A Monte Carlo evaluation of RapidArc dose calculations for oropharynx radiotherapy.

RapidArc, recently released by Varian Medical Systems, is a novel extension of IMRT in which an optimized 3D dose distribution may be delivered in a single gantry rotation of 360 degrees or less. The purpose of this study was to investigate the accuracy of the analytical anisotropic algorithm (AAA), the sole algorithm for photon dose calculations of RapidArc treatment plans. The clinical site chosen was oropharynx and the associated nodes involved. The VIMC-Arc system, which utilizes BEAMnrc and DOSXYZnrc for particle transport through the linac head and patient CT phantom, was used as a benchmarking tool. As part of this study, the dose for a single static aperture, typical for RapidArc delivery, was calculated by the AAA, MC and compared with the film. This film measurement confirmed MC modeling of the beam aperture in water. It also demonstrated that the AAA dosimetric error can be as high as 12% near isolated leaf edges and up to 5% at the leaf end. The composite effect of these errors in a full RapidArc calculation in water involving a C-shaped target and the associated organ at risk produced a 1.5% overprediction of the mean target dose. In our cohort of six patients, the AAA was found, on average, to overestimate the PTV60 coverage at the 95% level in the presence of air cavities by 1.0% (SD = 1.1%). Removing the air cavities from the target volumes reduced these differences by about a factor of 2. The dose to critical structures was also overestimated by the AAA. The mean dose to the spinal cord was higher by 1.8% (SD = 0.8%), while the effective maximum dose (D2%) was only 0.2% higher (SD = 0.6%). The mean dose to the parotid glands was overestimated by approximately 9%. This study has shown that the accuracy of the AAA for RapidArc dose calculations, performed at a resolution of 2.5 mm or better, is adequate for clinical use.

Azimuthal particle redistribution for the reduction of latent phase-space variance in Monte Carlo simulations.

It is well known that the use of a phase space in Monte Carlo simulation introduces a baseline level of variance that cannot be suppressed through the use of standard particle recycling techniques. This variance (termed latent phase-space variance by Sempau et al) can be a significant limiting factor in achieving accurate, low-uncertainty dose scoring results, especially near the surface of a phantom. A BEAMnrc component module (MCTWIST) has been developed to reduce the presence of latent variance in phase-space-based Monte Carlo simulations by implementing azimuthal particle redistribution (APR). For each recycled use of a phase-space particle a random rotation about the beam's central axis is applied, effectively utilizing cylindrical symmetry of the particle fluence and therefore providing a more accurate representation of the source. The MCTWIST module is unique in that no physical component is actually added to the accelerator geometry. Beam modifications are made by directly transforming particle characteristics outside of BEAMnrc/EGSnrc particle transport. Using MCTWIST, we have demonstrated a reduction in latent phase-space variance by more than a factor of 20, for a 10 x 10 cm(2) field, when compared to standard phase-space particle recycling techniques. The reduction in latent variance has enabled the achievement of dramatically smoother in-water dose profiles. This paper outlines the use of MCTWIST in Monte Carlo simulation and quantifies for the first time the latent variance reduction resulting from exploiting cylindrical phase-space symmetry.

Absolute dose calculations for Monte Carlo simulations of radiotherapy beams.

Monte Carlo (MC) simulations have traditionally been used for single field relative comparisons with experimental data or commercial treatment planning systems (TPS). However, clinical treatment plans commonly involve more than one field. Since the contribution of each field must be accurately quantified, multiple field MC simulations are only possible by employing absolute dosimetry. Therefore, we have developed a rigorous calibration method that allows the incorporation of monitor units (MU) in MC simulations. This absolute dosimetry formalism can be easily implemented by any BEAMnrc/DOSXYZnrc user, and applies to any configuration of open and blocked fields, including intensity-modulated radiation therapy (IMRT) plans. Our approach involves the relationship between the dose scored in the monitor ionization chamber of a radiotherapy linear accelerator (linac), the number of initial particles incident on the target, and the field size. We found that for a 10 x 10 cm2 field of a 6 MV photon beam, 1 MU corresponds, in our model, to 8.129 x 10(13) +/- 1.0% electrons incident on the target and a total dose of 20.87 cGy +/- 1.0% in the monitor chambers of the virtual linac. We present an extensive experimental verification of our MC results for open and intensity-modulated fields, including a dynamic 7-field IMRT plan simulated on the CT data sets of a cylindrical phantom and of a Rando anthropomorphic phantom, which were validated by measurements using ionization chambers and thermoluminescent dosimeters (TLD). Our simulation results are in excellent agreement with experiment, with percentage differences of less than 2%, in general, demonstrating the accuracy of our Monte Carlo absolute dose calculations.

Implementation of random set-up errors in Monte Carlo calculated dynamic IMRT treatment plans.

The fluence-convolution method for incorporating random set-up errors (RSE) into the Monte Carlo treatment planning dose calculations was previously proposed by Beckham et al, and it was validated for open field radiotherapy treatments. This study confirms the applicability of the fluence-convolution method for dynamic intensity modulated radiotherapy (IMRT) dose calculations and evaluates the impact of set-up uncertainties on a clinical IMRT dose distribution. BEAMnrc and DOSXYZnrc codes were used for Monte Carlo calculations. A sliding window IMRT delivery was simulated using a dynamic multi-leaf collimator (DMLC) transport model developed by Keall et al. The dose distributions were benchmarked for dynamic IMRT fields using extended dose range (EDR) film, accumulating the dose from 16 subsequent fractions shifted randomly. Agreement of calculated and measured relative dose values was well within statistical uncertainty. A clinical seven field sliding window IMRT head and neck treatment was then simulated and the effects of random set-up errors (standard deviation of 2 mm) were evaluated. The dose-volume histograms calculated in the PTV with and without corrections for RSE showed only small differences indicating a reduction of the volume of high dose region due to set-up errors. As well, it showed that adequate coverage of the PTV was maintained when RSE was incorporated. Slice-by-slice comparison of the dose distributions revealed differences of up to 5.6%. The incorporation of set-up errors altered the position of the hot spot in the plan. This work demonstrated validity of implementation of the fluence-convolution method to dynamic IMRT Monte Carlo dose calculations. It also showed that accounting for the set-up errors could be essential for correct identification of the value and position of the hot spot.

Modelling an extreme water-lung interface using a single pencil beam algorithm and the Monte Carlo method.

The goal of this study was to quantify, in a heterogeneous phantom, the difference between experimentally measured beam profiles and those calculated using both a commercial convolution algorithm and the Monte Carlo (MC) method. This was done by arranging a phantom geometry that incorporated a vertical solid water-lung material interface parallel to the beam axis. At nominal x-ray energies of 6 and 18 MV, dose distributions were modelled for field sizes of 10 x 10 cm(2) and 4 x 4 cm(2) using the CadPlan 6.0 commercial treatment planning system (TPS) and the BEAMnrc-DOSXYZnrc Monte Carlo package. Beam profiles were found experimentally at various depths using film dosimetry. The results showed that within the lung region the TPS had a substantial problem modelling the dose distribution. The (film-TPS) profile difference was found to increase, in the lung region, as the field size decreased and the beam energy increased; in the worst case the difference was more than 15%. In contrast, (film-MC) profile differences were not found to be affected by the material density difference. BEAMnrc-DOSXYZnrc successfully modelled the material interface and dose profiles to within 2%.

The effect of leaf width and sampling distance on the "stair-stepping" of field borders defined by multileaf collimators.

Standard multileaf collimators (MLCs) are now available with 1.0 and 0.5 cm leaf widths. The aim of this work is to compare the dose-undulation and effective penumbra of field edges formed by these MLC leaf widths and to determine how reducing the sampling distance (centre-to-centre distance between adjacent MLC leaves) for the 1.0 cm leaf width compares to the smaller leaf width. The undulation of the 50% isodose line and the 80-20% and 80-30% effective penumbra were compared for a field edge angled at 45 degrees to the MLC leaf motion direction at 8 cm depth. The larger leaf width field was also segmented to form field edges with 0.5, 0.33 and 0.2 cm sampling distance. Random setup variation of 2 mm standard deviation was also incorporated. Dose undulation was 1.5 mm for the 0.5 cm MLC leaf width compared to 4.5 mm for the 1.0 cm width. The 80-20% effective penumbra was 2 mm less for the 0.5 cm leaf width and the 80-30% effective penumbra was approximately 3 mm less. When random setup variation was incorporated the 0.5 cm leaf width isodoses were straight compared with approximately 3 mm undulation for the larger MLC. Reducing the sampling distance for the 1.0 cm MLC leaf width to 0.33 cm resulted in penumbra only slightly greater than the 0.5 cm leaf width and removed the undulation. Effective penumbra and dose undulation are reduced for the 0.5 cm leaf width compared to a 1.0 cm leaf width. Reducing the sampling distance for the 1.0 cm MLC leaf can approximate the 0.5 cm leaf width, at the expense of longer treatment times, and increased quality assurance investment.

A fluence-convolution method to calculate radiation therapy dose distributions that incorporate random set-up error.

The International Commission on Radiation Units and Measurements Report 62 (ICRU 1999) introduced the concept of expanding the clinical target volume (CTV) to form the planning target volume by a two-step process. The first step is adding a clinically definable internal margin, which produces an internal target volume that accounts for the size, shape and position of the CTV in relation to anatomical reference points. The second is the use of a set-up margin (SM) that incorporates the uncertainties of patient beam positioning, i.e. systematic and random set-up errors. We propose to replace the random set-up error component of the SM by explicitly incorporating the random set-up error into the dose-calculation model by convolving the incident photon beam fluence with a Gaussian set-up error kernel. This fluence-convolution method was implemented into a Monte Carlo (MC) based treatment-planning system. Also implemented for comparison purposes was a dose-matrix-convolution algorithm similar to that described by Leong (1987 Phys. Med. Biol. 32 327-34). Fluence and dose-matrix-convolution agree in homogeneous media. However, for the heterogeneous phantom calculations, discrepancies of up to 5% in the dose profiles were observed with a 0.4 cm set-up error value. Fluence-convolution mimics reality more closely, as dose perturbations at interfaces are correctly predicted (Wang et al 1999 Med. Phys. 26 2626-34, Sauer 1995 Med. Phys. 22 1685-90). Fluence-convolution effectively decouples the treatment beams from the patient. and more closely resembles the reality of particle fluence distributions for many individual beam-patient set-ups. However, dose-matrix-convolution reduces the random statistical noise in MC calculations. Fluence-convolution can easily be applied to convolution/superposition based dose-calculation algorithms.

Evaluation of the validity of a convolution method for incorporating tumour movement and set-up variations into the radiotherapy treatment planning system.

Modern radiotherapy techniques have developed to a point where the ability to conform to a particular tumour shape is limited by organ motion and set-up variations. The result is that dose distributions displayed by treatment planning systems based on static beam modelling are not representative of the dose received by the patient during a fractionated course of radiotherapy. The convolution-based method to account for these variations in radiation treatment planning systems has been suggested in previous work. The validity of the convolution method is tested by comparing the dose distribution obtained from this convolution method with the dose distribution obtained by summing the contribution to the total dose from each fraction of a fractionated treatment (for increasing numbers of fractions) and simulating random target position variations between fractions. For larger numbers of fractions (approximately or > 15) which are the norm for radical treatment schemes, it is clear that incorporation of movement by a convolution method could potentially produce a more accurate dose distribution. There are some limitations that have been identified, however, especially in relation to the heterogeneous nature of patient tissues, which require further investigation before the technique could be applied clinically.

A method to predict the effect of organ motion and set-up variations on treatment plans.

Dose distributions calculated by commercial treatment planning systems do not allow incorporation of the effects of patient position variation or organ motion throughout the course of radiation therapy treatment. We have established a convolution-based method, which enables us to display dose distributions using a commercial treatment planning system that can take into account target movement. An example of the method applied to a prostate treatment plan is presented. For the method to be of clinical use it requires assessment of the parameters leading to target movement in a scientific manner in the same treatment department that it is to be used. It is not sufficient to rely on published data especially that relating to set-up accuracy as this has been shown to vary widely from centre to centre. We believe that with appropriate movement data, a convolution-based approach can lead to more optimal radiation margins around clinical target volumes (CTV). Optimal margins will help prevent geometric misses as well as ensure that the amount of critical late reacting normal tissues surrounding the CTV irradiated is minimised. Optimal margins cannot be guaranteed with the more conventionally used "rule of thumb" techniques for placing a planning target volume around the CTV.

Photon buildup in orthovoltage X-ray beams.

Orthovoltage x-ray beams exhibit the characteristic of depth dose buildup which is not well described in the literature. The principal reason for this phenomenon is the increase in dose deposited due to electrons set in motion by secondary (Compton) scattered photons within the phantom, as depth is increased until longitudinal equilibrium is reached. This happens within a few millimetres of the surface and has been demonstrated both experimentally and by Monte Carlo methods. The Monte Carlo technique also enabled description of a second order primary dose buildup effect (due to longitudinal electronic disequilibrium) that would be impossible to detect with conventional detectors due to the short range of the electrons. The magnitude of buildup was observed to alter with various combinations of beam parameters. Variations will also occur with detectors used to measure buildup. It is recommended that radiation oncology departments assess this effect in the context of their clinical data in current use to ensure that there are not doses higher than prescribed being applied a few millimetres below the skin surface, especially if data was collected with a thin windowed, parallel plate ionisation chamber and/or that coarse steps for depth dose data collection were used along the beam central axis.

An independent check method of radiotherapy computer plan derived monitor units.

With the increasing complexity of radiotherapy computer treatment planning systems (CTPS), verification of the treatments is an important component of a quality assurance program. The radiation dose delivered to a patient is based on treatment machine monitor units (MUs) that are calculated either using data tables or within the CTPS. An independent method for checking radiotherapy MU calculations is proposed. This method involves calculating the dose at a point (usually ICRU reference point) using the derived MUs and independently measured treatment data and comparing to the dose at the same point as predicted by the CTPS. A program was developed using Microsoft Excel to assist in performing this check. The program contains the beam data and various correction factors and calculates the dose after input of the treatment setup parameters. This independent method is used to check all radiotherapy treatment fields at Liverpool Hospital and has found a number of significant errors in the planning process and in the CTPS calculations.

A method for producing uniform dose distributions in the junction regions of large hinge angle electron fields.

PURPOSE: The planning problems presented by abutting electron fields are well recognized. Junctioning electron fields with a large hinge angle compounds the problems because of the creation of closely situated high dose and low-dose regions. METHODS AND MATERIALS: The technique involving a compensated superficial x-ray (SXR) field to treat the junction region between electron fields was developed and used in a particular clinical case (treatment of a squamous cell carcinoma of the forehead/scalp). The superficial x-ray beam parameters were chosen and the compensator was designed to make the SXR field complementary to the electron fields. RESULTS: Application of a compensated SXR field eliminated low dose zones in the junction region and reduced high dose zones to 110%. In the clinical case discussed, the high-dose areas due to the SXR field would not appear because of increased attenuation of the soft X-rays in bone. CONCLUSION: The technique proposed produces uniform dose distribution up to 3 cm deep and can be considered as an additional tool for dealing with electron field junctioning problems.

Dose rate dependence of a PTW diamond detector in the dosimetry of a 6 MV photon beam.

The dose rate dependence and current/voltage characteristics of a PTW Riga diamond detector in the dosimetry of a 6 MV photon beam have been investigated. Diamond detectors are radiosensitive resistors whose conductivity (i) varies almost in proportion to dose rate and (ii) is almost independent of bias voltage for a constant dose rate. At the recommended bias of +100 V, and also at +200 V, the detector is operating with incomplete charge collection due to the electron-hole recombination time being shorter that the maximum time for an electron to be collected by the anode. As dose rate is varied by changing FSD or depth (changing dose per pulse), detector current and dose rate are related by the expression i alpha Ddelta where delta is approximately 0.98. This manifests itself in an overestimate in percentage depth-dose at a depth of 30 cm of approximately 1% when compared to ionization chamber results. A similar sublinearity is seen when pulse repetition frequency is varied, indicating that the dependence is an on average rather than an instantaneous dose rate. The dose rate dependence is attributed to the reduction in recombination time as dose rate increases.

Secondary electron scatter in radiotherapy: implications for treatment fields in proximity to the lens of the eye and the testes.

For external beam photon radiotherapy treatments, secondary electrons pose unique shielding problems. Dose due to these electrons can be deposited under shielding blocks and also outside of collimated beam edges. The sources of contamination electrons are many, but the dominant one is identified as the plastic accessory tray commonly used to support shielding blocks. Because these electrons only deposit dose superficially, the critical structures that have been established which are in range of them are the lens of the eye and the testes. The present work outlines a technique that involves a low atomic number absorber placed directly over either of these structures (outside of the primary beam) to provide the necessary shielding. Use of the technique will ensure that critical structures in proximity to treatment portals have the dose to them minimized as much as possible.

The use of a BF3 remmeter to calibrate neutron activation foils in the medical linear accelerator environment.

A technique has been developed for radiation protection purposes whereby it is possible to measure the dose-equivalent due to neutrons from a dual energy medical linear accelerator using a conventional BF3 neutron remmeter to calibrate gold foils. This technique has been applied to measuring the neutron dose-equivalent from a Philips SL75-20 medical linear accelerator. The tenth value distance for the entrance maze is 3.3 +/- 0.5m compared to a typically quoted value of 5m. This suggests that the use of wood and boron tiles in the construction of the entrance maze can reduce neutron propagation down the maze.