Incident Learning Systems for Radiation Oncology: Development and Value at the Local, National and International Level.

AIMS: To discuss the background for incident reporting and learning systems, as well as the infrastructure and operational aspects to run them. MATERIALS AND METHODS: Information from peer-reviewed literature, online resources and the authors' experience synthesised into a concise understanding of the topic. RESULTS: Incident learning systems can be local, national or international, each having the same basic goals but facilitating different audiences and environments. A key component of any reporting and learning system is timely and effective analysis of near-misses and incidents as well as feedback to the users of the system. It is important for staff to know that reports are acknowledged, analysed and acted upon. There is a need to comply with current European legislation and other national systems, which can be addressed together with the steps required for comprehensive management of an incident. CONCLUSION: Reporting and learning from incidents and near-misses is a key component of quality and safety in radiotherapy. A major benefit of the national or international systems is the potential for a larger database of incidents, supporting wider analysis and comparison, and sharing of knowledge across a larger community.

Consensus recommendations for incident learning database structures in radiation oncology.

PURPOSE: Incident learning plays a key role in improving quality and safety in a wide range of industries and medical disciplines. However, implementing an effective incident learning system is complex, especially in radiation oncology. One current barrier is the lack of technical standards to guide users or developers. This report, the product of an initiative by the Work Group on Prevention of Errors in Radiation Oncology of the American Association of Physicists in Medicine, provides technical recommendations for the content and structure of incident learning databases in radiation oncology. METHODS: A panel of experts was assembled and tasked with developing consensus recommendations in five key areas: definitions, process maps, severity scales, causality taxonomy, and data elements. Experts included representatives from all major North American radiation oncology organizations as well as users and developers of public and in-house reporting systems with over two decades of collective experience. Recommendations were developed that take into account existing incident learning systems as well as the requirements of outside agencies. RESULTS: Consensus recommendations are provided for the five major topic areas. In the process mapping task, 91 common steps were identified for external beam radiation therapy and 88 in brachytherapy. A novel feature of the process maps is the identification of "safety barriers," also known as critical control points, which are any process steps whose primary function is to prevent errors or mistakes from occurring or propagating through the radiotherapy workflow. Other recommendations include a ten-level medical severity scale designed to reflect the observed or estimated harm to a patient, a radiation oncology-specific root causes table to facilitate and regularize root-cause analyses, and recommendations for data elements and structures to aid in development of electronic databases. Also presented is a list of key functional requirements of any reporting system. CONCLUSIONS: Incident learning is recognized as an invaluable tool for improving the quality and safety of treatments. The consensus recommendations in this report are intended to facilitate the implementation of such systems within individual clinics as well as on broader national and international scales.

SU-E-T-249: Theoretical Analysis of the Effects Uncertainties Have On Treatment Outcomes.

PURPOSE: Mathematical models are used in Industrial & Systems Engineering to analyze complex integrated operational systems. Adapting this approach to radiation therapy can help quantify the precision and accuracy necessary to achieve optimal outcome of radiation treatment. The purpose of this work is to develop such a model using clinical data and assess the effect uncertainties have on treatment outcomes. METHODS: The Taguchi Loss Function (TLF) is adapted to radiation therapy using conventional radiobiological models for tumor control probabilities (TCP) and normal tissues complication probabilities (NTCP) based on the equivalent uniform dose. The TCP and NTCP curves are combined to create a failure probability function for a given treatment plan. The composite effects of all uncertainties involved in treating a patient are modeled by a normal distribution. The standard deviation and mean of the normal distribution represent the precision and accuracy of a treatment. The failure probability function is convolved with the normal distribution to arrive at an expected failure probability. Precision was varied from 0.5% to 25% while accuracy ranged from ±5% to investigate uncertainties effects on complication-free local tumor control. 3D 4-field box plans where compared to IMRT plans for 18 prostate patients using this method. RESULTS: The average expected failure probability at the prescription dose for the 3D 4-field box plans was 30.02% and 18.13% for the IMRT plans at zero uncertainty. At 25% uncertainty the expected failure probabilities were 76.85% and 64.36%, respectively. On average the IMRT plans failure probability was 14.84% less than the 3D 4-field box plans for all uncertainty levels. CONCLUSION: This study demonstrates that uncertainty in radiotherapy procedures has a quantifiable effect on treatment outcome. To further improve complication-free local tumor control we must both improve treatment technologies and improve quality to minimize the uncertainties in radiation therapy.

MO-F-211-01: Methods for Completing Practice Quality Improvement (PQI).

Practice Quality Improvement (PQI) is becoming an expected part of routine practice in healthcare as an approach to provide more efficient, effective and high quality care. Additionally, as part of the ABR's Maintenance of Certification (MOC) pathway, medical physicists are now expected to complete a PQI project. This session will describe the history behind and benefits of the ABR's MOC program, provide details of quality improvement methods and how to successfully complete a PQI project. PQI methods include various commonly used engineering and management tools. The Plan-Do-Study-Act (PDSA) cycle will be presented as one project planning and implementation tool. Other PQI analysis instruments such as flowcharts, Pareto charts, process control charts and fishbone diagrams will also be explained with examples. Cause analysis, solution development and implementation, and post-implementation measurement will be presented. Project identification and definition as well as appropriate measurement tool selection will be offered. Methods to choose key quality metrics (key quality indicators) will also be addressed. Several sample PQI projects and templates available through the AAPM and other organizations will be described. At least three examples of completed PQI projects will be shared. LEARNING OBJECTIVES: 1. Identify and define a PQI project 2. Identify and select measurement methods/techniques for use with the PQI project 3. Describe example(s) of completed projects.

MO-B-211-01: Linac-Based IMRT/VMAT Commissioning and QA Program Development.

Intensity modulated radiotherapy (IMRT) has been used clinically for many years. Reports from the RPC indicate that up to 30% of the institutions fail to pass RPC IMRT credentialing process on the first attempt. While volumetric modulated arc therapy (VMAT) has been introduced more recently, it has quickly gained wide clinical use. In spite of the long history with IMRT and rapid adoption of VMAT, commissioning and developing a quality assurance (QA) program continues to be a challenge especially in busy departments. These points indicate that a review of commissioning and quality assurance for IMRT is still very much needed. In this session, the development of an overall IMRT/VMAT QA program, the role of team members and on-going program functions will be described including aspects of both quality and safety. General issues and specifics of IMRT/VMAT commissioning and quality assurance will be covered. While the general principles of commissioning and QA apply to any device capable of intensity-modulation, specific examples will be provided for Elekta and Varian linear accelerators. Strategies for commission and useful checklists will be discussed as well as some differences between Elekta and Varian technologies. There will also be a focus on practical advice towards the implementation and on-going QA of linac-based IMRT and VMAT. Patient- specific QA strategies along with the comparison of different QA equipment and techniques will be presented. Lastly, differences will be highlighted between IMRT and VMAT for patient-specific QA. LEARNING OBJECTIVES: 1. Understand approaches to IMRT/VMAT commissioning and QA 2. Describe most relevant issues in patient-specific QA for IMRT/VMAT 3. Discuss issues with IMRT /VMAT QA equipment and techniques.

WE-G-213CD-08: Initial Experience with the Clinical Implementation of a Deep Inspiration Breath Hold for Left Breast Radiotherapy Using Surface Imaging and Visual Aid.

PURPOSE: To evaluate the clinical implementation of a deep inspiration breath hold (DIBH) treatment for left breast radiotherapy using surface imaging and visual aid. METHODS: A CT scan of the patient at DIBH is acquired and used for treatment planning. The plan and skin contour, containing isocenter and surface information are exported from the treatment planning system and imported into the surface imaging system (SIS). The skin contour constitutes the treatment reference surface or target DIBH position. A region of interest (ROI) consisting of the sternum and medial breasts is selected in the SIS. A set of video goggles allows the patient to view their breathing signal within the SIS, aiding in producing a reproducible and stable DIBH similar to simulation. Once the patient is set up at free breathing, she performs a DIBH while being monitored with the SIS. Shifts to minimize displacements from their reference DIBH surface are made. The surface image and patient setup are validated with weekly MV images. The beam is enabled when the two surfaces are within a predetermined tolerance. RESULTS: Data for evaluation of the implementation was acquired for 4 patients throughout treatment. Average treatment time was 16.8 minutes and 14.2 minutes for setup. The average displacement from the reference surface was 0.4 mm during DIBHs. The average reduction of heart mean dose and volume receiving 50% of the prescribed dose between DIBH and FB was 38% and 89% respectively. A total of 15 patients have completed this new treatment. 2 were excluded for inability to achieve reproducible and stable DIBH. CONCLUSION: The workflow we have implemented has proven to be effective and efficient for clinical purposes. Surface imaging provides adequate real time information valuable to the treatment process. Visual aid has helped patients achieve DIBH with high reproducibility and stability.

Target localization and toxicity in dose-escalated prostate radiotherapy with image-guided approach using daily planar kilovoltage imaging.

Dose escalation with intensity-modulated radiation therapy (IMRT) for carcinoma of the prostate has augmented the need for accurate prostate localization prior to dose delivery. Daily planar kilovoltage (kV) imaging is a low-dose image-guidance technique that is prevalent among radiation oncologists. However, clinical outcomes evaluating the benefit of daily kV imaging are lacking. The purpose of this study was to report our clinical experience, including prostate motion and gastrointestinal (GI) and genitourinary (GU) toxicities, using this modality. A retrospective analysis of 100 patients treated consecutively between December 2005 and March 2008 with definitive external beam IMRT for T1c-T4 disease were included in this analysis. Prescription doses ranged from 74-78 Gy (median, 76) in 2 Gy fractions and were delivered following daily prostate localization using on-board kV imaging (OBI) to localize gold seed fiducial markers within the prostate. Acute and late toxicities were graded as per the NCI CTCAEv3.0. The median follow-up was 22 months. The magnitude and direction of prostate displacement and daily shifts in three axes are reported. Of note, 9.1% and 12.9% of prostate displacements were ≥ 5 mm in the anterior-posterior and superior-inferior directions, respectively. Acute grade 2 GI and GU events occurred in 11% and 39% of patients, respectively, however no grade 3 or higher acute GI or GU events were observed. Regarding late toxicity, 2% and 17% of patients developed grade 2 toxicities, and similarly no grade 3 or higher events had occurred by last follow-up. Thus, kV imaging detected a substantial amount of inter-fractional displacement and may help reduce toxicity profiles, especially high grade events, by improving the accuracy of dose delivery.

Preoperative versus postoperative radiotherapy for locally advanced gastroesophageal junction and proximal gastric cancers: a comparison of normal tissue radiation doses.

Locoregional relapse occurs in over half of gastric cancer patients who undergo potentially curative resection. Adjuvant chemoradiation reduces locoregional relapse, but often requires irradiating large fields and is limited by poor patient tolerance. This study explores the potential dosimetric benefit in reducing the radiation dose to normal structures by treating gastroesophageal (GE) junction/proximal gastric cancers with preoperative rather than adjuvant radiotherapy. Five cases of GE junction/proximal gastric cancer patients treated postoperatively with curative intent were selected. The actual target contours were then modified to reflect hypothetical target volumes which would have been used had the patients been treated preoperatively. Hypothetical preoperative treatment plans were generated for each patient based on these modified contours. The hypothetical preoperative treatment plans were then compared to the actual postoperative plans with respect to dose-volume parameters including lung mean dose, lung V20, heart V20 and V30, and mean doses to abdominal structures. Target volumes were smaller with preoperative treatment, with an average reduction of 23%. Comparative dose-volume histogram (DVH) analysis showed the resultant composite lung doses were reduced in the preoperative plans by 50-79%. In all patients, the proportion of lungs receiving at least 20 Gy (V20) was substantially reduced using preoperative treatment (1.9% vs. 9.7% in the 3-D conformal patient; mean of 3.1% vs. 17.6% in the intensity modulated radiation therapy patients). Likewise, the volume of heart receiving at least 30 Gy was dramatically reduced in all preoperative plans (15.8% vs. 35.4%). Doses to the kidneys, liver and spinal cord were comparable in both approaches. Preoperative treatment of GE junction and proximal gastric cancer patients offers the potential to decrease the radiation dose received by normal thoracic structures.

Effect of statistical uncertainties on Monte Carlo treatment planning.

This paper reviews the effect of statistical uncertainties on radiotherapy treatment planning using Monte Carlo simulations. We discuss issues related to the statistical analysis of Monte Carlo dose calculations for realistic clinical beams using various variance reduction or time saving techniques. We discuss the effect of statistical uncertainties on dose prescription and monitor unit calculation for conventional treatment and intensity-modulated radiotherapy (IMRT) based on Monte Carlo simulations. We show the effect of statistical uncertainties on beamlet dose calculation and plan optimization for IMRT and other advanced treatment techniques such as modulated electron radiotherapy (MERT). We provide practical guidelines for the clinical implementation of Monte Carlo treatment planning and show realistic examples of Monte Carlo based IMRT and MERT plans.

A quality assurance phantom for IMRT dose verification.

This paper investigates a quality assurance (QA) phantom specially designed to verify the accuracy of dose distributions and monitor units (MU) calculated by clinical treatment planning optimization systems and by the Monte Carlo method for intensity-modulated radiotherapy (IMRT). The QA phantom is a PMMA cylinder of 30 cm diameter and 40 cm length with various bone and lung inserts. A procedure (and formalism) has been developed to measure the absolute dose to water in the PMMA phantom. Another cylindrical phantom of the same dimensions, but made of water, was used to confirm the results obtained with the PMMA phantom. The PMMA phantom was irradiated by 4, 6 and 15 MV photon beams and the dose was measured using an ionization chamber and compared to the results calculated by a commercial inverse planning system (CORVUS, NOMOS, Sewickley, PA) and by the Monte Carlo method. The results show that the dose distributions calculated by both CORVUS and Monte Carlo agreed to within 2% of dose maximum with measured results in the uniform PMMA phantom for both open and intensity-modulated fields. Similar agreement was obtained between Monte Carlo calculations and measured results with the bone and lung heterogeneity inside the PMMA phantom while the CORVUS results were 4% different. The QA phantom has been integrated as a routine QA procedure for the patient's IMRT dose verification at Stanford since 1999.

A comparative dosimetric study on tangential photon beams, intensity-modulated radiation therapy (IMRT) and modulated electron radiotherapy (MERT) for breast cancer treatment.

Recently, energy- and intensity-modulated electron radiotherapy (MERT) has garnered a growing interest for the treatment of superficial targets. In this work. we carried out a comparative dosimetry study to evaluate MERT, photon beam intensity-modulated radiation therapy (IMRT) and conventional tangential photon beams for the treatment of breast cancer. A Monte Carlo based treatment planning system has been investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We have compared breast treatment plans generated using this home-grown treatment optimization and dose calculation software forthese treatment techniques. The MERT plans were planned with up to two gantry angles and four nominal energies (6, 9, 12 and 16 MeV). The tangential photon treatment plans were planned with 6 MV wedged photon beams. The IMRT plans were planned using both multiple-gantry 6 MV photon beams or two 6 MV tangential beams. Our results show that tangential IMRT can reduce the dose to the lung, heart and contralateral breast compared to conventional tangential wedged beams (up to 50% reduction in high dose volume or 5 Gy in the maximum dose). MERT can reduce the maximum dose to the lung by up to 20 Gy and to the heart by up to 35 Gy compared to conventional tangential wedged beams. Multiple beam angle IMRT can significantly reduce the maximum dose to the lung and heart (up to 20 Gy) but it induces low and medium doses to a large volume of normal tissues including lung, heart and contralateral breast. It is concluded that MERT has superior capabilities to achieve dose conformity both laterally and in the depth direction, which will be well suited for treating superficial targets such as breast cancer.

A Monte Carlo dose calculation tool for radiotherapy treatment planning.

A Monte Carlo user code, MCDOSE, has been developed for radiotherapy treatment planning (RTP) dose calculations. MCDOSE is designed as a dose calculation module suitable for adaptation to host RTP systems. MCDOSE can be used for both conventional photon/electron beam calculation and intensity modulated radiotherapy (IMRT) treatment planning. MCDOSE uses a multiple-source model to reconstruct the treatment beam phase space. Based on Monte Carlo simulated or measured beam data acquired during commissioning, source-model parameters are adjusted through an automated procedure. Beam modifiers such as jaws, physical and dynamic wedges, compensators, blocks, electron cut-outs and bolus are simulated by MCDOSE together with a 3D rectilinear patient geometry model built from CT data. Dose distributions calculated using MCDOSE agreed well with those calculated by the EGS4/DOSXYZ code using different beam set-ups and beam modifiers. Heterogeneity correction factors for layered-lung or layered-bone phantoms as calculated by both codes were consistent with measured data to within 1%. The effect of energy cut-offs for particle transport was investigated. Variance reduction techniques were implemented in MCDOSE to achieve a speedup factor of 10-30 compared to DOSXYZ.

Monte Carlo simulation for MLC-based intensity-modulated radiotherapy.

This article describes photon beam Monte Carlo simulation for multi leaf collimator (MLC)-based intensity-modulated radiotherapy (IMRT). We present the general aspects of the Monte Carlo method for the non-Monte Carloist with an emphasis given to patient-specific radiotherapy application. Patient-specific application of the Monte Carlo method can be used for IMRT dose verification, inverse planning, and forward planning in conventional conformal radiotherapy. Because it is difficult to measure IMRT dose distributions in heterogeneous phantoms that approximate a patient, Monte Carlo methods can be used to verify IMRT dose distributions that are calculated using conventional methods. Furthermore, using Monte Carlo as the dose calculation method for inverse planning results in better-optimized treatment plans. We describe both aspects and present our recent results to illustrate the discussion. Finally, we present current issues related to clinical implementation of Monte Carlo dose calculation. Monte Carlo is the most recent, and most accurate, method of radiotherapy dose calculation. It is currently in the process of being implemented by various treatment planning vendors and will be available for clinical use in the immediate future.

Derivation of electron and photon energy spectra from electron beam central axis depth dose curves.

A method for deriving the electron and photon energy spectra from electron beam central axis percentage depth dose (PDD) curves has been investigated. The PDD curves of 6, 12 and 20 MeV electron beams obtained from the Monte Carlo full phase space simulations of the Varian linear accelerator treatment head have been used to test the method. We have employed a 'random creep' algorithm to determine the energy spectra of electrons and photons in a clinical electron beam. The fitted electron and photon energy spectra have been compared with the corresponding spectra obtained from the Monte Carlo full phase space simulations. Our fitted energy spectra are in good agreement with the Monte Carlo simulated spectra in terms of peak location, peak width, amplitude and smoothness of the spectrum. In addition, the derived depth dose curves of head-generated photons agree well in both shape and amplitude with those calculated using the full phase space data. The central axis depth dose curves and dose profiles at various depths have been compared using an automated electron beam commissioning procedure. The comparison has demonstrated that our method is capable of deriving the energy spectra for the Varian accelerator electron beams investigated. We have implemented this method in the electron beam commissioning procedure for Monte Carlo electron beam dose calculations.

The MLC tongue-and-groove effect on IMRT dose distributions.

We have investigated the tongue-and-groove effect on the IMRT dose distributions for a Varian MLC. We have compared the dose distributions calculated using the intensity maps with and without the tongue-and-groove effect. Our results showed that, for one intensity-modulated treatment field, the maximum tongue-and-groove effect could be up to 10% of the maximum dose in the dose distributions. For an IMRT treatment with multiple gantry angles (> or = 5), the difference between the dose distributions with and without the tongue-and-groove effect was hardly visible, less than 1.6% for the two typical clinical cases studied. After considering the patient setup errors, the dose distributions were smoothed with reduced and insignificant differences between plans with and without the tongue-and-groove effect. Therefore, for a multiple-field IMRT plan (> or = 5), the tongue-and-groove effect on the IMRT dose distributions will be generally clinically insignificant due to the smearing effect of individual fields. The tongue-and-groove effect on an IMRT plan with small number of fields (< 5) will vary depending on the number of fields in a plan (coplanar or non-coplanar), the MLC leaf sequences and the patient setup uncertainty, and may be significant (> 5% of maximum dose) in some cases, especially when the patient setup uncertainty is small (< or = 2 mm).

Validation of a Monte Carlo dose calculation tool for radiotherapy treatment planning.

A new EGS4/PRESTA Monte Carlo user code, MCDOSE, has been developed as a routine dose calculation tool for radiotherapy treatment planning. It is suitable for both conventional and intensity modulated radiation therapy. Two important features of MCDOSE are the inclusion of beam modifiers in the patient simulation and the implementation of several variance reduction techniques. Before this tool can be used reliably for clinical dose calculation, it must be properly validated. The validation for beam modifiers has been performed by comparing the dose distributions calculated by MCDOSE and the well-benchmarked EGS4 user codes BEAM and DOSXYZ. Various beam modifiers were simulated. Good agreement in the dose distributions was observed. The differences in electron cutout factors between the results of MCDOSE and measurements were within 2%. The accuracy of MCDOSE with various variance reduction techniques was tested by comparing the dose distributions in different inhomogeneous phantoms with those calculated by DOSXYZ without variance reduction. The agreement was within 1.0%. Our results demonstrate that MCDOSE is accurate and efficient for routine dose calculation in radiotherapy treatment planning, with or without beam modifiers.

Monte Carlo verification of IMRT dose distributions from a commercial treatment planning optimization system.

The purpose of this work was to use Monte Carlo simulations to verify the accuracy of the dose distributions from a commercial treatment planning optimization system (Corvus, Nomos Corp., Sewickley, PA) for intensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planning system has been implemented clinically to improve and verify the accuracy of radiotherapy dose calculations. Further modifications to the system were made to compute the dose in a patient for multiple fixed-gantry IMRT fields. The dose distributions in the experimental phantoms and in the patients were calculated and used to verify the optimized treatment plans generated by the Corvus system. The Monte Carlo calculated IMRT dose distributions agreed with the measurements to within 2% of the maximum dose for all the beam energies and field sizes for both the homogeneous and heterogeneous phantoms. The dose distributions predicted by the Corvus system, which employs a finite-size pencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations and measurements to within 4% in a cylindrical water phantom with various hypothetical target shapes. Discrepancies of more than 5% (relative to the prescribed target dose) in the target region and over 20% in the critical structures were found in some IMRT patient calculations. The FSPB algorithm as implemented in the Corvus system is adequate for homogeneous phantoms (such as prostate) but may result in significant under or over-estimation of the dose in some cases involving heterogeneities such as the air-tissue, lung-tissue and tissue-bone interfaces.

Removing the effect of statistical uncertainty on dose-volume histograms from Monte Carlo dose calculations.

Dose-volume histograms (DVHs) of the dose distributions calculated by the Monte Carlo method contain statistical uncertainties. The Monte Carlo DVH can be considered as blurred from the noiseless DVH by the statistical uncertainty. The focus of the present work is on the removal of the statistical uncertainty effect on the Monte Carlo DVHs and the reconstruction of the noiseless DVHs. We first study the effect of statistical uncertainty. It is found that the steeper the DVH, the more significant the effect. For typical critical structure DVHs the effect is usually negligible. For the target DVHs the effect could be clinically significant, depending on the value of uncertainty and the slope of the DVH. We then propose an iterative reconstruction algorithm. Using the DVHs and statistical uncertainties from the Monte Carlo simulations, we are able to reconstruct the noiseless DVHs. A hypothetical example and a number of clinical cases have been used to test the proposed algorithm. For each clinical case, two Monte Carlo simulations (denoted A and B) were performed. Simulation A has very large statistical uncertainties (about 10% of dose in the target volume) while simulation B has very small uncertainties (about 1%). DVHs from simulation B were used to approximate the noiseless DVHs. Using the proposed algorithm, the effect of statistical uncertainty can be removed from the DVHs of simulation A. The reconstructed DVHs were in good agreement with the DVHs from simulation B. The proposed approach is expected to be useful in removing the blurring effect on a quickly calculated Monte Carlo DVH when performing the iterative forward treatment planning.

Energy- and intensity-modulated electron beams for radiotherapy.

This work investigates the feasibility of optimizing energy- and intensity-modulated electron beams for radiation therapy. A multileaf collimator (MLC) specially designed for modulated electron radiotherapy (MERT) was investigated both experimentally and by Monte Carlo simulations. An inverse-planning system based on Monte Carlo dose calculations was developed to optimize electron beam energy and intensity to achieve dose conformity for target volumes near the surface. The results showed that an MLC with 5 mm leaf widths could produce complex field shapes for MERT. Electron intra- and inter-leaf leakage had negligible effects on the dose distributions delivered with the MLC, even at shallow depths. Focused leaf ends reduced the electron scattering contributions to the dose compared with straight leaf ends. As anticipated, moving the MLC position toward the patient surface reduced the penumbra significantly. There were significant differences in the beamlet distributions calculated by an analytic 3-D pencil beam algorithm and the Monte Carlo method. The Monte Carlo calculated beamlet distributions were essential to the accuracy of the MERT dose distribution in cases involving large air gaps, oblique incidence and heterogeneous treatment targets (at the tissue-bone and bone-lung interfaces). To demonstrate the potential of MERT for target dose coverage and normal tissue sparing for treatment of superficial targets, treatment plans for a hypothetical treatment were compared using photon beams and MERT.

Photon beam characterization and modelling for Monte Carlo treatment planning.

Photon beams of 4, 6 and 15 MV from Varian Clinac 2100C and 2300C/D accelerators were simulated using the EGS4/BEAM code system. The accelerators were modelled as a combination of component modules (CMs) consisting of a target, primary collimator, exit window, flattening filter, monitor chamber, secondary collimator, ring collimator, photon jaws and protection window. A full phase space file was scored directly above the upper photon jaws and analysed using beam data processing software, BEAMDP, to derive the beam characteristics, such as planar fluence, angular distribution, energy spectrum and the fractional contributions of each individual CM. A multiple-source model has been further developed to reconstruct the original phase space. Separate sources were created with accurate source intensity, energy, fluence and angular distributions for the target, primary collimator and flattening filter. Good agreement (within 2%) between the Monte Carlo calculations with the source model and those with the original phase space was achieved in the dose distributions for field sizes of 4 cm x 4 cm to 40 cm x 40 cm at source surface distances (SSDs) of 80-120 cm. The dose distributions in lung and bone heterogeneous phantoms have also been found to be in good agreement (within 2%) for 4, 6 and 15 MV photon beams for various field sizes between the Monte Carlo calculations with the source model and those with the original phase space.