Hugo Palmans

A new formalism for reference dosimetry of small and nonstandard fields

The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or nonuniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). This has been facilitated by the increased availability of standard and add-on multileaf collimators and a variety of new treatment units. For these fields, dosimetric errors have become considerably larger than in conventional beams mostly due to two reasons; (i) the reference conditions recommended by conventional Codes of Practice (CoPs) cannot be established in some machines and (ii) the measurement of absorbed dose to water in composite fields is not standardized. In order to develop standardized recommendations for dosimetry procedures and detectors, an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This paper outlines a new formalism for the dosimetry of small and composite fields with the intention to extend recommendations given in conventional CoPs for clinical reference dosimetry based on absorbed dose to water. This formalism introduces the concept of two new intermediate calibration fields: (i) a static machine-specific reference field for those modalities that cannot establish conventional reference conditions and (ii) a plan-class specific reference field closer to the patient-specific clinical fields thereby facilitating standardization of composite field dosimetry. Prior to progressing with developing a CoP or other form of recommendation, the members of this IAEA working group welcome comments from the international medical physics community on the formalism presented here.

Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: A comparison between radiochromic film measurements, Monte Carlo simulations, and collapsed cone convolution calculations

Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons, especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis dose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rim situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLiplus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm2 and a 10 x 1 cm2 field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata, Cahada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas, CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of 1 mm, the maximum dose overestimation was 14% or 24%.

Dosimetry for ion beam radiotherapy

Recently, ion beam radiotherapy (including protons as well as heavier ions) gained considerable interest. Although ion beam radiotherapy requires dose prescription in terms of iso-effective dose (referring to an iso-effective photon dose), absorbed dose is still required as an operative quantity to control beam delivery, to characterize the beam dosimetrically and to verify dose delivery. This paper reviews current methods and standards to determine absorbed dose to water in ion beam radiotherapy, including (i) the detectors used to measure absorbed dose, (ii) dosimetry under reference conditions and (iii) dosimetry under non-reference conditions. Due to the LET dependence of the response of films and solid-state detectors, dosimetric measurements are mostly based on ion chambers. While a primary standard for ion beam radiotherapy still remains to be established, ion chamber dosimetry under reference conditions is based on similar protocols as for photons and electrons although the involved uncertainty is larger than for photon beams. For non-reference conditions, dose measurements in tissue-equivalent materials may also be necessary. Regarding the atomic numbers of the composites of tissue-equivalent phantoms, special requirements have to be fulfilled for ion beams. Methods for calibrating the beam monitor depend on whether passive or active beam delivery techniques are used. QA measurements are comparable to conventional radiotherapy; however, dose verification is usually single field rather than treatment plan based. Dose verification for active beam delivery techniques requires the use of multi-channel dosimetry systems to check the compliance of measured and calculated dose for a representative sample of measurement points. Although methods for ion beam dosimetry have been established, there is still room for developments. This includes improvement of the dosimetric accuracy as well as development of more efficient measurement techniques.

Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams

PURPOSE The aim of the present study is to provide a comprehensive set of detector specific correction factors for beam output measurements for small beams, for a wide range of real time and passive detectors. The detector specific correction factors determined in this study may be potentially useful as a reference data set for small beam dosimetry measurements. METHODS Dose response of passive and real time detectors was investigated for small field sizes shaped with a micromultileaf collimator ranging from 0.6 × 0.6 cm(2) to 4.2 × 4.2 cm(2) and the measurements were extended to larger fields of up to 10 × 10 cm(2). Measurements were performed at 5 cm depth, in a 6 MV photon beam. Detectors used included alanine, thermoluminescent dosimeters (TLDs), stereotactic diode, electron diode, photon diode, radiophotoluminescent dosimeters (RPLDs), radioluminescence detector based on carbon-doped aluminium oxide (Al2O3:C), organic plastic scintillators, diamond detectors, liquid filled ion chamber, and a range of small volume air filled ionization chambers (volumes ranging from 0.002 cm(3) to 0.3 cm(3)). All detector measurements were corrected for volume averaging effect and compared with dose ratios determined from alanine to derive a detector correction factors that account for beam perturbation related to nonwater equivalence of the detector materials. RESULTS For the detectors used in this study, volume averaging corrections ranged from unity for the smallest detectors such as the diodes, 1.148 for the 0.14 cm(3) air filled ionization chamber and were as high as 1.924 for the 0.3 cm(3) ionization chamber. After applying volume averaging corrections, the detector readings were consistent among themselves and with alanine measurements for several small detectors but they differed for larger detectors, in particular for some small ionization chambers with volumes larger than 0.1 cm(3). CONCLUSIONS The results demonstrate how important it is for the appropriate corrections to be applied to give consistent and accurate measurements for a range of detectors in small beam geometry. The results further demonstrate that depending on the choice of detectors, there is a potential for large errors when effects such as volume averaging, perturbation and differences in material properties of detectors are not taken into account. As the commissioning of small fields for clinical treatment has to rely on accurate dose measurements, the authors recommend the use of detectors that require relatively little correction, such as unshielded diodes, diamond detectors or microchambers, and solid state detectors such as alanine, TLD, Al2O3:C, or scintillators.

Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.

Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams.

Small fields where electronic equilibrium is not achieved are becoming increasingly important in clinical practice. These complex situations give rise to problems and inaccuracies in both dosimetry and analytical/empirical dose calculation, and therefore require other than conventional methods. A natural diamond detector and a Markus parallel plate ionization chamber have been selected for clinical dosimetry in 6 MV photon beams. Results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry have been compared with dose measurements. A modification of the existing component module for multileaf collimators (MLCs) allowed the modeling of a linear accelerator SL 25 (Elekta Oncology Systems) equipped with a MLC with curved leaf-ends. A mechanical measurement method with spacer plates and a light-field edge detection technique are described as methods to obtain geometrical data of collimator openings for application in the Monte Carlo system. Generally a good agreement is found between measurements and calculations of depth dose distributions and deviations are typically less than 1%. Calculated lateral dose profiles slightly exceed measured dose distributions near the higher level of the penumbras for a 10x2 cm2 field, but agree well with the measurements for all other cases. The simulations are also able to predict variations of output factors and ratios of output factors as a function of field width and field-offset. The Monte Carlo results demonstrate that qualitative changes in energy spectra are too small to explain these variations and that especially geometrical factors affect the output factors and depth dose curves and profiles.

LET dependence of GafChromic films and an ion chamber in low-energy proton dosimetry

Dosimetry using a PMMA phantom was performed in 15 and 29 MeV proton beams from the Birmingham cyclotron, with a Markus parallel-plate ionization chamber and GafChromic EBT and MD-V2-55 film. Simulations of the depth-dose curves were performed with FLUKA 2008.3 and MCNPX 2.5.0, which agreed almost perfectly with each other in range and only differed by 2% in the Bragg peak (BP) region. FLUKA was also used to calculate k(Q) factors for Markus chamber measurements as an improvement to the IAEA TRS-398 values in low-energy beams. FLUKA depth-dose simulations overestimate the BP height measured by ion chamber by about 10%, where the initial proton energy spread was estimated by fitting to the slope of the measured BP distal edge. Both GafChromic films showed an under-response in the BP compared to ion chamber; however, EBT exhibits this effect at lower energies than MD-V2-55. A possible reason for this is attributed to the shape and arrangement of the monomer particles being different in the active components of EBT and MD-V2-55. Relative effectiveness (RE) of both films is presented as functions of residual range R(res) in water and peak proton energy determined by FLUKA, with considerations for the spatial separation of the two active layers in each film. The proton energies at which RE reduces to 90% of maximum film response are 6.7 and 3.2 MeV for MD-V2-55 and EBT, respectively. Additionally, a beam quality correction factor (g(Q,Q0)) is suggested for both GafChromic films, involving water-to-film stopping power ratios evaluated using ICRU recommendations, and a polymer yield factor G(Q0)/G(Q). RE in this work is equated to the reciprocal of the polymer yield factor. The calculated values of (S(w,film))Q/(S(w,film))Q0 are constant within 2.1% and 1.2% across the proton energy range of 1-300 MeV for EBT and MD-V2-55, respectively, so it is concluded that the polymer yield factor is the dominant factor causing the LET quenching effect.

Detector comparison for small field output factor measurements in flattening filter free photon beams

PURPOSE The applicability of various detectors for small field dosimetry and whether there are differences in the detector response when irradiated with FF- and FFF-beams was investigated. MATERIALS AND METHODS Output factors of 6 and 10 MV FF- and FFF-beams were measured with 14 different online detectors using field sizes between 10 × 10 and 0.6 × 0.6 cm(2) at a depth of 5 cm of water in isocentric conditions. Alanine pellets with a diameter of 5 and 2.5mm were used as reference dosimeters for field sizes down to 1.2 × 1.2 and 0.6 × 0.6 cm(2), respectively. The ratio of the relative output measured with the online detectors to the relative output measured with alanine was evaluated (referred to as dose response ratio). RESULTS The dose response ratios of two different shielded diodes measured with 10 MV FF-beams deviated substantially by 2-3% compared to FFF-beams at a field size of 0.6 × 0.6 cm(2). This difference was less pronounced for 6 MV FF- and FFF-beams. For all other detectors the dose response ratios of FF- and FFF-beams showed no significant difference. CONCLUSION The dose response ratios of the majority of the detectors agreed within the measurement uncertainty when irradiated with FF- and FFF-beams. Of all investigated detectors, the microDiamond and the unshielded diodes would require only small corrections which make them suitable candidates for small field dosimetry in FF- and FFF-beams.

Parameter dependence of the MCNP electron transport in determining dose distributions

In this paper, a detailed study of the electron transport in MCNP is performed, separating the effects of the energy binning technique on the energy loss rate, the scattering angles, and the sub-step length as a function of energy. As this problem is already well known, in this paper we focus on the explanation as to why the default mode of MCNP can lead to large deviations. The resolution dependence was investigated as well. An error in the MCNP code in the energy binning technique in the default mode (DBCN 18 card = 0) was revealed, more specific in the updating of cross sections when a sub-step is performed corresponding to a high-energy loss. This updating error is not present in the ITS mode (DBCN 18 card = 1) and leads to a systematically lower dose deposition rate in the default mode. The effect is present for all energies studied (0.5-10 MeV) and depends on the geometrical resolution of the scoring regions and the energy grid resolution. The effect of the energy binning technique is of the same order of that of the updating error for energies below 2 MeV, and becomes less important for higher energies. For a 1 MeV point source surrounded by homogeneous water, the deviation of the default MCNP results at short distances attains 9% and remains approximately the same for all energies. This effect could be corrected by removing the completion of an energy step each time an electron changes from an energy bin during a sub-step. Another solution consists of performing all calculations in the ITS mode. Another problem is the resolution dependence, even in the ITS mode. The higher the resolution is chosen (the smaller the scoring regions) the faster the energy is deposited along the electron track. It is proven that this is caused by starting a new energy step when crossing a surface. The resolution effect should be investigated for every specific case when calculating dose distributions around beta sources. The resolution should not be higher than 0.85*(1-EFAC)*CSDA, where EFAC is the energy loss per energy step and CSDA a continuous slowing down approximation range. This effect could as well be removed by determining the cross sections for energy loss and multiple scattering at the average energy of an energy step and by sampling the cross sections for each sub-step. Overall, we conclude that MCNP cannot be used without a caution due to possible errors in the electron transport. When care is taken, it is possible to obtain correct results that are in agreement with other Monte Carlo codes.

Detector dose response in megavoltage small photon beams. I. Theoretical concepts

PURPOSE To explain the reasons for significant quality correction factors in megavoltage small photon fields and clarify the underlying concepts relevant to dosimetry under such conditions. METHODS The validity of cavity theory and the requirement of charged particle equilibrium (CPE) are addressed from a theoretical point of view in the context of nonstandard beams. Perturbation effects are described into four main subeffects, explaining their nature and pointing out their relative importance in small photon fields. RESULTS It is demonstrated that the failure to meet classical cavity theory requirements, such as CPE, is not the reason for significant quality correction factors. On the contrary, it is shown that the lack of CPE alone cannot explain these corrections and that what matters most, apart from volume averaging effects, is the relationship between the lack of CPE in the small field itself and the density of the detector cavity. The density perturbation effect is explained based on Fanos theorem, describing the compensating effect of two main contributions to cavity absorbed dose. Using the same approach, perturbation effects arising from the difference in atomic properties of the cavity medium and the presence of extracameral components are explained. Volume averaging effects are also discussed in detail. CONCLUSIONS Quality correction factors of small megavoltage photon fields are mainly due to differences in electron density between water and the detector medium and to volume averaging over the detector cavity. Other effects, such as the presence of extracameral components and differences in atomic properties of the detection medium with respect to water, can also play an accentuated role in small photon fields compared to standard beams.