Radiation transmission data for radionuclides and materials relevant to brachytherapy facility shielding.

To address the limited availability of radiation shielding data for brachytherapy as well as some disparity in existing data, Monte Carlo simulation was used to generate radiation transmission data for 60Co, 137CS, 198Au, 192Ir 169Yb, 170Tm, 131Cs, 125I, and 103pd photons through concrete, stainless steel, lead, as well as lead glass and baryte concrete. Results accounting for the oblique incidence of radiation to the barrier, spectral variation with barrier thickness, and broad beam conditions in a realistic geometry are compared to corresponding data in the literature in terms of the half value layer (HVL) and tenth value layer (TVL) indices. It is also shown that radiation shielding calculations using HVL or TVL values could overestimate or underestimate the barrier thickness required to achieve a certain reduction in radiation transmission. This questions the use of HVL or TVL indices instead of the actual transmission data. Therefore, a three-parameter model is fitted to results of this work to facilitate accurate and simple radiation shielding calculations.

A dosimetric study on the Ir-192 high dose rate flexisource.

In this work, the dose rate distribution of a new Ir-192 high dose rate source (Flexisource used in the afterloading Flexitron system, Isodose Control, Veenendaal, The Netherlands) is studied by means of Monte Carlo techniques using the GEANT4 code. The dosimetric parameters of the Task Group No. 43 Report (TG43) formalism and two-dimensional rectangular look-up tables have been obtained.

Quality assurance of EORTC trial 22922/10925 investigating the role of internal mammary--medial supraclavicular irradiation in stage I-III breast cancer: the individual case review.

To assess consistency among participants in an European Organisation for Research and Treatment of Cancer (EORTC) phase III trial randomising between irradiation and no irradiation of the internal mammary and medial supraclavicular (IM-MS) lymph nodes, all participating institutes were invited to send data from 3 patients in each arm as soon as they started accrual. The evaluation focused on eligibility, compliance with the radiotherapy guidelines, treatment techniques and dose prescription to the IM-MS region. Nineteen radiotherapy departments provided a total of 111 cases, all being eligible. Minor discrepancies were found in the surgery and pathology data in almost half the patients. Major radiotherapy protocol deviations were very limited: 2 cases of unwarranted irradiation of the supraclavicular region and a significant dose deviation to the internal mammary region in 5 patients. The most frequently observed minor protocol deviation was the absence of delineation of the target volumes in 80% of the patients. By detecting systematic protocol deviations in an early phase of the trial, recommendations made to all the participating institutes should improve the interinstitutional consistency and promote a high-quality treatment.

Verification of dose calculations with a treatment planning system for open and wedged fields of a 60Co unit with a new asymmetric collimator.

A Cobalt-60 treatment unit was equipped with a new collimator with asymmetrical capability of both the X-and Y-jaws. This new collimator design opens possibilities for treatment techniques with this apparatus, which, until now, were only achievable with linear accelerators. Before accepting the device and taking it into clinical routine, a dosimetric verification was performed, which compared results of dose measurements with the results of dose calculations of the treatment planning system. For this purpose, the approach of Report 55 of the AAPM Task Group 23 for testing a treatment planning system was followed, with modifications to comply with the asymmetrical settings of the fields. The study shows that criteria for acceptance of the treatment planning algorithm were met for the asymmetrical open fields and for asymmetrical fields with a 22 degrees and a 45 degrees wedge. However, deviations tended to be too high under the thick part of the thickest wedge.

Formalisms for MU calculations, ESTRO booklet 3 versus NCS report 12.

Although the relevance and importance of quality assurance and quality control in radiotherapy is generally accepted, only recently, methods for monitor unit (MU) calculation and verification have been addressed in recognized recommendations, published by the European Society of Therapeutic Radiation Oncology (ESTRO) and by the Netherlands Commission on Radiation Dosimetry (Dutreix A, Bjärngard BE, Bridier A, Mijnheer B, Shaw JE, Svensson H. Monitor unit calculation for high-energy photon beams. Physics for clinical radiotherapy. ESTRO Booklet No. 3. Leuven: Garant, 1997; Netherlands Commission on Radiation Dosimetry (NCS). Determination and use of scatter correction factors of megavoltage photon beams. NCS report 12. Deift: NCS, 1998). Both documents are based on the same principles: (i) the separation of the output factor into a head and a volume (or phantom) scatter component; (ii) the use of a so-called mini-phantom to measure and verify the head scatter component; and (iii) the recommendation to use a single reference depth of 10 cm for all photon beam qualities. However, there are substantial differences between the approach developed in the IAEA-ESTRO task group and the NCS approach for MU calculations, which might lead to confusion and/or misinterpretation if both reports are used simultaneously or if data from the NCS report is applied in the algorithms of the ESTRO report without careful consideration. The aim of the present paper is to discuss and to clearly point out these differences (e.g. field size definitions, phantom scatter parameters, etc.). Additionally, corresponding quantities in the two reports are related where possible and several aspects concerning the use of a mini-phantom (e.g. size, detector position, composition) are addressed.

Tolerances for the accuracy of photon beam dose calculations of treatment planning systems.

BACKGROUND AND PURPOSE: To design a consistent set of criteria for acceptability of photon beam dose calculations of treatment planning systems. The set should be applicable in combination with a test package used for evaluation of a treatment planning system, such as the ones proposed by the AAPM Task Group 23 or by the Netherlands Commission on Radiation Dosimetry. RESULTS: Tolerances have been defined for the accuracy with which a treatment planning system should be able to calculate the dose in different parts of a photon beam: the central beam axis and regions with large and small dose gradients. For increasing complexity of the geometry, wider tolerances are allowed, varying between 2 and 5%. For the evaluation of a large number of data points an additional quantity, the confidence limit, has been introduced, which combines the influence of systematic and random deviations. The proposed tolerances have been compared with other recommendations for a number of clinically relevant examples, showing considerable differences, which are partly due to the way the complexity of the geometry is taken into account. Furthermore differences occur if criteria for acceptability of dose calculations are related either to the local dose value or to a normalized dose value. CONCLUSIONS: Although it is acknowledged that the general aim must be to have good agreement between dose calculation and the actual dose value, e.g. within 2% or 2 mm, current day algorithms and their implementation into commercial treatment planning systems result often in larger deviations. A high accuracy can at present only be achieved in relatively simple cases. The new set of tolerances and the quantity confidence limit have proven to be useful tools for the acceptance of photon beam dose calculation algorithms of treatment planning systems.

Application of a test package in an intercomparison of the photon dose calculation performance of treatment planning systems used in a clinical setting.

BACKGROUND AND PURPOSE: Testing the performance of treatment planning systems by using the AAPM Task Group 23 test package is a useful approach, but has its limitations. To be able to include technical developments, such as the asymmetric collimator, it was decided to remeasure the AAPM data set on more modern radiotherapy equipment, to extend the test geometries, and to evaluate the use of the new package. MATERIALS AND METHODS: A coherent set of beam data of 6, 10 and 18 MV photon beams was measured on two modern linear accelerators. These data served as input data in seven commercially available treatment planning systems, which were clinically in use in different radiotherapy departments. Next, a test package was measured which included a missing tissue geometry and fields with asymmetrical collimator setting, with and without a wedge. RESULTS: The absolute dose prediction from the different treatment planning systems in which the measured beam data were entered, was compared for all test points with the results of direct measurements. The criteria of acceptability were exceeded by some systems in cases of irregular field geometry and missing tissue geometry. The majority of the systems had difficulties with accurate dose calculation for asymmetrically wedged fields. CONCLUSIONS: The application of the new test package did not introduce insuperable difficulties and was highly appreciated by the participating centres. Most systems performed reasonably well for the majority of the beam geometries, with the exception of asymmetrically wedged beams. The extended test package is available for other users or user groups for the purpose of commissioning new treatment planning systems, or new releases of existing systems.

Determination of the accuracy of implant reconstruction and dose delivery in brachytherapy in The Netherlands and Belgium.

PURPOSE: To gain insight into the accuracy of brachytherapy treatments, the accuracy of implant reconstruction and dose delivery was investigated in 33 radiotherapy institutions in The Netherlands and Belgium. MATERIALS AND METHODS: The accuracy of the implant reconstruction method was determined using a cubic phantom containing 25 spheres at well-known positions. Reconstruction measurements were obtained on 41 brachytherapy localizers, 33 of which were simulators. The reconstructed distances between the spheres were compared with the true distances. The accuracy of the dose delivery was determined for high dose rate (HDR), pulsed dose rate (PDR) and low dose rate (LDR) afterloading systems using a polymethyl methacrylate cylindrical phantom containing a NE 2571 ionization chamber in its centre. The institutions were asked to deliver a prescribed dose at the centre of the phantom. The measured dose was compared with the prescribed dose. RESULTS: The average reconstruction accuracy was -0.07 mm (+/-0.4 mm, 1 SD) for 41 localizers. The average deviation of the measured dose from the prescribed dose was +0.9% (+/-1.3%, 1 SD) for 21 HDR afterloading systems, +1.0% (+/-2.3%, 1 SD) for 12 PDR afterloaders, and +1.8% (+/-2.5%, 1 SD) for 15 LDR afterloaders. CONCLUSIONS: This comparison showed a good accuracy of brachytherapy implant reconstruction and dose delivery in The Netherlands and Belgium.

The potential impact of treatment variations on the results of radiotherapy of the internal mammary lymph node chain: a quality-assurance report on the dummy run of EORTC Phase III randomized trial 22922/10925 in Stage I--III breast cancer(1).

PURPOSE: To present the results of the dummy run of the European Organization for Research and Treatment of Cancer (EORTC) trial investigating the role of adjuvant internal mammary and medial supraclavicular (IM-MS) irradiation in Stage I--III breast cancer. METHODS AND MATERIALS: All participating institutions were asked to produce a treatment plan without (Arm 1) and with (Arm 2) simultaneous IM-MS irradiation of 1 patient after mastectomy and of 1 patient after lumpectomy. Thirty-two dummy runs have been evaluated for compliance to protocol guidelines, with respect to treatment technique and dose prescription. RESULTS: A number of more or less important deviations in treatment setup and prescription have been found. The dose in the IM-MS region deviated significantly from the prescribed dose in 10% of the cases for Arm 1, and in 21% for Arm 2. Assuming a true 5% 10-year survival benefit from optimal IM-MS irradiation, an increase of only 3.8% will be found due to this suboptimal dose distribution. CONCLUSION: In the dummy run, a number of potential systematic protocol deviations that might lead to false-negative results were detected. By providing recommendations to the participating institutions, we expect to improve the interinstitutional consistency and to promote a high quality irradiation in all institutions participating in the trial.

Dependence of the tray transmission factor on collimator setting and source-surface distance.

When blocks are placed on a tray in megavoltage x-ray beams, generally a single correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assumed to be independent of field size and source-surface distance (SSD). Analysis of a set of measurements performed in beams of 13 different linear accelerators demonstrates that there is, however, a slight variation of the tray transmission factor with field size and SSD. The tray factor changes about 1.5% for collimator settings varying between 4x4 cm and 40 x 40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The variation with field size is smaller if the source-surface distance is increased. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerating potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source-surface distance can easily be taken into account in the dose calculation by considering the volume of the irradiated tray material and the position of the tray in the beam. A relation is presented which can be used to calculate the numerical value of the tray transmission factor directly. These calculated values can be checked with only a few measurements using a cylindrical beam coaxial miniphantom.

Effect of electron contamination on scatter correction factors for photon beam dosimetry.

Physical quantities for use in megavoltage photon beam dose calculations which are defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimator and phantom scatter correction factors, Sc and Sp, defined at a reference depth of 10 cm. The data can be obtained from measurements at that depth in a miniphantom and in a full scatter phantom. Equations are presented that show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown that conversion of Sc and Sp determined at a 10 cm depth to quantities defined at the dose maximum such as (normalized) peak scatter factor, (normalized) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in Sc at dmax resulting from this electron contamination compared with Sc values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, Scel, for a number of photon beams of different accelerator types. It is shown that Scel may vary up to 5%. Because in the new formalisms output factors are defined at a reference depth of 10 cm, they do not require Scel data. The use of Sc and Sp values, defined at a 10 cm depth, combined with relative depth-dose data or tissue-phantom ratios is therefore recommended. For a transition period the use of the equations provided in this article and Scel data might be required, for instance, if treatment planning systems apply Sc data normalized at d(max).

A single-variable method for the derivation of collimator scatter correction factors in symmetrical and asymmetrical X-ray beams.

Using a minimal set of measured data, the collimator scatter correction factor of an asymmetrical collimated rectangular field (X1,X2;Y1,Y2) can be calculated from the product of one-dimensional factors, in combination with a correction term: Sc(X1,X2;Y1,Y2) = S(cx1)(X1)S(cx2)(X2)S(cy1)(Y1)S(cy2)(Y2) + c delta(X;Y). Two forms of the function delta(X;Y) were investigated.

A consistent formalism for the application of phantom and collimator scatter factors.

A coherent system for the use of scatter correction factors, determined at 10 cm depth, is described for dose calculations on the central axis of arbitrarily shaped photon beams. The system is suitable for application in both the fixed source-surface distance (SSD) and in the isocentric treatment set-up. This is in contrast to some other proposals where only one of these approaches forms the basis of the calculation system or where distinct quantities and data sets are needed. In order to derive the relations in the formalism, we introduced a separation of the phenomena related to the energy fluence in air and to the phantom scatter contribution to the dose. Both are used relative to quantities defined for the reference irradiation set-up. It is shown that dose calculations can be performed with only one set of basic beam data, obtained at a reference depth of 10 cm. These data consist for each photon beam quality of measured collimator and phantom scatter correction factors, in combination with a set of (percentage/relative) depth-dose or tissue-phantom ratio values measured along the central axis of the beam. Problems related to measurements performed at the depth of maximum absorbed dose, due to the electron contamination of the beam, are avoided in this way. Collimator scatter correction factors are obtained by using a mini-phantom, while phantom scatter correction factors are derived from measurements in a full scatter phantom in combination with the results of the mini-phantom measurements. For practical reasons the fixed SSD system was chosen to determine the data. Then, dose calculations in a fixed SSD treatment set-up itself are straightforward. Application in the isocentric treatment set-up needs simple conversion steps, while the inverse approach, from isocentric to fixed SSD, is described as well. Differences between the two approaches are discussed and the equations for the conversions are given.

A solution for the treatment of small lesions using electron beams from a Saturne linear accelerator with continuous variable trimmers; dosimetrical aspects.

For treatment of superficial lesions with ionizing radiation, a number of treatment techniques can be used. We use a standardized technique by applying electron beams from GE-MS Saturne-41 linear accelerators; this also works in cases where very small target areas have to be irradiated. In our institute linear accelerators of this type are equipped with continuous variable trimmer systems. Very small electron fields show specific dosimetric problems, which in general necessitates performing measurements for each individual patient. A solution is presented, using a specially designed set of 17 templates, which can be placed on the skin of a patient. The trimmers are set to a predefined field size. Output factors, profiles, percentage depth dose (PDD) and dose outside the template are determined for each combination of template and beam energy. Look-up tables are produced, which can be used by the radiation oncologist and in a monitor unit calculation program.

A three-Gaussian fit of phantom scatter correction data.

The phantom scatter correction factor Sp of megavoltage photon beams can be accurately described using a three-Gaussian fit. The model leads to six parameters, with which Sp(r) is described as a smooth function of the field radius r for beam qualities in the range from 60Co up to 25 MV. The parameters allow Sp values to be calculated at intermediate beam energies and for any field shape. Calculated Sp(X, Y) values for rectangular fields (X, Y) can be subsequently used as reference values to compare with measured Sp(X, Y) values, for example when appraising a new beam.

Evaluation of late morbidity in patients with carcinoma of the uterine cervix following a dose rate change.

BACKGROUND: A retrospective analysis of late complications for patients with cervical cancer treated with two different brachytherapy schedules in one institute, using the French-Italian glossary. MATERIALS AND METHODS: From 1979 until 1986, a total of 77 patients were treated with external radiation followed by two intracavitary applications with the Gynatron Cs-137 afterloading (dose rate 0.54 Gy/h). After 1986, 66 patients received intracavitary applications with Selectron-LDR (dose rate 1.07 Gy/h). Because of the expected increase in complications with increasing dose rate, the dose per application was reduced from 25 Gy to 20 Gy. RESULTS: 49/77 late gastrointestinal and urinary complications were scored in the Gynatron group and 46/68 in the Selectron group. Actuarial estimates at 5 years showed 42% and 54.1% late gastrointestinal complications and 16.9% and 24.1% for late urinary complications in the patients treated with, respectively, the Gynatron and Selectron. CONCLUSIONS: Despite the dose reduction, there remains a clear dose rate influence on the late morbidity. Correction for this influence is essential.

Comparison of parametrization methods of the collimator scatter correction factor for open rectangular fields of 6-25 MV photon beams.

PURPOSE: To facilitate the use of the collimator scatter correction factor, Sc, parametrization methods that relate Sc to the field size by fitting were investigated. MATERIALS AND METHODS: Sc was measured with a mini-phantom for five types of dual photon energy accelerators with energies varying between 6 and 25 MV. Using these Sc-data six methods of parametrizing Sc for square fields were compared, including a third-order polynomial of the natural logarithm of the field size normalized to the field size of 10 cm2. Also five methods of determining Sc for rectangular fields were considered, including one which determines the equivalent field size by extending Sterling's method. RESULTS: The deviations between measured and calculated Sc-values were determined for all photon beams and methods investigated in this study. The resulting deviations of the most accurate method varied between 0.07 and 0.42% for square fields and between 0.26 and 0.79% for rectangular fields. A recommendation is given as to how to limit the number of fields for which Sc should be measured in order to be able to accurately predict it for an arbitrary field size.

Is there a need for a revised table of equivalent square fields for the determination of phantom scatter correction factors?

The use of the British Journal of Radiology (BJR) (supplement 17) tables of equivalent square fields for dose calculations is widespread. A revised version of the supplement was published recently, with a more elaborate discussion, but without changes in data given in these tables (Br. J. Radiol. suppl 25). The tables were generated for use in dose calculations, with relative beam data such as PDD, BSF, PSF, all with d(max) as the reference depth. However, the current philosophy in dose calculational methods is based on quantities defined at a reference depth, d(ref) = 10 cm, on a separation of phantom and head scatter, and on the use of the relative depth-dose or tissue-phantom ratios normalized at d(ref). By using these quantities as a starting point, problems at shallow depths related to the influence of contaminating electrons in the beam can be eliminated. Recently, a comprehensive set of phantom scatter factor data with d(ref) = 10 cm has been published for a set of square field sizes and a wide range of photon beam energies, showing that phantom scatter is a smoothly varying function of field size and quality index. It is not a priori evident that the conventional concept of equivalent squares for rectangular fields is also fully applicable for phantom scatter factors and phantom scatter related quantities at a depth of 10 cm. It was questioned whether or not new tables of equivalent square fields are needed for this purpose. In this paper, new tables have been constructed for four photon beam energies in the range of Co-60 to 25 MV (quality index from 0.572 to 0.783). The small differences between the outcome of these new tables allowed the construction of one averaged table of equivalent square fields. Phantom scatter factors were calculated for rectangular fields based on the use of the BJR table and on the use of the newly constructed tables and the differences were quantified. For Co-60 no improvements could be shown when using the new averaged table, but for beam energies of 6 to 10 MV small improvements of the order of 0.5 to 1.0% were found. For a higher beam energy of 25 MV the improvement is smaller. Deviations resulting from the BJR table are within the limits of accuracy as stated by the authors. Therefore, for clinical use, the continued use of the BJR table of equivalent squares for phantom scatter factors and phantom scatter related quantities of rectangular fields is justified, irrespective of photon beam energy.