Out-of-field dose measurements in radiotherapy – An overview of activity of EURADOS Working Group 9: Radiation protection in medicine

This review of dosimetry for second cancer risk estimation introduces work carried out by Working Group 9 (WG9: Radiation Protection Dosimetry in Medicine) of the European Radiation Dosimetry Group (EURADOS). The work concentrates on the measurement of out-of-field doses in water phantoms using a variety of dosimeters to measure photon and neutron doses. These include optically stimulated luminescence (OSL), radiophotoluminescence (RPL) and thermoluminescence (TLD) dosimeters for photon dosimetry (together with ion chambers for reference measurements) and track etch and superheated emulsion detectors for neutron measurements. The motivation of WG 9 was to assess undue, non-target patient doses in radiotherapy and the related risks of second malignancy. Improvements in cancer treatment have increased survival times and thus increased incidence of second cancer may be expected in the future. In addition, increased whole body exposure may result from some developments in radiotherapy. This means that radiotherapy clinics will need to simulate their treatments in order to estimate and minimise doses to healthy tissues and organs. The proposed work is designed to generate a robust dataset of out-of-field dose measurements which can be used for the development and validation of dose algorithms.     

Superheated emulsions and track etch detectors for photoneutron measurements

This paper describes the criteria behind the selection of neutron detection techniques for photoneutron dosimetry as well as the methods adopted to obtain dosimetric readouts. The work was conducted within the framework of Working Group 9 (WG9 Radiation Protection Dosimetry in Medicine), coordinated by the European Radiation Dosimetry Group (EURADOS). WG9 research aims at estimating the risk of second cancer induction due to radiation therapy. Therefore, a comprehensive experimental programme was devised to measure doses received by non-target organs-at-risk (OAR) during radiation therapy. The techniques described in this work were selected and used for the neutron dosimetric assessment during in-phantom simulations of clinical prostate radiotherapy treatments, carried out in three European facilities. Non-conformal standard fields were used as a common reference between different facilities.Performing neutron measurements near linacs is a complex task, because of the intense pulsed photon primary field. Therefore, photon insensitive dosimeters such as superheated emulsions (SE) and solid state nuclear track detectors (SSNTD) were chosen. Their readout procedures were carefully assessed. Methods were developed to count the large number of tracks and bubbles in SE. These are described in detail in the present work, along with a brief introduction to the detector physics.     

Introduction to dosimetry and risk estimation of second cancer induction following radiotherapy

This brief review of dosimetry in second cancer dosimetry introduces work carried out by Working Group 9 (Radiation Protection Dosimetry in Medicine) of the European Radiation Dosimetry Group (EURADOS). The work described in the following papers in this edition was presented at a Workshop on Dosimetry for Second Cancer Risk Estimation given at the EURADOS Annual meeting in Vienna on February 8th 2012. The work concentrates on the measurement of out-of-field doses in water tanks and BOMAB-like phantoms using a variety of dosimeters to measure photon and neutron doses. These include optically stimulated luminescence (OSL), radiophotoluminescence (RPL) and thermoluminescence (TLD) dosimeters for photon dosimetry (together with ion chambers for reference measurements traceable to primary standards) and track etch and bubble detectors for neutron measurements. A discussion of the various phantoms available for these measurements is presented together with a brief introduction to a model for the relationship between organ doses and the risk of induction of second cancers. The estimation of second cancer risks is not trivial and involves processes which are currently incompletely understood. However, progress in this field requires a robust foundation and methodology for the measurement or calculation of organ doses following radiotherapy, so that risks can be placed in perspective, algorithms for out-of-field doses can be compared with measured data, and future epidemiological studies may have a reliable foundation of organ dosimetry for retrospective dosimetry studies.     

Photon dosimetry methods outside the target volume in radiation therapy: Optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry

Dosimetry methods outside the target volume are still not well established in radiotherapy. Luminescence detectors due to their small dimensions, very good sensitivity, well known dose and energy response are considered as an interesting approach in verification of doses outside the treated region. The physical processes of thermoluminescence (TL), radiophotoluminescence (RPL) and optically stimulated luminescence (OSL) are very similar and can be described in terms of the energy band model of electron-hole production following irradiation.This work is a review of the main dosimetric characteristics of luminescence detectors which were used in experiments performed by EURADOS Working Group 9 for in-phantom measurements of secondary radiation (scattered and leakage photons). TL LiF:Mg,Ti detectors type MTS-7 (IFJ PAN, Poland), types TLD-100 and TLD-700 (Harshaw), OSL Al₂O₃:C detectors type nanoDot™ (Landauer Inc.) and RPL rod glass elements type GD-352M (Asahi Techno Glass Coorporation) are described. The main characteristics are discussed, together with the readout and calibration procedures which lead to a determination of absorbed dose to water.All dosimeter types used show very good uniformity, batch reproducibility and homogeneity. For improved accuracy, individual sensitivity correction factors should be applied for TL and OSL dosimeters while for RPL dosimeters there is no need for individual sensitivity corrections.The dose response of all dosimeters is linear for a wide range of doses.The energy response of GD-352M type dosimeters (with Sn filter) used for out-of-field measurements is flat for medium and low energy X-rays.The energy dependence for TLDs is low across the range of photon energies used and the energy correction was neglected. A significant over response of Al₂O₃:C OSLDs irradiated in kilovoltage photon beams was taken into account. The energy correction factor f_en was calculated by using the 2006 PENELOPE Monte Carlo code.With suitable calibration, all dosimeter types are appropriate for out-of-field dose measurements as well as for the in-phantom measurements of radiotherapy MV X-rays beams.     

Peripheral doses in radiotherapy: A comparison between IMRT,VMAT and Tomotherapy

The goal of this intercomparison is to determine the peripheral doses during treatment of prostate and head and neck (H&N) cancers. In the case of prostate cancer, two different treatment techniques are compared: intensity-modulated radiation therapy (IMRT – 10 MV and 18 MV), on a Varian Clinac 2100 C/D and Tomotherapy. VMAT (also on a Varian Clinac 2100 C/D) was compared to Tomotherapy, for H&N cancer. The treatment devices are located at the university hospitals of Leuven and Brussels, respectively. A common treatment protocol was agreed between the two clinical centers and this same protocol was used by each partner. For the higher energy modalities (10 MV and 18 MV) we also assessed the neutron contribution to the total dose, by using bubble detectors. In this way, the performance (in terms of peripheral doses) of the different treatment techniques, when faced with the same dose distribution constraints, was evaluated. The doses were evaluated with an anthropomorphic phantom loaded with TLD detectors. Summarizing our results, we can conclude that low energy radiation techniques, namely VMAT and Tomotherapy, have more interesting performances when compared to IMRT at energies of 10 MV and 18 MV, with respect to peripheral dose. On the one hand the former are associated with lower photon doses and, on the other hand, there is no contribution from neutrons to the total dose.     

Radiotherapy out-of-field dosimetry: Experimental and computational results for photons in a water tank

The first objective of this work was to check and select a set of four kinds of passive photon, dosimeters (two thermo-luminescence dosimeter (TLD) types, one radiophotoluminescence (RPL) dosimeter and one optically stimulated luminescence (OSL) dosimeter) together with a common measurement protocol. Dosimeters were calibrated in a reference clinical linear acccelerator beam in a water tank at a reference facility at the Laboratoire National Henri Becquerel (CEA LIST/LNE LNHB, Saclay. Radiation qualities of 6, 12 and 20 MV were used with standard calibration conditions described in IAEA TRS 398 and non-standard conditions. Profile and depth dose ion chamber measurements were also made to provide reference values. Measurements were made in a water tank into which pipes could be inserted which held dosimeters in pre-determined and reproducible positions. The water tank was built to enable investigation of doses up to 60 cm from the beam axis. A first set of experiments was carried out with the beam passing through the tank. From this first experiment, penumbra and out-of-field dose profiles including water and collimator scatter and leakage were found over three orders of magnitude. Two further sets of experiments using the same experimental arrangement with the beam outside the tank, to avoid water scatter, were designed to measure collimator scatter and leakage by closing the jaws of the collimator. Depending on the energy, typical leakage and collimator scatter represents 10–40% and 30–50% of the total out-of-field doses respectively. It was concluded that all dosimeters can be used for out-of-field photon dosimetry. All show good uniformity, good reproducibility, and can be used down to low doses expected at distances remote from the subsequent radiotherapy target volume.     

Intercomparison of Gafchromic™ films,TL detectors and TL foils for the measurements of skin dose in interventional radiology

Several passive solid state dosemeters, such as Gafchromic™ films and thermoluminescence (TL) detectors, are used to estimate and monitor patient skin doses in interventional radiology. To determine the suitability of XR-TypeR Gafchromic™ films and of detectors based on TL materials: pellets, chips and foils to measure skin dose, an intercomparison exercise has been organized within European Dosimetry Radiation Group – Working Group 12 “European Medical ALARA Network” (EURADOS WG12). To test response detectors were exposed to X-ray beams of energies and qualities applied clinically. A blind test was also performed to investigate the accuracy of the dose estimate by detectors exposed to unknown doses. We found the response of films to be strongly dependent on beam quality and filtration (increasing by up to 80% with respect to reference beam quality). The response of TL detectors was found to be less dependent on beam quality (less than 25% variation), with TL foils showing less than 10% variation with respect to reference beam quality. To accurately estimate patient skin doses in interventional radiology it is important to choose the quality of the calibration beam to be as close as possible to the quality of beams actually applied in clinical work.     

Evaluation of the applicability of MOSFET detectors in radiotherapy

MOSFET detectors of 1 mVcGy⁻¹ sensitivity were tested for the accuracy of absorbed dose measurements in radiation therapy with the use of photon and electron beams. Before a detector was used in the study, several calibration coefficients were determined to allow for different factors affecting its operation. Then, the detector exposure response was compared with the dose calculated in the anthropomorphic phantom by the Monaco and MasterPlan treatment planning systems. MOSFET detectors were placed inside the phantom during the irradiation. Three different plans for thorax and pelvis areas were studied. The paper presents the differences between planned and MOSFET measured doses delivered to the selected target areas using conventional and IMRT techniques.     

Sensitization of the analytical methods for photoneutron calculations to the wall concrete composition in radiation therapy

The effect of wall material on photoneutron production in radiation therapy rooms was studied using Monte Carlo (MC) simulations. An analytical formula was proposed to take into account the concrete composition in photoneutron dose calculations. Using the MCNPX MC code, the 18 MV photon beam of the Varian Clinac 2100 and a typical treatment room with concrete compositions according to report No. 144 of National Council of Radiation Protection (NCRP) were simulated. Number of room produced photoneutrons per Gray of X-ray at the isocenter was determined for different types of concrete and named as “Q_W”. This new factor was inserted in the used formula for photoneutron fluence calculations at the inner entrance of maze. The photoneutron fluence was calculated using new proposed formula at the inner entrance of maze for all studied concretes. The difference between conventional and proposed equations varied from 11% to 46% for studied concretes. It was found that room produced photoneutrons could be significant for high density concretes. Additionally, applying the new proposed formula can consider the effect of wall material composition on the photoneutron production in high energy radiation therapy rooms. Further studies to confirm the accuracy of newly developed method is recommended.     

Performance comparison of OSLD (Al2O3:C) and TLD (LiF:Mg,Cu,P) in accreditation proficiency testing

The U.S. Navy uses a dosimetric system, which employs the LiF:Mg,Cu,P thermoluminescence dosimeters (TLDs), developed and produced by Thermo Fisher Scientific. Every two years, the Naval Dosimetry Center (NDC) performs proficiency testing to maintain its national accreditation. Since 2007, the U.S. Navy has also tested InLight Basic - OSLN Optically Stimulated Luminescence Al₂O₃:C dosimeters (OSLD) manufactured by Landauer. In 2011 and 2013, the Naval Dosimetry Center performed proficiency testing for both systems. Here we present a comparison of the performance of TLDs (LiF:Mg,Cu,P) and OSLDs (Al₂O₃:C) in five categories of proficiency testing. The testing included irradiation with photons, neutrons, beta particles and selected mixtures of these radiations. All irradiations were performed at the Pacific Northwest National Laboratory (PNNL). The delivered doses were not reported to the NDC. The official comparison of delivered and reported doses was conducted by PNNL in terms of dose bias and its standard deviation for each category of accreditation. In total, the NDC reported to the PNNL doses for 147 dosimeters of each type (TLD and OSLD). Both NDC tested dosimetric systems have passed established limits. The comparison of OSLD and TLD system performance in each category is discussed. Advantages and disadvantages of both systems are analyzed.     

Measurement of rectum dose by in vivo alanine/ESR dosimetry in gynecological 192Ir HDR brachytherapy

High dose rate (HDR) brachytherapy (BT) used in treatments of gynecological cancer often results in high doses in the pelvic organs at risk (OARs) and the complications in the rectum are a serious concern. Dosimetry procedures in vivo can be used as an evaluation method of calculated dose in treatment planning. One dosimetric method is the use of alanine with electron spin resonance (ESR) that has been used in different clinical practices. The aim of this study was to indicate the dose level in the female rectum volume, using alanine dosimeters during ¹⁹²Ir HDR gynecological BT, for cervical cancer. Doses were compared with the values obtained using the computational treatment planning system based on two orthogonal radiographic images. Firstly a phantom study in water was performed, enabling the in vivo study. Ten patients had the dose in rectum measured, resulting from 10 points properly referred; variations found were in the range of +60% and −50% of the delivered doses compared to the treatment planning system. Differences between planned and measured doses can be mainly due to uncertainty of dosimeter position determination, averaging of dose points specified over the whole dosimeter position, uncontrolled changes in detector position during treatment due to rectum movement and to a simplified treatment system planning, that do not take into account the details of the patient anatomy and the difference among the tissues. Results show that improvements of the protocol treatment should be done to enhance the relation between treatment planning system and experimental results, nevertheless the dose at the OARs was lower than the recommended by the ICRU Report 38.     

In-phantom neutron dose distribution for bladder cancer cases treated with high-energy photons

This work presents an estimation of the neutron dose distribution for common bladder cancer cases treated with high-energy photons of 15 MV therapy accelerators. Neutron doses were measured in an Alderson phantom, using TLD 700 and 600 thermoluminescence dosimeters, resembling bladder cancer cases treated with high-energy photons from 15 MV LINAC and having a treatment plan using the four-field pelvic box technique. Thermal neutron dose distribution in the target area and the surrounding tissue was estimated. The sensitivity of all detectors for both gamma and neutrons was estimated and used for correction of the TL reading. TLD detectors were irradiated with a Co⁶⁰ gamma standard source and thermal neutrons at the irradiation facility of the National Institute for Standards (NIS). The TL to dose conversion factor was estimated in terms of both Co⁶⁰ neutron equivalent dose and thermal neutron dose. The dose distribution of photo-neutrons throughout each target was estimated and presented in three-dimensional charts and isodose curves. The distribution was found to be non-isotropic through the target. It varied from a minimum of 0.23 mSv/h to a maximum of 2.07 mSv/h at 6 cm off-axis. The mean neutron dose equivalent was found to be 0.63 mSv/h, which agrees with other published literature. The estimated average neutron equivalent to the bladder per administered therapeutic dose was found to be 0.39 mSv Gy^?1, which is also in good agreement with published literature. As a consequence of a complete therapeutic treatment of 50 Gy high-energy photons at 15 MV, the total thermal neutron equivalent dose to the abdomen was found to be about 0.012 Sv.     

Franco-Russian comparison of mixed neutron and gamma radiation field dosimeters at the Silène reactor

This paper gives the results of dosimetry measurements carried out in the Silène reactor at Valduc (France) with neutron and photon dosimeters in mixed neutron and gamma radiation fields, in the frame of a Franco-Russian comparison of dosimeters. Neutron dosimetry was supplied by passive semiconductors, activation detectors and nuclear track detectors. For photon dosimetry, thermoluminescent and passive semiconductor detectors were used. The experiments were located at 3 m from the reactor core, in free air and also at the front and back of a tissue-equivalent phantom. The pulse operating mode of the reactor was used to simulate a criticality accident with solid fissile material, while the free evolution mode simulated a criticality accident in a fissile solution. The photon absorbed dose showed a slight increase on entering the phantom compared to measurements in free air, probably due to backscattering by the phantom. At the rear of the phantom, the neutron kerma was four times lower than on the front, whereas the photon dose was only two times lower. The heterogeneity of dose inside the phantom was far greater for neutrons than for photons.     

New developments on electrochemical etching processes at the atomic energy organization of Iran

Some highlights of new developments made in our laboratory at the Atomic Energy Organization of Iran on chemical and electrochemical etching (ECE) of polymer track detectors like polycarbonate (PC) are presented. They include introduction of new ECE chamber systems and methods for production of ECE signs and symbols, and a new versatile ECE chamber (VECE) system for multi-purpose, multi-size, and/or multi-shape detector processing; determination of photoneutron doses in and around high-energy X-ray beams of a 20 MV medical accelerator; verification of the Smythe and Mason equations for ECE of tracks in polymers; ECE of alpha and recoil tracks in PC using PMW, PEW and PEMW etchants; introduction of a novel method using ethylene diamine for treatment of PC detectors with its applications, for example in precision removal of surface layers of PC (e.g. bulk removal rates of about 0.04, 0.15, 0.36, 0.66, and 1.33 mm min⁻¹ for 60%, 65%, 70%, 75% and 80% ethylene diamine solution (v/v) in water respectively, with no effects on its transparency), and in significant reduction of background track density of PC detectors, in alpha energy discrimination and alpha spectrometry; and development of an image processing system for track counting and measurements; etc. Some main results are reviewed and discussed.     

Photon contributions from the 252Cf and 241Am–Be neutron sources at the PSI Calibration Laboratory

At the accredited PSI Calibration Laboratory neutron reference fields traceable to the national standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany are available for the calibration of ambient and personal dose equivalent (rate) meters and passive dosimeters. The photon contribution to the ambient dose equivalent in the neutron fields of the ²⁵²Cf and ²⁴¹Am–Be sources was measured using various photon dose rate meters and active and passive dosimeters. Measuring photons from a neutron source usually involves considerable uncertainties due to the presence of neutron induced photons in the room, due to a non-zero neutron sensitivity of the photon detector, and last but not least due to the energy response of the photon detectors. Therefore eight independent detectors and methods were used to obtain a reliable estimate for the photon contribution of the two sources as an average of the individual methods. For the ²⁴¹Am–Be source a photon contribution of approximately 4.9% was determined and for the ²⁵²Cf source a contribution of 3.6%.     

The comparison of the proton dose distribution calculated with the treatment planning system and measured with alanine detectors in the eye phantom irradiated under therapeutic conditions

The paper describes the applicability of commercially available alanine detectors produced by Synergy Health for verification of the dose distribution calculated by the treatment planning system (TPS) used in proton eye radiotherapy – Eclipse Ocular Proton Planning (EOPP) program, version 8.9.06, Varian Medical Systems. The TPS-planned dose distribution at selected points in the eye phantom is compared to the dose registered by alanine detectors at these points during a simulated therapeutic irradiation at the proton eye radiotherapy facility in the Henryk Niewodniczanski Institute of Nuclear Physics (IFJ PAN), Krakow, Poland. The phantom was irradiated to obtain, a typical for choroidal melanoma, fraction dose of 15 CGE (13,64 Gy) at the tumor location. The dose registered with alanine pellets located inside the simulated tumor volume demonstrates a good agreement with the TPS-planned dose. The typical for proton radiotherapy, steep dose fall-off outside the treated area is registered by the alanine pellets however, it is difficult to assess it quantitatively, because the dose related EPR signal is registered from the entire pellet volume.     

In-phantom dose verification of prostate IMRT and VMAT deliveries using plastic scintillation detectors

The goal of this work was to demonstrate the feasibility of using a plastic scintillation detector (PSD) incorporated into a prostate immobilization device to verify doses in vivo delivered during intensity-modulated radiation therapy (IMRT) and volumetric modulated-arc therapy (VMAT) for prostate cancer. The treatment plans for both modalities had been developed for a patient undergoing prostate radiation therapy. First, a study was performed to test the dependence, if any, of PSD accuracy on the number and type of calibration conditions. This study included PSD measurements of each treatment plan being delivered under quality assurance (QA) conditions using a rigid QA phantom. PSD results obtained under these conditions were compared to ionization chamber measurements. After an optimal set of calibration factors had been found, the PSD was combined with a commercial endorectal balloon used for rectal distension and prostate immobilization during external beam radiotherapy. This PSD-enhanced endorectal balloon was placed inside of a deformable anthropomorphic phantom designed to simulate male pelvic anatomy. PSD results obtained under these so-called “simulated treatment conditions” were compared to doses calculated by the treatment planning system (TPS). With the PSD still inserted in the pelvic phantom, each plan was delivered once again after applying a shift of 1 cm anterior to the original isocenter to simulate a treatment setup error.The mean total accumulated dose measured using the PSD differed the TPS-calculated doses by less than 1% for both treatment modalities simulated treatment conditions using the pelvic phantom. When the isocenter was shifted, the PSD results differed from the TPS calculations of mean dose by 1.2% (for IMRT) and 10.1% (for VMAT); in both cases, the doses were within the dose range calculated over the detector volume for these regions of steep dose gradient. Our results suggest that the system could benefit prostate cancer patient treatment by providing accurate in vivo dose reports during treatment and verify in real-time whether treatments are being delivered according to the prescribed plan.     

Incident contamination lepton doses measured using radiochromic film in radiotherapy

Measurement of lepton contamination is achieved across a radiotherapy photon beam and peripheral doses using radiochromic film. An extrapolation technique is used where several layers are suspended in air to measure incident contamination without the effects of phantom scatter. Surface dose was measured as 11% of D_max for 6 MV beams at central axis and 9% for 10 MV photons for a 10×10 cm field size. Peripheral lepton doses were found to decrease compared to central, however, were still measurable. Peripheral lepton dose was found to increase with field size and was 12% and 15%, 2 cm outside the geometric field edge of a 30×30 cm field size at 6 and 10 MV respectively. Radiochromic film is a suitable dosimeter for measurement of lepton contamination absorbed dose to surface layers of skin.     

Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters

Lithium magnesium phosphate LiMgPO₄ (LMP) doped with Tb and B is one of new materials intended for use in optically stimulated luminescence (OSL) dosimetry. LMP doped with Tb and B luminophors were synthetized at IFJ PAN in Krakow. The investigations were carried out on self-developed dosimeters consisting of a slide with four LMP detectors and a light tight cover. LMP detectors were investigated in regard to their OSL properties using OSL reader named HELIOS adopted to the readouts of dosimetric cards. New LMP detectors showed high sensitivity to the ionizing radiation, good repeatability of OSL signal and good dose response, 25% of fading in the first two weeks after irradiation. Also, the pronounced dependence of OSL response on the energy of the measured radiation requires to apply the compensation filters.     

TRUS-probe integrated MOSkin detectors for rectal wall in vivo dosimetry in HDR brachytherapy: In phantom feasibility study

The increasing complexity and high amount of dose per fraction delivered in prostate high dose rate (HDR) brachytherapy treatments call for the implementation of accurate and effective methods for the systematic and independent quality control of the overall treatment procedure. In this study, MOSkin detectors were placed on a trans-rectal ultrasound (TRUS) probe with the aim of performing both imaging and real time rectal wall in vivo dosimetry with the use of just one single instrument. After an adequate calibration of the detectors, which was carried out in a solid water phantom, the use of MOSkins integrated to the TRUS probe was studied in a gel phantom with a typical (simplified) prostate implant. Measured and calculated doses from the treatment planning system were compared, with a resulting very low average discrepancy of −0.6 ± 2.6%. The results are very promising and of particular clinical importance, however, further in vivo investigation is planned to validate the proposed method.