A.S. Guerra

Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse

The correction for charge recombination was determined for different plane-parallel ionization chambers exposed to clinical electron beams with low and high dose per pulse, respectively. The electron energy was nearly the same (about 7 and 9 MeV) for any of the beams used. Boags two-voltage analysis (TVA) was used to determine the correction for ion losses, k(s), relevant to each chamber considered. The presence of free electrons in the air of the chamber cavity was accounted for in determining k(s) by TVA. The determination of k(s) was made on the basis of the models for ion recombination proposed in past years by Boag, Hochhäuser and Balk to account for the presence of free electrons. The absorbed dose measurements in both low-dose-per-pulse (less than 0.3 mGy per pulse) and high-dose-per-pulse (20-120 mGy per pulse range) electron beams were compared with ferrous sulphate chemical dosimetry, a method independent of the dose per pulse. The results of the comparison support the conclusion that one of the models is more adequate to correct for ion recombination, even in high-dose-per-pulse conditions, provided that the fraction of free electrons is properly assessed. In this respect the drift velocity and the time constant for attachment of electrons in the air of the chamber cavity are rather critical parameters because of their dependence on chamber dimensions and operational conditions. Finally, a determination of the factor k(s) was also made by zero extrapolation of the 1/Q versus 1/V saturation curves, leading to the conclusion that this method does not provide consistent results in high-dose-per-pulse beams.

Dosimetric characteristics of electron beams produced by a mobile accelerator for IORT

Energy and angular distributions of electron beams with different energies were simulated by Monte Carlo calculations. These beams were generated by the NOVAC7 system (Hitesys, Italy), a mobile electron accelerator specifically dedicated to intra-operative radiation therapy (IORT). The electron beam simulations were verified by comparing the measured dose distributions with the corresponding calculated distributions. As expected, a considerable difference was observed in the energy and angular distributions between the IORT beams studied in the present work and the electron beams produced by conventional accelerators for non-IORT applications. It was also found that significant differences exist between the IORT beams used in this work and other IORT beams with different collimation systems. For example, the contribution from the scattered electrons to the total dose was found to be up to 15% higher in the NOVAC7 beams. The water-to-air stopping power ratios of the IORT beams used in this work were calculated on the basis of the beam energy distributions obtained by the Monte Carlo simulations. These calculated stopping power ratios, s(w,air), were compared with the corresponding s(w,air) values recommended by the TRS-381 and TRS-398 IAEA dosimetry protocols in order to estimate the deviations between a dosimetry based on generic parameters and a dosimetry based on parameters specifically obtained for the actual IORT beams. The deviations in the s(w,air) values were found to be as large as up to about 1%. Therefore, we recommend that a preliminary analysis should always be made when dealing with IORT beams in order to assess to what extent the possible differences in the s(w,air) values have to be accounted for or may be neglected on the basis of the specific accuracy needed in clinical dosimetry.

Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes

High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane–parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%.

A Joint Research Project to improve the accuracy in dosimetry of brachytherapy treatments in the framework of the European Metrology Research Programme.

This paper outlines the joint research project “Increasing cancer treatment efficacy using 3D brachytherapy” co-funded in the FP7, according to the iMERA-Plus Grant Agreement No. 217257 between the EC and EURAMET e. V. (European Association of National Metrology Institutes). The project brings together the capabilities of ten major European National Metrology Institutes in the ionizing radiation field and it is focused on the targeted programme “Health”. It aims at establishing across Europe a more accurate metrological basis in brachytherapy by developing methods for the direct measurement of the quantity absorbed dose to water in brachytherapy dosimetry with an uncertainty on the dose delivered to the target volume less than 5% (k=1) at clinical level. In fact, in the current brachytherapy practice, the procedures to determine the absorbed dose imparted to the patient are affected by an uncertainty that could reduce the treatment success. Most of this uncertainty is due to a lacking metrology. No absorbed-dose primary standards are so far available to assure direct traceability in dosimetry of brachytherapy sources. In order to optimize the brachytherapy treatments there is also a need for more accurate dosimetry with high spatial resolution. The present research project is expected to increase the accuracy of brachytherapy to a level comparable to that typical of radiotherapy with external accelerator beams.

A graphite calorimeter for absolute measurements of absorbed dose to water: application in medium-energy x-ray filtered beams

The Italian National Institute of Ionizing Radiation Metrology (ENEA-INMRI) has designed and built a graphite calorimeter that, in a water phantom, has allowed the determination of the absorbed dose to water in medium-energy x-rays with generating voltages from 180 to 250 kV. The new standard is a miniaturized three-bodies calorimeter, with a disc-shaped core of 21 mm diameter and 2 mm thickness weighing 1.134 g, sealed in a PMMA waterproof envelope with air-evacuated gaps. The measured absorbed dose to graphite is converted into absorbed dose to water by means of an energy-dependent conversion factor obtained from Monte Carlo simulations. Heat-transfer correction factors were determined by FEM calculations. At a source-to-detector distance of 100 cm, a depth in water of 2 g cm(-2), and at a dose rate of about 0.15 Gy min(-1), results of calorimetric measurements of absorbed dose to water, D(w), were compared to experimental determinations, D wK, obtained via an ionization chamber calibrated in terms of air kerma, according to established dosimetry protocols. The combined standard uncertainty of D(w) and D(wK) were estimated as 1.9% and 1.7%, respectively. The two absorbed dose to water determinations were in agreement within 1%, well below the stated measurement uncertainties. Advancements are in progress to extend the measurement capability of the new in-water-phantom graphite calorimeter to other filtered medium-energy x-ray qualities and to reduce the D(w) uncertainty to around 1%. The new calorimeter represents the first implementation of in-water-phantom graphite calorimetry in the kilovoltage range and, allowing independent determinations of D(w), it will contribute to establish a robust system of absorbed dose to water primary standards for medium-energy x-ray beams.

Heat loss through connecting thermistor wires in a three-body graphite calorimeter

The main aim of this paper is to calculate the small but significant amount of heat lost from a graphite calorimeter absorber through connecting thermistor wires during electrical calibration. Taking into account the electro-thermal interaction between the heating thermistor and its surrounding environment, a more realistic approach to the problem was developed and estimative numerical results were obtained. It was found that the wires contribute about 0.01% in extracting heat from the calorimeter core (which corresponds to a correction factor kwcore = 0.9999). The total correction factor for heat loss through the connecting thermistor wires during the electrical calibration of the calorimeter (the total combined effect of the heater and the sensor leads due to conduction, radiation and Joule effect) was determined: kw = 0.9989.

Comparison between small radiation therapy electron beams collimated by Cerrobend and tubular applicators

The purpose of this study was to compare the dosimetric properties of small field electron beams shaped by circular Cerrobend blocks and stainless steel tubular applicators. Percentage depth dose curves, beam profiles, and output factors of small‐size circular fields from 2 to 5 cm diameter, obtained either by tubular applicators and Cerrobend blocks, were measured for 6, 10, and 15 MeV electron beam energies. All measurements were performed using a PTW microDiamond 60019 premarket prototype. An overall similar behavior between the two collimating systems can be observed in terms of PDD and beam profiles. However, Cerrobend collimators produce a higher bremsstrahlung background under irradiation with high‐energy electrons. In such irradiation condition, larger output factors are observed for tubular applicators. Similar dosimetric properties are observed using circular Cerrobend blocks and stainless steel tubular applicators at lower beam energies. However, Cerrobend collimators allow the delivery of specific beam shapes, conformed to the target area. On the other hand, in high‐energy irradiation conditions, tubular applicators produce a lower bremsstrahlung contribution, leading to lower doses outside the target volume. In addition, the higher output factors observed at high energies for tubular applicators lead to reduced treatment times. PACS number: 87.53.Bn, 87.55.Qr, 87.56.Fc

Corrigendum: Experimental determination of the dose rate constant for selected 125I- and 192Ir-brachytherapy sources (2012 Metrologia 49 S219–22)

The consensus value for Λ reported in the last column of table 2 should be Λ = 1.012 × 104 not Λ = 1.021 × 104.

Feasibility of using a dose-area product ratio as beam quality specifier for photon beams with small field sizes

PURPOSE To investigate the feasibility of using the ratio of dose-area product at 20 cm and 10 cm water depths (DAPR20,10) as a beam quality specifier for radiotherapy photon beams with field diameter below 2 cm. METHODS Dose-area product was determined as the integral of absorbed dose to water (Dw) over a surface larger than the beam size. 6 MV and 10 MV photon beams with field diameters from 0.75 cm to 2 cm were considered. Monte Carlo (MC) simulations were performed to calculate energy-dependent dosimetric parameters and to study the DAPR20,10 properties. Aspects relevant to DAPR20,10 measurement were explored using large-area plane-parallel ionization chambers with different diameters. RESULTS DAPR20,10 was nearly independent of field size in line with the small differences among the corresponding mean beam energies. Both MC and experimental results showed a dependence of DAPR20,10 on the measurement setup and the surface over which Dw is integrated. For a given setup, DAPR20,10 values obtained using ionization chambers with different air-cavity diameters agreed with one another within 0.4%, after the application of MC correction factors accounting for effects due to the chamber size. DAPR20,10 differences among the small field sizes were within 1% and sensitivity to the beam energy resulted similar to that of established beam quality specifiers based on the point measurement of Dw. CONCLUSIONS For a specific measurement setup and integration area, DAPR20,10 proved suitable to specify the beam quality of small photon beams for the selection of energy-dependent dosimetric parameters.

Study of a CVD diamond detector for absorbed dose measurement in photon beams with small field sizes

A new diamond detector to perform absorbed-dose- to-water measurement in radiotherapy photon beams with small field sizes is being developed in the framework of the EURAMET/EC FP7 project “External Beam Cancer Therapy”. An objective of the project is to obtain detectors capable of ensuring traceability of absorbed dose measurements in radiotherapy photon beams with field size down to 1 cm x 1 cm. To this end the CVD diamond detectors up to now developed were found not adequate. The present project requires detectors with high spatial resolution, good stability and low energy dependence. Diamond detectors were chosen for their high spatial resolution and good tissue equivalence (Z = 6). The detector studied in this work is based on a single crystal CVD diamond with dimensions 3 mm x 3 mm x 0.5 mm on which 0.2 μm electrodes were thermally evaporated. Particular care was addressed to ensure the tissue-equivalence of the detector by using appropriate materials. A thorough analysis of the priming procedure, stability and signal reproducibility was carried out in a Co-60 gamma beam at dose rates in the range from 0.3 Gy min− 1 to 1.38 Gy min− 1. Moreover the detector response was studied by Monte Carlo calculations as a function both of the beam quality, from Co-60 to 10 MV photon beams, and field size, from 10 cm x 10 cm to 1 cm x 1 cm. The perturbation due to the non-water equivalence of electrodes was also determined by Monte Carlo simulations.