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R van der Laarse

Publications and source records attributed to R van der Laarse.

At least 19 recordsLinked to original sources
PubMed2000

The combined use of the natural and the cumulative dose-volume histograms in planning and evaluation of permanent prostatic seed implants.

BACKGROUND AND PURPOSE: To investigate prostate dose coverage and overdosage in planned and realized permanent iodine seed prostate implants and to explore the use of the natural dose-volume histogram (NDVH) and the cumulative dose-volume histogram (CDVH) as tools to optimize prostate implants. MATERIALS AND METHODS: The optimal prescription dose (PD) or natural prescription dose (NPD) was derived from the NDVH. The mismatch between the NPD and the given PD was called the natural dose ratio (NDR). For an ideal implant the NDR should be 1. The target is overdosed if NDR >1 and underdosed if NDR <1. The NDR and prostate coverage were evaluated in implants of nine patients. Prostate coverage was determined from the CDVH based on pre-implant ultrasound or post-implant MRI for the planned and realized implants, respectively. The use of the NDVH to further optimize the planned prostate implants was also explored. RESULTS: The mean values of the NDRs were 1.30+/-0.34 (range 0.76-1.79), 1.22+/-0.31 (0.76-1.74) and 1.22+/-0.12 (0.98-1.33) for the planned, realized and optimized seed distributions, respectively. The realized prostatic implants showed smaller prostate coverage than the planned implants. The prostate volume fractions receiving 100% of the prescription dose were V(100)=79+/-6% and V(100)=97+/-3% for the realized and the planned implants, respectively. CONCLUSIONS: The NDVH and the CDVH proved to be valuable tools in plan evaluation. The NDVH and its derived parameter NDR quantify the risk of under or overdosage for a given PD. The CDVH is valuable in evaluation of prostate coverage realized prostate. Our strategy to implant just the prostate and not the prostate plus a margin led to NDR values between 1.1 and 1.3 and a prostate coverage of V(100)=79+/-6% in the nine patients. The planned coverage of V(100)=95% was not realized, mainly due to inadequate coverage of the base of the prostate.

Brachytherapy↗
PubMed2000

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.

Models, Theoretical↗
PubMed1999

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.

Cobalt Radioisotopes↗
PubMed1999

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).

Electrons↗
PubMed1998

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.

Biophysical Phenomena↗
PubMed1997

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.

Mathematical Computing↗
PubMed1997

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.

Calibration↗
PubMed1997

CT-based software for 3-D localization and reconstruction in stepping source brachytherapy.

This paper describes innovative software for catheter localization and three-dimensional (3-D) reconstruction in stepping source brachytherapy applications. Patient information is a set of computed tomography (CT) slices scanned during the implantation of brachytherapy catheters. Catheter geometry and patient anatomy are exported for use with dose calculation software modules. The errors produced by the system are also encouragingly low. Time saving was achieved, in terms of other traditional reconstruction techniques. Various automated procedures, 3-D graphics and a user-friendly GUI, have contributed to providing a powerful, comprehensive software tool, directly useable in the clinical practice.

Brachytherapy↗
PubMed1991

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.

Dose-Response Relationship, Radiation↗
PubMed1988

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.

Brachytherapy↗
PubMed1986

[Standardized low-dose-rate afterloading therapy of cervical cancer using a ring applicator].

At our hospital, the Munich method has already proved its value in gynecologic brachytherapy. In order to use this method for the afterloading technique (LDR), too, we developed a computer program for a standardized therapy with the circular applicator. A certain arrangement of sources is indicated for all applicator sizes. So the dose prescribed (point A) as well as the treatment times and the maximum rectum doses can be given in tabular form. Furthermore, another reduction of the exposure of the rectum could be achieved by means of a certain symmetric arrangement of the sources. To our opinion, this standardized treatment scheme allows in most cases a sufficiently exact definition of the dose distribution in the pelvis minor. If necessary, a more individualized irradiation planning can be established later using CT scans.

Brachytherapy↗
PubMed1985

Relation between local cure and dose-time-volume factors in interstitial implants.

The dose and volume data of 119 patients treated with radium or iridium-192 implants for cancer of the tongue, bladder or perineum are presented. The computer dosimetry system used in the Antoni van Leeuwenhoek Hospital permitted analysis of data regarding treated volume, dose variation inside the treated volume, patterns of geographical defects and their impact on the result of treatment. Dose was expressed in a number of ways, including prescribed (reference) and average dose and the CRE (Cumulative Radiation Effect) level attained. The CRE value included a time correction and a volume correction. The average dose proved to be the best predictor of local result in case of tongue and perineal implants. Great care is needed in case of tongue carcinoma to avoid a geographical miss by an inadequate treatment volume or geometric defects. The hypothesis that a dose reduction factor is necessary, in case of a high dose rate, could not be validated.

Brachytherapy↗
PubMed1985

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.

Brachytherapy↗
PubMed1984

Wedge filters for megavoltage roentgen ray beams.

The aim of this investigation, to construct a range of fixed wedge filters and to simulate these with a motorized wedge, led to the derivation of 5 equations. These equations can be used to construct and test a consistent set of fixed wedge filters, eliminating elaborate trial and error experiments. The fixed wedge filters already in existence for the Philips SL75-10 and SL75-20 linear accelerators fitted these equations rather well. After adapting the motorized wedge of the SL75-14 according to these equations, it simulated the fixed wedges accurately.

Equipment Design↗