R. van der Laarse

The determination of phantom and collimator scatter components of the output of megavoltage photon beams: measurement of the collimator scatter part with a beam-coaxial narrow cylindrical phantom.

The separation of the total scatter correction factor Sc,p in a collimator scatter component, Sc, and a phantom scatter component, Sp, has proven to be an useful concept in megavoltage photon beam dose calculations in situations which differ from the standard treatment geometry. A clinically applicable method to determine Sc is described. Measurements are carried out with an ionization chamber, placed at a depth beyond the range of contaminant electrons, in a narrow cylindrical polystyrene phantom with a diameter of 4 cm of which the axis coincides with the beam axis. Sc,p is measured in a full-scatter phantom and Sp can be derived from Sc,p and Sc. In order to obtain a reliable separation, i.e. excluding the influence of contaminant electrons, measurements of Sc,p have been carried out at depths of 5 cm for photon beams with a quality index (QI) up to and including 0.75 and a depth of 10 cm with QI larger than 0.75. These depths are in accordance with recommendations given in recent dosimetry protocols. The consistency of the method was checked by comparing calculated and measured values of Sc,p for a set of blocked fields for a range of photon beam energies from 60Co up to 25 MV showing a maximum deviation of 2%. The method can easily be implemented in existing procedures for the calculation of the number of monitor units to deliver a specified dose to a target volume.

Design and evaluation of a HDR skin applicator with flattening filter

The purposes of this study are: (i) to design field flattening filters for the Leipzig applicators of 2 and 3 cm of inner diameter with the source traveling parallel to the applicator contact surface, which are accessories of the microSelectron-HDR afterloader (Nucletron, Veenendaal, The Netherlands). These filters, made of tungsten, aim to flatten the heterogeneous dose distribution obtained with the Leipzig applicators. (ii) To estimate the dose rate distributions for these Leipzig+filter applicators by means of the Monte Carlo (MC) method. (iii) To experimentally verify these distributions for prototypes of these new applicators, and (iv) to obtain the correspondence factors to measure the output of the applicators by the user using an insert into a well chamber. The MC GEANT4 code has been used to design the filters and to obtain the dose rate distributions in liquid water for the two Leipzig+filter applicators. In order to validate this specific application and to guarantee that realistic source-applicator geometry has been considered, an experimental verification procedure was implemented in this study, in accordance with the updated recommendations of the American Association of Physicists in Medicine Task Group No. 43 U1 Report. Thermoluminescent dosimeters, radiochromic film, and a pin-point ionization chamber in a plastic [polymethylmethacrylate (PMMA)] phantom were used to verify the MC results for the two applicators of a microSelectron-HDR afterloader with the mHDR-v2 source. To verify the output of the Leipzig +filter applicators, correspondence factors were deduced for the well chambers HDR100-plus (Standard Imaging, Inc., Middleton, WI) and TM33004 (PTW, Freiburg, Germany) using a specific insert for both applicators. The doses measured in the PMMA phantom agree within experimental uncertainties with the dose obtained by the MC calculations. Percentage depth dose and off-axis profiles were obtained normalized at a depth of 3 mm along the central applicator axis in a cylindrical 20 x 20 cm water phantom. A table of output factors, normalized to 1 U of source air kerma strength at this depth, is presented. Correspondence factors were obtained for the two well chambers considered. The matrix data obtained in the MC simulation with a grid separation of 0.5 mm has been used to build a data set in a convenient format to model these distributions for routine use with a brachytherapy treatment planning system.

Calculation of electron beam dose distributions for arbitrarily shaped fields

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is presented. Isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.

Screens in ovoids of a selectron cervix applicator

The addition of screens in the vaginal source holders of a cervix applicator for intracavitary brachytherapy reduces the dose to rectum and bladder and therefore diminishes the number of rectal and vesical complications. Shielding properties of tungsten rectal and bladder screens of a Selectron cervix applicator, loaded with spherical cesium sources, were determined for verification of dose calculations. Transmission characteristics of half-disk shaped tungsten screen segments in a single ovoid were measured in a water phantom. The minimum transmission ratios are 60, 70 and 80% for segment thickness of 5.0, 3.5 and 2.0 mm, respectively. The accuracy of the new screen correction algorithm of the Selectron Planning System was assessed by comparing measured and calculated dose rates and was found to be better than +/- 4%. The correction algorithm provides a method to analyse the efficacy of screens in the ovoids for various segment geometries and orientations without extensive phantom measurements. Isotransmission and isodose calculations were made for a typical clinical applicator set-up and source distribution. The dose reduction to rectum and bladder, near the bottom and top of the ovoids was analysed in detail. A 3.5 mm thick rectum and bladder screen in each ovoid reduces the dose approximately by 20% to the rectum and by 15% to the bladder. A distance enlargement of about 5 mm between ovoid and rectum or bladder, e.g. by packing, results in a comparable dose reduction. Shielding properties of a Selectron cervix applicator, provided with screens, were compared with those of some Fletcher-type applicators. Significant differences between the transmission ratios and shielded areas of the screens of both systems near rectum and bladder were observed.

Wall-Scattering Effects in Electron Beam Collimation

This report describes now a set of applicators, convering fields with dimensions of 4 to 20 cm, for the 6 to 20 MeV electron beams of a MEL SL75-20 linear accelerator was developed. The electron scatter contribution of the applicator walls to the treatment field was investigated, varying the applicator entrance opening and the scattering foil, with the aim of optimizing the resulting field flatness, with a minimum loss of depth dose. Experiments with field defining end frames and additional perspex scatterers for large field sizes are also reported.

Technique for routine output verification of Leipzig applicators with a well chamber

The H-type Leipzig applicators are accessories of the microSelectron-HDR system (Nucletron, Veenendaal, The Netherlands) for treatment of superficial malignancies. Recently, the dose rate distributions in liquid water for the whole set of applicators using both source models available for the microSelectron-HDR afterloaders have been obtained by means of the experimentally validated Monte Carlo (MC) code GEANT4. Also an output table (cGy/hU) at 3 mm depth on the applicator central axis was provided. The output verification of these applicators by the user, prior to their clinical use, present practical problems: small detectors such as thermoluminescent dosimeters or parallel-plate ionization chambers are not easily used for verification in a clinical environment as they require a rigid setup with the Leipzig applicator and a phantom. In contrast, well-type ionization chambers are readily available in radiotherapy departments. This study presents a technique based on the HDR1000Plus well chamber (Standar Imaging) measurements with a special insert, which allows the output verification of the H-type Leipzig applicators on a routine basis. This technique defines correspondence factors (CF) between the in water dose rate output of the Leipzig applicators (cGy/hU) obtained with MC and the reading on the well chamber with the special insert, normalized to the HDR calibration factor with the HDR insert and to the source strength. To commission the applicators (with the well chamber and the special insert used), the physicist should check if the CF value agrees with its tabulated values presented in this work. If the differences are within 5% the tabulated output values can be used in clinical dosimetry. This technique allows the output validation of the Leipzig applicators with a well chamber widely used for HDR Ir-192 source strength measurements. It can easily be adapted to other types of well chambers for HDR source output verification.

Dosimetric characterization of Ir‐192 LDR elongated sources

Ir-192 wires have been used in low-dose-rate brachytherapy for many years. Commercially available treatment planning systems approximate the dose rate distribution of the straight or curved wires applying the superposition principle using one of the following methods: (i) The wire is modeled as a set of point sources, (ii) the wire is modeled as a set of small straight segment wires, (iii) the values of the parameters and functions of the American Association of Physicists in Medicine (AAPM) Task Group 43 protocol are obtained for wire lengths between 3 and 7 cm assuming some simplifications. The dose rate distributions obtained using these methods for linear wires of different lengths and U-shaped wires present significant deviations compared to those obtained by Monte Carlo. In the present study we propose a new method to model 192Ir wires of any length and shape, named the Two Lengths based Segmented method. This method uses the formalism stated in the AAPM Task Group 43 protocol for two straight wires only, 0.5 and 1 cm, to obtain the dose rate distribution around wires of any length (down to 0.3 cm and up to 10 cm) improving on the results of the aforementioned ones. This method can easily be applied to dose calculations around other wires, such as Pd-103 ones.

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.

Discrepancies in dose and dose specification in interstitial implants

The first step in the execution of an interstitial implant is the decision on size and location of the target volume. Several implant systems, e.g. the Paterson-Parker system and the Paris system, give instructions for the optimal arrangement of sources to assure that the planned target volume is adequately covered. They also give guidelines to calculate the reference dose rate encompassing the planned target volume. These systems provide different solutions for the source arrangement for the same planned target volume, and vice versa, resulting in different reference dose rates. The problem of dose specification is discussed. For a number of theoretical implants predicted reference dose rates for the planned target volume were compared with the computer calculated dose rates for that volume. Discrepancies increase when moderate digressions from the adopted implant system rules are allowed, such as could commonly occur clinically. For a number of examples the degree of change in dose rate, if over 10%, and the position where this deviation is likely to occur are described. For optimal results the clinician should be well aware of these variations.

Design and characterization of a new high-dose-rate brachytherapy Valencia applicator for larger skin lesions

PURPOSE The aims of this study were (i) to design a new high-dose-rate (HDR) brachytherapy applicator for treating surface lesions with planning target volumes larger than 3 cm in diameter and up to 5 cm in size, using the microSelectron-HDR or Flexitron afterloader (Elekta Brachytherapy) with a (192)Ir source; (ii) to calculate by means of the Monte Carlo (MC) method the dose distribution for the new applicator when it is placed against a water phantom; and (iii) to validate experimentally the dose distributions in water. METHODS The penelope2008 MC code was used to optimize dwell positions and dwell times. Next, the dose distribution in a water phantom and the leakage dose distribution around the applicator were calculated. Finally, MC data were validated experimentally for a (192)Ir mHDR-v2 source by measuring (i) dose distributions with radiochromic EBT3 films (ISP); (ii) percentage depth-dose (PDD) curve with the parallel-plate ionization chamber Advanced Markus (PTW); and (iii) absolute dose rate with EBT3 films and the PinPoint T31016 (PTW) ionization chamber. RESULTS The new applicator is made of tungsten alloy (Densimet) and consists of a set of interchangeable collimators. Three catheters are used to allocate the source at prefixed dwell positions with preset weights to produce a homogenous dose distribution at the typical prescription depth of 3 mm in water. The same plan is used for all available collimators. PDD, absolute dose rate per unit of air kerma strength, and off-axis profiles in a cylindrical water phantom are reported. These data can be used for treatment planning. Leakage around the applicator was also scored. The dose distributions, PDD, and absolute dose rate calculated agree within experimental uncertainties with the doses measured: differences of MC data with chamber measurements are up to 0.8% and with radiochromic films are up to 3.5%. CONCLUSIONS The new applicator and the dosimetric data provided here will be a valuable tool in clinical practice, making treatment of large skin lesions simpler, faster, and safer. Also the dose to surrounding healthy tissues is minimal.