Michael C. Lee

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.

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.

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.

Monte Carlo and experimental investigations of multileaf collimated electron beams for modulated electron radiation therapy.

Modulated electron radiation therapy (MERT) has been proposed as a means of delivering conformal dose to shallow tumors while sparing distal structures and surrounding tissues. Conventional systems for electron beam collimation are labor and time intensive in their construction and are therefore inadequate for use in the sequential delivery of multiple complex fields required by MERT. This study investigates two proposed methods of electron beam collimation: the use of existing photon multileaf collimators (MLC) in a helium atmosphere to reduce in-air electron scatter, and a MLC specifically designed for electron beam collimation. Monte Carlo simulations of a Varian Clinac 2100C were performed using the EGS4/BEAM system and dose calculations performed with the MCDOSE code. Dose penumbras from fields collimated by photon MLCs both with air and with helium at 6, 12, and 20 MeV at a range of SSDs from 70 to 90 cm were examined. Significant improvements were observed for the helium based system. Simulations were also performed on an electron specific MLC located at the level of the last scraper of a 25x25 cm2 applicator. A number of leaf materials, thicknesses, end shapes, and widths were simulated to determine optimal construction parameters. The results demonstrated that tungsten leaves 15 mm thick and 5 mm wide with unfocused ends would provide sufficient collimation for MERT fields. A prototype electron MLC was constructed and comparisons between film measurements and simulation demonstrate the validity of the Monte Carlo model. Further simulations of dose penumbras demonstrate that such an electron MLC would provide improvements over the helium filled photon MLC at all energies, and improvements in the 90-10 penumbra of 12% to 45% at 20 MeV and 6 MeV, respectively. These improvements were also seen in isodose curves when a complex field shape was simulated. It is thus concluded that an MLC specific for electron beam collimation is required for MERT.

Monte Carlo based treatment planning for modulated electron beam radiation therapy

A Monte Carlo based treatment planning system for modulated electron radiation therapy (MERT) is presented. This new variation of intensity modulated radiation therapy (IMRT) utilizes an electron multileaf collimator (eMLC) to deliver non-uniform intensity maps at several electron energies. In this way, conformal dose distributions are delivered to irregular targets located a few centimetres below the surface while sparing deeper-lying normal anatomy. Planning for MERT begins with Monte Carlo generation of electron beamlets. Electrons are transported with proper in-air scattering and the dose is tallied in the phantom for each beamlet. An optimized beamlet plan may be calculated using inverse-planning methods. Step-and-shoot leaf sequences are generated for the intensity maps and dose distributions recalculated using Monte Carlo simulations. Here, scatter and leakage from the leaves are properly accounted for by transporting electrons through the eMLC geometry. The weights for the segments of the plan are re-optimized with the leaf positions fixed and bremsstrahlung leakage and electron scatter doses included. This optimization gives the final optimized plan. It is shown that a significant portion of the calculation time is spent transporting particles in the leaves. However, this is necessary since optimizing segment weights based on a model in which leaf transport is ignored results in an improperly optimized plan with overdosing of target and critical structures. A method of rapidly calculating the bremsstrahlung contribution is presented and shown to be an efficient solution to this problem. A homogeneous model target and a 2D breast plan are presented. The potential use of this tool in clinical planning is discussed.

Monte Carlo characterization of clinical electron beams in transverse magnetic fields

Monte Carlo simulations were employed to study the characteristics of the electron beams of a clinical linear accelerator in the presence of 1.5 and 3.0 T transverse magnetic fields and to assess the possibility of using magnetic fields in conjunction with modulated electron radiation therapy (MERT). The starting depth of the magnetic field was varied over several centimetres. It was found that peak doses of as much as 2.7 times the surface dose could be achieved with a 1.5 T magnetic field. The magnetic field was shown to reduce the 80% and 20% dose drop-off distance by 50% to 80%. The distance between the 80% dose levels of the pseudo-Bragg peak induced by the magnetic field was found to be extremely narrow, generally less than 1 cm. However, by modulating the energy and intensity of the electron fields while simultaneously moving the magnetic field, a homogeneous dose distribution with low surface dose and a sharp dose fall-off was generated. Heterogeneities are shown to change the effective range of the electron beams, but not eliminate the advantages of a sharp depth dose drop-off or high peak-to-surface dose ratio. This suggests the applicability of MERT with magnetic fields in heterogeneous media. The results of this study demonstrate the ability to use magnetic fields in MERT to produce highly desirable dose distributions.

Characterization of electron beams for modulated electron beam radiotherapy

A set of clinical electron beams have been characterized in anticipation of their use in modulated electron radiotherapy (MERT). Using the EGS4/BEAM and DOSXYZ simulation codes, a Monte Carlo model of a Varian Clinac 2100C was generated for electron beams of nominal energies of 6, 9, 12, 16, and 20 MeV. Dose calculations using the Monte Carlo generated phase space data demonstrate profile and depth-dose agreement with measured data to within 2% of the maximum dose values. Simulations were then performed using a proposed electron multileaf collimator to examine beam characteristics relevant to MERT. In particular, abutting fields have been constructed and the presence of hot and cold spots due to penumbral mismatches is shown to be significant. The effect of field setup errors is also considered and it is shown that small setup uncertainties of 3 mm may lead to significant hot and cold spots of up to 25%. Furthermore, electron beamlets were shown to differ in penumbras and symmetry based on where they were located in the field. Intensity modulated electron fields were then constructed, demonstrating the ability to create fields with tilted dose profiles using a sequential abutting field method and a dynamic delivery method.

A quality assurance phantom for IMRT dose verification

We have investigated a quality assurance (QA) phantom that was specially designed to verify the accuracy of dose distributions calculated by a commercial inverse planning optimization system (CORVUS) 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 was developed to measure the absolute dose at any point inside the QA 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 CORVUS and by the Monte Carlo method. The results show that the dose distributions calculated by both CORVUS and Monte Carlo agreed well (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 calculation and measured results with the bone and lung heterogeneities inside the PMMA phantom. Following the positive results of this study, our QA phantom has been integrated as a routine QA procedure for patients IMRT dose verification at Stanford since 1999.

Monte Carlo Simulations of Multileaf Collimated Electrons

There has been growing interest in the use of intensity modulated electron beams in conjuction with intensity modulated photon beams to obtain optimal dose distributions for radiation therapy. A number of methods have been proposed, including the use of scanned beam systems1 as well as utilizing the existing photon multileaf collimators for electron therapy. Studies by Jansson et al. have used matched photon and electron beams from the intrinsic MLC of a microtron (MM22, Scanditronix Medical AB, Uppsala, Sweden) to improve dose distributions for breast cancer treatment.2 Experimental work by Klein et al. has shown that in a standard Varian 2100C, an SSD of no greater that 70 cm is required for a clinically acceptable field3. Recently, Karlsson et al. utilized Monte Carlo simulations to conclude that for a 9 MeV beam, an acceptable dose penumbra could be acheived by replacing the beam axis atmosphere with helium to reduce in-air scatter, or by lowering the photon MLC by at least 11cm4. This study uses a combination of Monte Carlo simulations and film measurements to extend these studies to a greater energy range (6, 12, and 20 MeV) and presents data that suggets that for low energies, an electron specific multileaf collimator is necessary for intensity modulated electron beam therapy.

Modulated Electron Beams for Treatment of Breast Cancer

Photon beams have been an effective modality for breast cancer treatment in radiation therapy while electron beams are occasionally used for a boost dose when the photon beams are intentionally placed to miss part of the target in order to reduce the dose to the lung and heart. Although such conventional treatment with tangential photon fields has been successful, the following problems (or potential areas of improvement) remain: (1) the inclusion of the lung and other normal tissues, and sometimes of a small volume of the heart in the high-dose volume due to tumor location, patient size or in the case of chest-wall treatments; (2) lower dose near the skin surface due to lack of electron build-up in a photon beam; and (3) high exit or scatter dose to normal structures such as the lung and heart, and more importantly the contralateral breast, which may be a major cause for the occurrence of secondary cancer in the contralateral breast for women under the age of 45.