7% and 1.2% below the expected response, respectively. For a graphite spherical chamber at the same air density, the calculated response is 3.8% below unity for 40 kV and 60 kV beam qualities. Calculated responses of chambers with C-552 plastic walls are all within 0.5% of the expected response at air densities as low as 0.84 kg/m3. Comparisons of calculated air kerma calibration coefficients at different air densities indicate that the breakdown of the PTP correction factor should be easily detected experimentally. Conclusion: Variations in the PTP-corrected response indicate that for low-energy x-rays the PTP correction factor inadequately accounts for the dependence of ion chamber response on the temperature and pressure. Additional correction factors are therefore necessary under these circumstances.

On the effective point of measurement in megavoltage photon beams

This paper presents a numerical investigation of the effective point of measurement of thimble ionization chambers in megavoltage photon beams using Monte Carlo simulations with the EGSNRC system. It is shown that the effective point of measurement for relative photon beam dosimetry depends on every detail of the chamber design, including the cavity length, the mass density of the wall material, and the size of the central electrode, in addition to the cavity radius. Moreover, the effective point of measurement also depends on the beam quality and the field size. The paper therefore argues that the upstream shift of 0.6 times the cavity radius, recommended in current dosimetry protocols, is inadequate for accurate relative photon beam dosimetry, particularly in the build-up region. On the other hand, once the effective point of measurement is selected appropriately, measured depth-ionization curves can be equated to measured depth-dose curves for all depths within +/- 0.5%.

Wall correction factors, , for thimble ionization chambers

The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall, for thimble ionization chambers in photon and electron beams. CSnrc calculated values of Pwall give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV. The sleeve effect, which is not currently taken into account by the TG-51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the Pwal1 correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show Pwall values of up to 0.6% at the reference depth, depending on the wall material. Pwall is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite-walled cylindrical Farmer-like chamber between a depth of 0.5 cm and R50 in a 6 MeV electron beam.

Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beams

Shielded p-silicon diodes, frequently applied in general photon-beam dosimetry, show certain imperfections when applied in the small photon fields occurring in stereotactic or intensity modulated radiotherapy (IMRT), in electron beams and in the buildup region of photon beam dose distributions. Using as a study object the shielded p-silicon diode PTW 60008, well known for its reliable performance in general photon dosimetry, we have identified these imperfections as effects of electron scattering at the metallic parts of the shielding. In order to overcome these difficulties a new, unshielded diode PTW 60012 has been designed and manufactured by PTW Freiburg. By comparison with reference detectors, such as thimble and plane-parallel ionization chambers and a diamond detector, we could show the absence of these imperfections. An excellent performance of the new unshielded diode for the special dosimetric tasks in small photon fields, electron beams and build-up regions of photon beams has been observed. The new diode also has an improved angular response. However, due to its over-response to low-energy scattered photons, its recommended range of use does not include output factor measurements in large photon fields, although this effect can be compensated by a thin auxiliary lead shield.

The use of plane-parallel chambers in electron dosimetry without any cross-calibration

Current dosimetry protocols from AAPM, DIN and IAEA recommend a cross-calibration for plane-parallel chambers against a calibrated thimble chamber for electron dosimetry. The rationale for this is the assumed chamber-to-chamber variation of plane-parallel chambers and the large uncertainty in the wall perturbation factor (pwall60Co)pp at 60Co for plane-parallel chambers.We have confirmed the results of other authors that chamber-to-chamber variation of the investigated chambers of types Roos, Markus, Advanced Markus and Farmer is less than 0.3%. Starting with a calibration factor for absorbed dose to water and on the basis of the three dosimetry protocols AAPM TG-51, DIN 6800-2 (slightly modified) and IAEA TRS-398, values for (pwall60Co)Roos of 1.024 ± 0.005, (pwall60Co)Markus of 1.016 ± 0.005 and (pwall60Co)Advanced Markus of 1.014 ± 0.005 have been determined. In future this will permit electron dosimetry with the above-listed plane-parallel chambers having a calibration factor ND,w60Co without the necessity for cross-calibration against a thimble chamber.

The design and evaluation of a phantom for the audit of the treatment chain for prostate radiotherapy

Background and Purpose: A phantom has been designed and built for a multi-institutional technique audit of the planning and delivery for radiotherapy to the prostate. The phantom was designed to test both the geometric and dosimetric accuracy of each aspect of the process. Materials and Methods: The phantom consists of two curved water filled perspex tanks either side of a central block of solid water equivalent material. There are two options for the central section; a target defining block and a dose measurement block. The target defining block uses air holes to define a 3-D target volume for imaging via a CT scanner or a simulator. These holes can subsequently be filled with steel pins to allow megavoltage imaging. The dose measurement block allows thimble chamber measurements to be made at pre-selected points in a 5×5mm array. Five dose measurement points, typical for a prostate planning target volume (PTV) were selected. Initial evaluation of the phantom was performed by auditing the prostate radiotherapy planning and treatment chain at one institution. Results: Agreement between the phantom and planned geometry confirmed that the stages of image acquisition, transfer and manipulation were accurately performed. Agreement within 0.5% was found between phantom and water tank measurements for dose calibration at a reference point. The measured dose delivered was within 2% of the dose calculated by the planning computer for all of the selected measurement points. The target volume marked by the steel pins was visible using electronic portal imaging. Conclusions: The phantom is a useful tool for the technique audit of prostate radiotherapy.

Reference dosimetry for fast neutron and proton therapy

Fast neutrons and protons undergo fundamentally different interactions in tissue. The former interact with nuclei, while the latter, as in the case of photons, interact mainly with atomic electrons. Protons do, however, also undergo some nuclear reactions, the probability of which increases with proton energy. The determination of the reference absorbed dose in a patient for the two modalities is described. Two internationally accepted protocols have been formulated and have been published in ICRU reports. For both modalities the practical means for determining the reference absorbed dose in a patient involves the use of thimble ionization chambers calibrated in 60Co beams.

Dosimetry of rotational partial-skin electron irradiation

Background and purpose: Often, the most appropriate treatment for superficially and extensively spreading tumors of the skin is to use electron irradiation at enlarged distances. Rotational skin electron irradiation is a proven method for the treatment of the entire skin surface. We here report modifications of this technique in the set-up of partial-skin electron irradiation and the results of dosimetric examinations with regard to optimal shielding, dose profiles and depth dose curves under various irradiation conditions.Materials and methods: Irradiation was performed using electron beams with nominal energies of 6 MeV from a linear accelerator. The phantom was located on a rotating platform at a source-surface distance SSD=300 cm. A horizontal slit aperture (height: 32 cm) within a 2 cm thick polymethylmethacrylate (PMMA) shielding plate near the phantom was used to define the size of the irradiated region. Influences on dose distributions due to scattering processes on the PMMA edges were investigated using a flat ionization chamber and films. Absolute dose measurements and film calibration were made with the flat chamber. The quality of bremsstrahlung radiation behind the shielding was determined with a thimble ionization chamber in the phantom.Results and conclusions: The results of rotational partial-skin electron irradiation reveal some of the investigated shielding geometries to be optimal. Depth dose distributions and dose rates correspond to the results obtained in total skin electron rotational irradiation. It is possible to apply the dose superficially in the first millimeters of the skin; the dose maximum is located at a depth of 0–2 mm, the 80% isodose at 9 mm. The amount of bremsstrahlung contamination is 2.5%. The local amount of absorbed dose per monitor unit depends strongly on patient/phantom cross-section geometry. At our institute, rotational partial-skin electron irradiation was implemented into clinical routine in 1997.

Polarity effects of ionization chambers used in TBI dosimetry due to cable irradiation

This paper presents the results of an investigation on polarity effects in total-body irradiation (TBI) dosimetry. Thimble (NE2571, 0.6 cc) and plane-parallel (Markus NE2534 0.055 cc) chambers were investigated in a 30 × 30 × 30-cm 3 acrylic phantom in TBI conditions (6-MV x-rays). The thimble chamber was positioned at the midline and at the entrance and exit D max (1.5 cm from the phantom surface) levels. The Markus chamber, which is generally used for skin dose estimations, was positioned at various depths from the entrance surface of the phantom (from 0- to 2-cm depth). The polarity factor (P pol) was defined as (Q + + Q −)/2Q −, where Q + and Q − were the collected charges at positive and negative bias voltage, respectively. The variations of P pol with many parameters (absorbed dose, dose rate, the presence or absence of a 1-cm acrylic spoiler, irradiated cable length) were investigated. Results show that P pol is quite small (within 1.002 for on-axis measurements and 1.005 for off-axis measurements) for the NE2571 chamber when the beam spoiler is placed. P pol was significantly higher without the beam spoiler (within 1.008 for on-axis measurements, up to 1.02 for off-axis measurements). Concerning the Markus chamber, for on-axis skin dose measurements, P pol was found to be less than unity (around 0.988) or more than unity (around 1.0035), respectively, with and without the beam spoiler. Possible “directional effects” of the currents generated in the cable were investigated for both chambers and found to be insignificant. This shows that the application of P pol correction has to be considered a reliable procedure in minimizing these effects. When the beam spoiler is placed, the cable has to be drawn to minimize the portion of cable just outside the beam; if this is not the case, P pol may significantly vary (for the NE2571 chamber values up to 1.0035 were found for on-axis measurements).