Supplementary values of the dosimetric parameters kNR and Em for various types of detectors in 6 and 15 MV photon fields

The present communication broadens the data base for determinations of the non-reference condition correction factor k_NR needed in high-energy photon dosimetry to accomplish the use of various detectors under non-reference conditions. Following our previous strategy of calculating semiempirical values of k_NR and correlating them with the mean photon energy E_m at the point of measurement in a large water phantom, the values of E_m are now stated for 6 and 15 MV photon radiations of accelerators with and without flattening filters, square field sizes from 1 to 30 cm side length and depths from 0 to 28 cm. The unambiguity of the k_NR-E_m correlation is again confirmed and is quantified by fitting formulae for air-filled ionization chambers, TLD detectors and Si diodes. This survey provides a practicable access to the k_NR values, particularly for the non-water equivalent detectors to be used in small-field dosimetry     

A new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry – An experimental and Monte Carlo study

The frequently applied narrow and non-standard transverse dose profiles of intensity modulated photon-beam radiotherapy, lacking lateral secondary electron equilibrium, require the use of high-resolution dosimetry detectors, and small air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors. The present study focuses on the dosimetric properties of a novel cylindrical ionization chamber, the PTW Semiflex 3D 31021. The chamber's effective point of measurement was found to lie at (0.41 ± 0.04) r downstream the tip of the inner surface of the spherical front wall in the axial orientation and (0.46 ± 0.04) r upstream the chamber axis in the radial orientation. Due to its symmetrical design, the sigma values of its lateral dose response functions for all chamber's orientations are the same (2.10 ± 0.05 mm). The polarity correction factors obtained in this work do not exceed 0.1% and the saturation correction factor was below 1% up to a dose-per-pulse value of 0.956 mGy. The radiation quality correction factor k_Q of the chamber as a function of the tissue-phantom-ratio, TPR_20,10, has been calculated by Monte Carlo simulation and has been determined experimentally at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). The values of the non-reference condition correction factor k_NR have been Monte-Carlo-calculated for use of the chamber at various depths and field sizes.     

Mapping radiation quality inside photon-irradiated absorbers by means of a twin-chamber method

In photon-beam radiotherapy, the absorbed dose in an irradiated object contains a contribution by energy-degraded photons originating from Compton scatter processes at parts of the treatment head and within the absorber itself. These low-energy spectral components may lead to changes in the response of non-ideally water-equivalent radiation detectors, such as Si diodes and radiographic films, in the water/tissue dose conversion factors and in the relative biological effectiveness (RBE). As a simple means of accounting for these changes in spectral quality, the Monte Carlo calculated fraction of the kerma or absorbed dose contributed by scattered photons with energies not exceeding a certain cut-off value has previously been proposed as a useful parameter.In this paper, we present an equivalent experimental approach, providing a means for the spatial mapping of radiation quality. Its applicability will be demonstrated for the case of ⁶⁰Co and 6 MV photons. A twin-chamber combination of a Farmer type ionization chamber, equipped with a graphited PMMA outer electrode, and a chamber of the same design, but with an outer electrode made from copper, has been developed. The measured quantity is the signal ratio (SR) of the copper wall and graphited wall chambers. A correlation between the SR and the fraction of the air kerma respectively of the absorbed dose to water, contributed by photons with energies not exceeding 200 keV, has been established at a Theratron 780-C ⁶⁰Co teletherapy unit and at a Siemens Primus 6 MV linear accelerator. We also describe a two-dimensional version of the twin-chamber method using the PTW 2D-Array 256. Typical trends of parameter SR with depth and off-axis distance in water-equivalent phantoms have been observed. Thereby, a simple experimental method for the space-resolved assessment of the dose fraction attributable to low-energy Compton scattered photons can be presented as an innovative instrument of describing radiation quality in radiotherapy.     

Low-energy photons in high-energy photon fields--Monte Carlo generated spectra and a new descriptive parameter

The varying low-energy contribution to the photon spectra at points within and around radiotherapy photon fields is associated with variations in the responses of non-water equivalent dosimeters and in the water-to-material dose conversion factors for tissues such as the red bone marrow. In addition, the presence of low-energy photons in the photon spectrum enhances the RBE in general and in particular for the induction of second malignancies. The present study discusses the general rules valid for the low-energy spectral component of radiotherapeutic photon beams at points within and in the periphery of the treatment field, taking as an example the Siemens Primus linear accelerator at 6 MV and 15 MV. The photon spectra at these points and their typical variations due to the target system, attenuation, single and multiple Compton scattering, are described by the Monte Carlo method, using the code BEAMnrc/EGSnrc. A survey of the role of low energy photons in the spectra within and around radiotherapy fields is presented. In addition to the spectra, some data compression has proven useful to support the overview of the behaviour of the low-energy component. A characteristic indicator of the presence of low-energy photons is the dose fraction attributable to photons with energies not exceeding 200 keV, termed P(D)(200 keV). Its values are calculated for different depths and lateral positions within a water phantom. For a pencil beam of 6 or 15 MV primary photons in water, the radial distribution of P(D)(200 keV) is bellshaped, with a wide-ranging exponential tail of half value 6 to 7 cm. The P(D)(200 keV) value obtained on the central axis of a photon field shows an approximately proportional increase with field size. Out-of-field P(D)(200 keV) values are up to an order of magnitude higher than on the central axis for the same irradiation depth. The 2D pattern of P(D)(200 keV) for a radiotherapy field visualizes the regions, e.g. at the field margin, where changes of detector responses and dose conversion factors, as well as increases of the RBE have to be anticipated. Parameter P(D)(200 keV) can also be used as a guidance supporting the selection of a calibration geometry suitable for radiation dosimeters to be used in small radiation fields.     

Resolution and sensitivity of a two-dimensional ionisation chamber array (PTW type 10024)

The two-dimensional verification of intensity-modulated radiation plans is one of the major requirements for the safe application of this technique. The present study examines the resolution and sensitivity of a two-dimensional ionisation-chamber array (PTW2D-Array, type 10024), which can be used for plan verification instead of films. According to the Shannon-Nyquist theorem, the resolution of the 2D-Array is sufficient for dose distributions with a minimal field size of 2 cm x 2 cm. The minimal field size can be reduced to 1 cm x 1 cm by shifting the array 5 mm in the direction of the MLC movement and by repeating the measurements. The high sensitivity against a monitor decalibration for a single field of a sequence is demonstrated on the basis of an individual case. The minimal threshold for MLC misalignment detected by a chamber of the array is less than 1 mm. Therefore, the resolution capabilities of the 2D-Array are sufficient for most intensity-modulated radiation therapy (IMRT)fields.     

Internal scatter, the unavoidable major component of the peripheral dose in photon-beam radiotherapy

In clinical photon beams, the dose outside the geometrical field limits is produced by photons originating from (i) head leakage, (ii) scattering at the beam collimators and the flattening filter (head scatter) and (iii) scattering from the directly irradiated region of the patient or phantom (internal scatter). While the first two components can be modified, e.g. by reinforcement of shielding components or by re-modeling the filter system, internal scatter remains an unavoidable contributor to the peripheral dose. Its relative magnitude compared to the other components, its numerical variation with beam energy, field size and off-axis distance as well as its spectral distribution are evaluated in this study. We applied a detailed Monte Carlo (MC) model of our 6/15 MV Siemens Primus linear accelerator beam head, provided with ideal head leakage shielding conditions (multi-leaf collimator without gaps) to assess the head scatter contribution. Experimental values obtained under real shielding conditions were used to evaluate the head leakage contribution. It was found that the MC-computed internal scatter doses agree with the results of our previous measurements, that internal scatter is the major contributor to the peripheral dose in the near periphery while head leakage prevails in the far periphery, and that the lateral decline of the internal scatter dose can be represented by the sum of two exponentials, with an asymptotic tenth value of 18 to 19 cm. Internal scatter peripheral doses from various elementary beams are additive, so that their sum increases approximately in proportion with field size. The ratio between normalized internal scatter doses at 6 and 15 MV is approximately 2:1. The energy fluence spectra of the internal scatter component at all points of interest outside the field have peaks near 500 keV. The fact that the energy-shifted internal scatter constitutes the major contributor to the dose in the near periphery has a general bearing for dosimetry, i.e. for energy-dependent detector responses and dose conversion factors, for the relative biological effectiveness and for second primary malignancy risk estimates in the peripheral region.     

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

PURPOSE: To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. MATERIAL AND METHODS: Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. RESULTS: Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). CONCLUSION: Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.     

A direction-selective flattening filter for clinical photon beams. Monte Carlo evaluation of a new concept

A new concept for the design of flattening filters applied in the generation of 6 and 15 MV photon beams by clinical linear accelerators is evaluated by Monte Carlo simulation. The beam head of the Siemens Primus accelerator has been taken as the starting point for the study of the conceived beam head modifications. The direction-selective filter (DSF) system developed in this work is midway between the classical flattening filter (FF) by which homogeneous transversal dose profiles have been established, and the flattening filter-free (FFF) design, by which advantages such as increased dose rate and reduced production of leakage photons and photoneutrons per Gy in the irradiated region have been achieved, whereas dose profile flatness was abandoned. The DSF concept is based on the selective attenuation of bremsstrahlung photons depending on their direction of emission from the bremsstrahlung target, accomplished by means of newly designed small conical filters arranged close to the target. This results in the capture of large-angle scattered Compton photons from the filter in the primary collimator. Beam flatness has been obtained up to any field cross section which does not exceed a circle of 15 cm diameter at 100 cm focal distance, such as 10 × 10 cm(2), 4 × 14.5 cm(2) or less. This flatness offers simplicity of dosimetric verifications, online controls and plausibility estimates of the dose to the target volume. The concept can be utilized when the application of small- and medium-sized homogeneous fields is sufficient, e.g. in the treatment of prostate, brain, salivary gland, larynx and pharynx as well as pediatric tumors and for cranial or extracranial stereotactic treatments. Significant dose rate enhancement has been achieved compared with the FF system, with enhancement factors 1.67 (DSF) and 2.08 (FFF) for 6 MV, and 2.54 (DSF) and 3.96 (FFF) for 15 MV. Shortening the delivery time per fraction matters with regard to workflow in a radiotherapy department, patient comfort, reduction of errors due to patient movement and a slight, probably just noticable improvement of the treatment outcome due to radiobiological reasons. In comparison with the FF system, the number of head leakage photons per Gy in the irradiated region has been reduced at 15 MV by factors 1/2.54 (DSF) and 1/3.96 (FFF), and the source strength of photoneutrons was reduced by factors 1/2.81 (DSF) and 1/3.49 (FFF).     

Non-reference condition correction factor k(NR) of typical radiation detectors applied for the dosimetry of high-energy photon fields in radiotherapy

According to accepted dosimetry protocols, the "radiation quality correction factor"k(Q) accounts for the energy-dependent changes of detector responses under the conditions of clinical dosimetry for high-energy photon radiations. More precisely, a factor k(QR) is valid under reference conditions, i.e. at a point on the beam axis at depth 10 cm in a large water phantom, for 10×10 cm(2) field size, SSD 100 cm and the given radiation quality with quality index Q. Therefore, a further correction factor k(NR) has been introduced to correct for the influences of spectral quality changes when detectors are used under non-reference conditions such as other depths, field sizes and off-axis distances, while under reference conditions k(NR) is normalized to unity. In this paper, values of k(NR) are calculated for 6 and 15 MV photon beams, using published data of the energy-dependent responses of various radiation detectors to monoenergetic photon radiations, and weighting these responses with validated photon spectra of clinical high-energy photon beams from own Monte-Carlo-calculations for a wide variation of the non-reference conditions within a large water phantom. Our results confirm the observation by Scarboro et al. [26] that k(NR) can be represented by a unique function of the mean energy Em, weighted by the spectral photon fluence. Accordingly, the numerical variations of Em with depth, field size and off-axis distance have been provided. Throughout all considered conditions, the deviations of the k(NR) values from unity are at most 2% for a Farmer type ion chamber, and they remain below 15% for the thermoluminescent detectors LiF:Mg,Ti and LiF:Mg,Cu,P. For the shielded diode EDP-10, k(NR) varies from unity up to 20%, while the unshielded diode EDD-5 shows deviations up to 60% in the peripheral region. Thereby, the restricted application field of unshielded diodes has been clarified. For small field dosimetry purposes k(NR) can be converted into k(NCSF), the non-calibration condition correction factor normalized to unity for a 4×4 cm(2) calibration field. For the unshielded Si diodes needed in small-field dosimetry, the values of k(NCSF) are closer to unity than the associated k(NR) values.     

Reference conditions for ion-chamber based HDR brachytherapy dosimetry and for the calibration of high-resolution solid detectors

The aim of this study has been to develop a two-step method of in-phantom dosimetry around a brachytherapy ¹⁹²Ir photon source. The first step is to measure the absorbed dose rate to water with a calibrated ionization chamber under reference conditions, the second to cross-calibrate, under these conditions, small solid-state detectors such as silicon diodes, synthetic diamond or scintillation detectors suited for spatially resolved dose rate measurements at other, particularly at smaller source axis distances in the water phantom. This two-step approach constitutes a method for in-phantom dosimetry in brachytherapy, analogous to the “small calibration field” commonly used in teletherapy to provide the reference conditions for the cross-calibration of high-resolution detectors.Under reference conditions, all known corrections for radiation quality, volume averaging and position of the chamber's effective point of measurement (EPOM) have to be applied. The study is therefore particularly devoted to (1) the experimental determination of the position of the source axis, (2) a general formulation for the volume averaging correction factor of small ionization chambers and (3) the experimental determination of the EPOM positions for the PinPoint chamber 31014 and the 3D-PinPoint chamber PTW 31022 (both PTW Freiburg, Germany). The distance of 30 mm from the source axis was chosen as the reference condition for cross calibrations. This concept is realized with the instrumentation available in a hospital, a scanning-type water phantom, a software package for small field dosimetry and detectors typically used in clinical routine dosimetry.The present development of a method of in-phantom dose measurement under ¹⁹²Ir brachytherapy conditions was performed in recognition of the primary role of dose calculations, e.g. according to the AAPM TG43 recommendations. But in addition, the methodology tested here is paving a practicable way for the experimental check of typical dose values under clinical conditions, should the need arise.