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.

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.

Consistency in quality control programmes for electron accelerators in radiotherapy centres.

BACKGROUND AND PURPOSE: To gain insight into the current practice of quality control (QC) of medical electron accelerators and to reduce possible variations in test frequencies and test procedures. MATERIALS AND METHODS: An extensive questionnaire on QC procedures of medical electron accelerators was distributed and completed by all (21) radiotherapy institutions in The Netherlands. The questions were related to safety systems, mechanical parameters, beam profiles, beam energy, absolute dosimetry, wedge filters, the dose monitor system and radiation leakage. The data of the questionnaire were compared with recommendations given in national and international reports on QC of electron accelerators. RESULTS: Large variations in time spent on QC exist, especially for accelerators having dual energy photon beams and several electron beam energies. This diversity is mainly due to differences in philosophy with regard to QC and the differences in resources and machine time available. Furthermore, large variations in test frequencies and test methodologies were observed. The staffing level involved in the QC measurements was evaluated and compared with recent recommendations provided by EFOMP-ESTRO. CONCLUSIONS: From these recommendations and the results of the questionnaire, a set of minimum guidelines for a QC programme could be formulated and implemented in all radiotherapy institutions in The Netherlands.

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.

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.

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 intercomparison at the radiotherapy centers in The Netherlands. 2. Accuracy of locally applied computer planning systems for external photon beams.

As part of a quality assurance program in The Netherlands, the performance of computer planning systems was tested. Relative dose values, determined with an ionization chamber, were compared with dose values obtained from locally applied computer planning systems. Several clinically relevant situations were investigated: perpendicular incident beams, wedged beams, oblique incident beams, variable source-surface distances (SSD) and off-axis planes. The mean value of the ratios of calculated to measured dose values is 0.994, with an uncertainty of 2.4% (1 S.D.) and a maximum deviation of 9%, for all combinations of energies, planning systems and geometries investigated. The uncertainty for each situation separately was less than 2% (1 S.D.), except for the wedged beams and off-axis plane, which showed uncertainties of 2.6% (1 S.D.). Part of the additional uncertainty for the wedged beams originates from the value chosen for the wedge factor. Systematic deviations between calculated and measured dose values were investigated for three commercially available planning systems, separately. The mean deviation was smaller than 1% (1 S.D.), for most situations. Only for the wedged beams, larger deviations, up to a mean deviation of 2.6%, were observed.

Dose intercomparison at the radiotherapy centres in The Netherlands. 1. Photon beams under reference conditions and for prostatic cancer treatment.

In 1985, a dosimetry intercomparison was performed at all 20 radiotherapy centres in The Netherlands. Absorbed dose was determined with an ionization chamber under reference conditions in a water phantom for cobalt-60 gamma-ray and megavoltage X-ray beams. The mean difference between measured and stated dose values was 0.5% with a standard deviation of 1.9%, but up to 6% at maximum. As soon as all institutes apply a common dosimetry protocol, this maximum difference will reduce to about 2%. In addition, an anthropomorphic phantom was irradiated to simulate the treatment of a prostatic cancer. The dose, determined with an ionization chamber at the isocentre and thermoluminescent dosimeters (TLD) powder at several points situated in the target volume, the bladder and the rectum, was compared with the stated dose calculated with the local planning system. Only small differences were found between the measured and stated dose at the isocentre: on the average 1.5%, with a standard deviation of 1.5%. The difference between stated and measured dose at several points situated in the target volume was on the average 0.4%, with a standard deviation of 5.2%. Almost the same result was found for a point situated in the bladder. In the rectum, the average difference was about 4%, however, with a large standard deviation, 18%, due to the relatively steep dose gradient at these points.

The "Ring" method: a semi-empirical method to calculate dose distributions of irregularly shaped photon beams.

The "Ring" method provides a fast dose calculation and isodose presentation for photon beams with blocks. The method takes into account the change in scatter due to the blocks at each calculation point. Firstly, the dose in a point is calculated assuming that no blocks are present. Secondly, the scatter reduction caused by the blocks is calculated and subtracted. To determine the scatter reduction the irradiated surface is divided in concentric rings around a point at the surface at the intersection with a ray line between focus and calculation point. The scatter reduction caused by blocks for each ring is calculated. The effect of scatter for rings with an outer radius greater than 15 cm where the scatter contribution is less than 1.0% is neglected. Results of the method for 4 MV photons using eight rings are presented. Comparison of dose measurements with calculations in an arrow-shaped photon field showed maximum deviations of 4.0%, using the IRREG program of Cunningham, 6.5% using the BLKINP program of Schlegel, which is based on Clarkson's method, 5.0% using the method of Wrede and 2.2% using the "Ring" method. Contrary to the first two calculation programs, the programs using the last two calculation methods provide isodose lines dose values at points.

Investigation of changes in CT-number in the prostate after radiotherapy.

From October 1980 until September 1982, 60 patients with a prostatic carcinoma were studied with computed tomography (CT). Evaluation of the CT numbers was one of the purposes. Patients were scanned before, directly after, 3 months after and 6 months after radiotherapy. Complete data were available for 41 patients. The mean CT number in the prostate from the first 32 patients without reference measurements showed no systematic changes. For the last 9 patients, reference measurements were carried out to eliminate a possible effect of machine fluctuations. For all patients the conclusion remained the same: no significant changes occur in the mean CT number in the prostate before and after radiation of the pelvis.

Determination of the accuracy of different computer planning systems for treatment with external photon beams.

The performance of computer planning systems has been tested by comparing calculations using the local beam data and computer facilities with measurements at the local installations. Relative absorbed dose distributions have been determined in a water phantom, irradiated with megavoltage photon beams with qualities from 60Co up to 25 MV. Three clinically relevant situations were studied: oblique incidence, tangential beams and wedged fields. For 45 degrees oblique incidence the mean deviation between the calculated and measured relative absorbed dose was less than 1%. Individual deviations, however, ranged from -2% up to +7%. A systematic difference, due to a straightforward application of the modified effective SSD method, was observed. For tangential irradiation the planning systems which do not consider the lack of scattering material showed deviations up to 8% between calculated and measured relative absorbed dose. For wedged beams, especially when they impinge obliquely on the phantom surface, differences were found up to 10%. Differences up to 20% were found for a point in the build-up region of an obliquely impinging wedged beam. From this study it can be concluded that planning systems may produce clinically unacceptable errors.

SIRAD: a program for automatic dosimetry and data transfer for radiotherapy planning.

A program is described which performs automatic dosimetry. The measured data is stored in such a way that it can be used immediately by a treatment planning system.

Evaluation of Cartesian coordinates and radiation doses in points determined with stereo x-ray techniques.

A Fortran computer program STEV (stereo evaluation) is described. The principles of the stereo techniques together with the calculation method of the stereo coordinates are given briefly. The determination of the rectangular coordinates from mean stereo coordinates is described. Radiation doses in anatomical points, during intracavitary and interstitial radiation therapy, are calculated, taking into account a statistical evaluation of the measurement errors.