Theoretical considerations of monitor unit calculations for intensity modulated beam treatment planning

Independent calculation of monitor units for VMAT and SPORT

PURPOSE

Dose and monitor units (MUs) represent two important facets of a radiation therapy treatment. In current practice, verification of a treatment plan is commonly done in dose domain, in which a phantom measurement or forward dose calculation is performed to examine the dosimetric accuracy and the MU settings of a given treatment plan. While it is desirable to verify directly the MU settings, a computational framework for obtaining the MU values from a known dose distribution has yet to be developed. This work presents a strategy to calculate independently the MUs from a given dose distribution of volumetric modulated arc therapy (VMAT) and station parameter optimized radiation therapy (SPORT).

METHODS

The dose at a point can be expressed as a sum of contributions from all the station points (or control points). This relationship forms the basis of the proposed MU verification technique. To proceed, the authors first obtain the matrix elements which characterize the dosimetric contribution of the involved station points by computing the doses at a series of voxels, typically on the prescription surface of the VMAT/SPORT treatment plan, with unit MU setting for all the station points. An in-house Monte Carlo (MC) software is used for the dose matrix calculation. The MUs of the station points are then derived by minimizing the least-squares difference between doses computed by the treatment planning system (TPS) and that of the MC for the selected set of voxels on the prescription surface. The technique is applied to 16 clinical cases with a variety of energies, disease sites, and TPS dose calculation algorithms.

RESULTS

For all plans except the lung cases with large tissue density inhomogeneity, the independently computed MUs agree with that of TPS to within 2.7% for all the station points. In the dose domain, no significant difference between the MC and Eclipse Anisotropic Analytical Algorithm (AAA) dose distribution is found in terms of isodose contours, dose profiles, gamma index, and dose volume histogram (DVH) for these cases. For the lung cases, the MC-calculated MUs differ significantly from that of the treatment plan computed using AAA. However, the discrepancies are reduced to within 3% when the TPS dose calculation algorithm is switched to a transport equation-based technique (Acuros™). Comparison in the dose domain between the MC and Eclipse AAA/Acuros calculation yields conclusion consistent with the MU calculation.

CONCLUSIONS

A computational framework relating the MU and dose domains has been established. The framework does not only enable them to verify the MU values of the involved station points of a VMAT plan directly in the MU domain but also provide a much needed mechanism to adaptively modify the MU values of the station points in accordance to a specific change in the dose domain.

Dosimetric validation and clinical implementation of two 3D dose verification systems for quality assurance in volumetric-modulated arc therapy techniques

A pretreatment quality assurance program for volumetric techniques should include redundant calculations and measurement-based verifications. The patient-specific quality assurance process must be based in clinically relevant metrics. The aim of this study was to show the commission, clinical implementation, and comparison of two systems that allow performing a 3D redundant dose calculation. In addition, one of them is capable of reconstructing the dose on patient anatomy from measurements taken with a 2D ion chamber array. Both systems were compared in terms of reference calibration data (absolute dose, output factors, percentage depth-dose curves, and profiles). Results were in good agreement for absolute dose values (discrepancies were below 0.5%) and output factors (mean differences were below 1%). Maximum mean discrepancies were located between 10 and 20 cm of depth for PDDs (-2.7%) and in the penumbra region for profiles (mean DTA of 1.5 mm). Validation of the systems was performed by comparing point-dose measurements with values obtained by the two systems for static, dynamic fields from AAPM TG-119 report, and 12 real VMAT plans for different anatomical sites (differences better than 1.2%). Comparisons between measurements taken with a 2D ion chamber array and results obtained by both systems for real VMAT plans were also performed (mean global gamma passing rates better than 87.0% and 97.9% for the 2%/2 mm and 3%/3 mm criteria). Clinical implementation of the systems was evaluated by comparing dose-volume parameters for all TG-119 tests and real VMAT plans with TPS values (mean differences were below 1%). In addition, comparisons between dose distributions calculated by TPS and those extracted by the two systems for real VMAT plans were also performed (mean global gamma passing rates better than 86.0% and 93.0% for the 2%/2 mm and 3%/ 3 mm criteria). The clinical use of both systems was successfully evaluated.

Motion as perturbation. II. Development of the method for dosimetric analysis of motion effects with fixed-gantry IMRT

PURPOSE

In this work, the feasibility of implementing a motion-perturbation approach to accurately estimate volumetric dose in the presence of organ motion--previously demonstrated for VMAT--is studied for static gantry IMRT. The method's accuracy is improved for the voxels that have very low planned dose but acquire appreciable dose due to motion. The study describes the modified algorithm and its experimental validation and provides an example of a clinical application.

METHODS

A contoured region-of-interest is propagated according to the predefined motion kernel throughout time-resolved 4D phantom dose grids. This timed series of 3D dose grids is produced by the measurement-guided dose reconstruction algorithm, based on an irradiation of a static ARCCHECK (AC) helical dosimeter array (Sun Nuclear Corp., Melbourne, FL). Each moving voxel collects dose over the dynamic simulation. The difference in dose-to-moving voxel vs dose-to-static voxel in-phantom forms the basis of a motion perturbation correction that is applied to the corresponding voxel in the patient dataset. A new method to synchronize the accelerator and dosimeter clocks, applicable to fixed-gantry IMRT, was developed. Refinements to the algorithm account for the excursion of low dose voxels into high dose regions, causing appreciable dose increase due to motion (LDVE correction). For experimental validation, four plans using TG-119 structure sets and objectives were produced using segmented IMRT direct machine parameters optimization in Pinnacle treatment planning system (v. 9.6, Philips Radiation Oncology Systems, Fitchburg, WI). All beams were delivered with the gantry angle of 0°. Each beam was delivered three times: (1) to the static AC centered on the room lasers; (2) to a static phantom containing a MAPCHECK2 (MC2) planar diode array dosimeter (Sun Nuclear); and (3) to the moving MC2 phantom. The motion trajectory was an ellipse in the IEC XY plane, with 3 and 1.5 cm axes. The period was 5 s, with the resulting average motion speed of 1.45 cm/s. The motion-perturbed high resolution (2 mm voxel) volumetric dose grids on the MC2 phantom were generated for each beam. From each grid, a coronal dose plane at the detector level was extracted and compared to the corresponding moving MC2 measurement, using gamma analysis with both global (G) and local (L) dose-error normalization.

RESULTS

Using the TG-119 criteria of (3%G/3 mm), per beam average gamma analysis passing rates exceeded 95% in all cases. No individual beam had a passing rate below 91%. LDVE correction eliminated systematic disagreement patterns at the beams' aperture edges. In a representative example, application of LDVE correction improved (2%L/2 mm) gamma analysis passing rate for an IMRT beam from 74% to 98%.

CONCLUSIONS

The effect of motion on the moving region-of-interest IMRT dose can be estimated with a standard, static phantom QA measurement, provided the motion characteristics are independently known from 4D CT or otherwise. The motion-perturbed absolute dose estimates were validated by the direct planar diode array measurements, and were found to reliably agree with them in a homogeneous phantom.

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D'0, that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D'0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D'0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of dm, with D'0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.

Optimized point dose measurement for monitor unit verification in intensity modulated radiation therapy using 6 MV photons by three different methodologies with different detector-phantom combinations: A comparative study

The study was aimed to compare accuracy of monitor unit verification in intensity modulated radiation therapy (IMRT) using 6 MV photons by three different methodologies with different detector phantom combinations. Sixty patients were randomly chosen. Zero degree couch and gantry angle plans were generated in a plastic universal IMRT verification phantom and 30×30×30 cc water phantom and measured using 0.125 cc and 0.6 cc chambers, respectively. Actual gantry and couch angle plans were also measured in water phantom using 0.6 cc chamber. A suitable point of measurement was chosen from the beam profile for each field. When the zero-degree gantry, couch angle plans and actual gantry, couch angle plans were measured by 0.6 cc chamber in water phantom, the percentage mean difference (MD) was 1.35%, 2.94 % and Standard Deviation (SD) was 2.99%, 5.22%, respectively. The plastic phantom measurements with 0.125 cc chamber Semiflex ionisation chamber (SIC) showed an MD=4.21% and SD=2.73 %, but when corrected for chamber-medium response, they showed an improvement, with MD=3.38 % and SD=2.59 %. It was found that measurements with water phantom and 0.6cc chamber at gantry angle zero degree showed better conformity than other measurements of medium-detector combinations. Correction in plastic phantom measurement improved the result only marginally, and actual gantry angle measurement in a flat- water phantom showed higher deviation.

Independent calculation of dose from a helical TomoTherapy unit

A new calculation algorithm has been developed for independently verifying doses calculated by the TomoTherapy Hi.Art treatment planning system (TPS). The algorithm is designed to confirm the dose to a point in a high dose, low dose-gradient region. Patient data used by the algorithm include the radiological depth to the point for each projection angle and the treatment sinogram file controlling the leaf opening time for each projection. The algorithm uses common dosimetric functions [tissue phantom ratio (TPR) and output factor (Scp)] for the central axis combined with lateral and longitudinal beam profile data to quantify the off-axis dose dependence. Machine data for the dosimetric functions were measured on the Hi.Art machine and simulated using the TPS. Point dose calculations were made for several test phantoms and for 97 patient treatment plans using the simulated machine data. Comparisons with TPS-predicted point doses for the phantom treatment plans demonstrated agreement within 2% for both on-axis and off-axis planning target volumes (PTVs). Comparisons with TPS-predicted point doses for the patient treatment plans also showed good agreement. For calculations at sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Calculations within lung and superficial PTVs overestimated the dose by an average of 3.1% (sigma=2.4%) and 3.2% (sigma=2.2%), respectively. Systematic errors within lung are probably due to the weakness of the algorithm in correcting for missing tissue and/or tissue density heterogeneities. Errors encountered within superficial PTVs probably result from the algorithm overestimating the scatter dose within the patient. Our results demonstrate that for the majority of cases, the algorithm could be used without further refinement to independently verify patient treatment plans.

Optimal matching of 3D film-measured and planned doses for intensity-modulated radiation therapy quality assurance

Intensity-modulated radiation therapy (IMRT) is one of the most complex applications of radiotherapy that requires patient-specific quality assurance (QA). Here, we describe a novel method of 3-dimensional (3D) dose-verification using 12 acrylic slabs in a 3D phantom (30 x 30 x 12 cm(3)) with extended dose rate (EDR2) films, which is both faster than conventionally used methods, and clinically useful. With custom-written software modules written in Microsoft Excel Visual Basic Application, the measured and planned dose distributions for the axial, coronal, and sagittal planes were superimposed by matching their origins, and the point doses were compared at all matched positions. Then, an optimization algorithm was used to correct the detected setup errors. The results show that this optimization method significantly reduces the average maximum dose difference by 7.73% and the number of points showing dose differences of more than 5% by 8.82% relative to the dose differences without an optimization. Our results indicate that the dose difference was significantly decreased with optimization and this optimization method is statistically reliable and effective. The results of 3D optimization are discussed in terms of various patient-specific QA data obtained from statistical analyses.

Optimized matching of film dosimetry with calculated doses for IMRT quality assurance

We develop a new method with a global optimization for registering films to calculate doses for intensity-modulated radiation therapy (IMRT) and intensity-modulated radiosurgery (IMRS) quality assurance (QA). Both absolute point dosimetry and two-dimensional (2D) film dosimetry are performed through the IMRT and IMRS using Clinac 21EX's 120 millenium MLC and BrainLab's micro-MLC, respectively. The measured and calculated dose distributions are superimposed by coincidence of their origins, followed by comparison of the point doses at all matched positions. Then, with the optimization algorithm the setup error of the dosimeter is corrected. An example of IMRT cases shows that the average percentage showing 3% of dose difference for 10 patients has been reduced from 19% to 9%, before and after optimization and weight, respectively. Similar results are obtained for IMRS. This method dramatically reduces the difference between measured and calculated dose distributions in all cases investigated.

Cálculo independente de dose para tratamentos de arco dinâmico com colimador micromultilâminas

OBJETIVO: Em técnicas de tratamento como o arco dinâmico, a verificação manual dos cálculos do sistema de planejamento é muito difícil. Assim, a utilização de ferramentas computacionais é de utilidade e torna-se componente essencial do programa de controle de qualidade. MATERIAIS E MÉTODOS: Foi criado um programa computacional de tipo planilha eletrônica para realizar cálculo independente da dose, ou equivalente das unidades monitoras, nos tratamentos realizados pela técnica de arco dinâmico com micromultilâminas. Os valores de dose calculados, por arco e por tratamento completo, foram comparados aos valores obtidos do sistema de planejamento BrainScan v5.3. O programa desenvolvido foi testado com 229 campos de arco dinâmico que representam 42 tratamentos de crânio. Desses, 109 campos foram calculados em reconstrução tridimensional feita a partir das imagens de tomografia dos pacientes, 109 na reconstrução de um objeto simulador de polimetilmetacrilato e 21 na de um objeto simulador sólido equivalente à água. RESULTADOS: A diferença média de doses totais encontrada nos 42 tratamentos (compostos de um ou mais arcos dinâmicos), entre o programa de verificação e o sistema de planejamento, foi de +1,73%, com desvio-padrão de 0,76%. A diferença máxima encontrada foi de 3,32% e a mínima, de -0,20%. No caso dos 229 arcos testados um a um, a diferença média encontrada foi de 1,61%, com desvio-padrão de 1,04%. Os valores máximos e mínimos das diferenças foram de 4,01% e -2,04%, respectivamente. Em 80,35% dos arcos testados, as doses calculadas acham-se na faixa de ± 2,5% de diferença com relação às doses geradas pelo sistema de planejamento. CONCLUSÃO: O programa apresentado é recomendado para a verificação da dose pontual dos planos de tratamento, como parte do procedimento de garantia de qualidade em radioterapia e radiocirurgia estereotáxica quando se utiliza a técnica de arco dinâmico por meio de um colimador micromultilâminas, nos quais um cálculo manual é muito difícil ou inviável, pela complexidade da técnica.

Inclusion of compensator-induced scatter and beam filtration in pencil beam dose calculations

Compensators can be used as beam intensity modulation devices for intensity-modulated radiation therapy applications. In contrast with multileaf collimators, compensators introduce scatter and beam hardening into the therapeutic x-ray beam. The degree of scatter and beam filtering depends on the compensator material and beam energy. Pencil beam dose calculation models can be used to derive the shape of the compensator. In this study a novel way of incorporating the effect of compensator-induced scatter and beam filtration is presented. The study was conducted using 6, 8, and 15 MV polyenergetic pencil beams (PBs). The compensator materials that were studied included wax, brass, copper, and lead. The perturbation effects of the compensators on the PB dose profiles were built in the PB dose profiles and tested for regular fields containing a step compensator and benchmarked against DOSXYZnrc Monte Carlo calculated dose profiles. These effects include compensator beam filtration and Compton-scattered photons generated in the compensator materials that influence the resulting PB dose profiles. These data were obtained from DOSXYZnrc simulations. A Gaussian function was used to model off-axis scatter and an exponential function was used to model beam hardening at any radius, r. Dose profiles were calculated under a step compensator using the method that can model beam hardening and off-axis scatter, as well as a conventional method where the PB profiles are not adjusted, but a single effective attenuation coefficient is used instead to best match the dose profiles. Both sets of data were compared to the DOSXYZnrc data. Depth and profile dose data for 10 x 10 cm2 and 20 x 20 cm2 fields indicated that at 2 cm depth in water the method that takes compensator scatter into account agrees more closely with the DOSXYZnrc data compared to the data using only an effective attenuation coefficient. Further, it was found that the effective attenuation method can only replicate the DOSXYZnrc data at 10 cm depth where it was chosen to do so. At shallower depths the effective attenuation method overestimates the dose and beyond 10 cm depth it causes an underestimation in the dose. The scatter and beam hardening inclusion method does not exhibit such properties. The exclusion of scatter can lead to dose errors of up to 4 percent with a copper compensator at 5 cm depth for a 10 X 10 cm2 field under a thickness of 5 cm at 6 MV. For materials such as lead this discrepancy could be as high as 7 to 8 percent at 6 MV. For larger fields (20 X 20 cm2) the effect of in-phantom scatter reduces the differences between the dose profiles calculated with the mentioned methods.

A separated primary and scatter model for independent dose calculation of intensity modulated radiotherapy

BACKGROUND AND PURPOSE

Independent checking of beam monitor units is an essential step in the preparation of any radiotherapy plan. The present work describes a simple model for calculating a point dose from an arbitrary number of irregular, asymmetric fields, typical of beam segments used for IMRT.

MATERIALS AND METHODS

Primary and scatter dose contributions were separated using a two-parameter exponential fit to measured beam data, from which differential scatter is determined. A total of 60 IMRT patient plans for a five-field prostate class solution were investigated to validate the model.

RESULTS

The average difference between the model's prediction and direct measurement of reference dose was found to be -0.6% (ranging from -2.9% to +1.6%), with a standard deviation of 1.0%. This compares well with the observed average difference between treatment planning system prediction and direct measurement of +0.8% (SD 0.6%).

CONCLUSIONS

The model is shown to provide a reliable and accurate independent check of planning system monitor units for the prostate IMRT plans studied. Implementation of the model could significantly reduce the time needed for point dose verification of IMRT plans currently performed by direct measurement.

History of tomotherapy

Tomotherapy is the delivery of intensity modulated radiation therapy using rotational delivery of a fan beam in the manner of a CT scanner. In helical tomotherapy the couch and gantry are in continuous motion akin to a helical CT scanner. Helical tomotherapy is inherently capable of acquiring CT images of the patient in treatment position and using this information for image guidance. This review documents technological advancements of the field concentrating on the conceptual beginnings through to its first clinical implementation. The history of helical tomotherapy is also a story of technology migration from academic research to a university-industrial partnership, and finally to commercialization and widespread clinical use.

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.

Dosimetric verification of IMRT treatment planning using Monte Carlo simulations for prostate cancer

The purpose of this work is to investigate the accuracy of dose calculation of a commercial treatment planning system (Corvus, Normos Corp., Sewickley, PA). In this study, 30 prostate intensity-modulated radiotherapy (IMRT) treatment plans from the commercial treatment planning system were recalculated using the Monte Carlo method. Dose-volume histograms and isodose distributions were compared. Other quantities such as minimum dose to the target (D(min)), the dose received by 98% of the target volume (D98), dose at the isocentre (D(iso)), mean target dose (D(mean)) and the maximum critical structure dose (D(max)) were also evaluated based on our clinical criteria. For coplanar plans, the dose differences between Monte Carlo and the commercial treatment planning system with and without heterogeneity correction were not significant. The differences in the isocentre dose between the commercial treatment planning system and Monte Carlo simulations were less than 3% for all coplanar cases. The differences on D98 were less than 2% on average. The differences in the mean dose to the target between the commercial system and Monte Carlo results were within 3%. The differences in the maximum bladder dose were within 3% for most cases. The maximum dose differences for the rectum were less than 4% for all the cases. For non-coplanar plans, the difference in the minimum target dose between the treatment planning system and Monte Carlo calculations was up to 9% if the heterogeneity correction was not applied in Corvus. This was caused by the excessive attenuation of the non-coplanar beams by the femurs. When the heterogeneity correction was applied in Corvus, the differences were reduced significantly. These results suggest that heterogeneity correction should be used in dose calculation for prostate cancer with non-coplanar beam arrangements.

Dosimetry validation of treatment room shielding design

Intensity-modulated radiation therapy (IMRT) can lead to an increase in leakage radiation. The total number of monitor units (MUs) for IMRT is typically 2-5 times that for conventional treatments [the ratio of the two is used to derive the effective modulation scaling factor (MSFeff)]. Shielding calculations for IMRT can be done by applying the MSFeff to measured exposures under conservative conditions (standard beam setup 40 cm x 40 cm field, 45 degrees collimator angle) to account for the increased leakage. In this work, we verified this approach for two existing vaults housing a Siemens Primart 6 MV linac and a Varian 21Ex 10 MV linac. We measured the cumulative exposures at various locations around the vaults for typical IMRT cases and for the standard beam setup using the same MUs. For the standard beam setup, the IMRT gantry angles and eight equally spaced angles were used. Estimations of weekly exposures for IMRT were carried out using exposure rates measured under standard beam setup and the MSFeff averaged over 20 treatment cases. The accumulated exposures under realistic IMRT conditions were 30%-50% lower than the estimated values using equally spaced gantry angles except for two locations where the real IMRT leakage was higher than the estimated value by approximately 10%. Measurements using the same gantry angles yielded similar results. Our results indicate that it is adequate to use the MSFeff and previously measured exposures to estimate the leakage increase due to IMRT for an existing vault. Different approaches should be followed when considering primary or secondary barriers since the standard beam setup is overestimating the exposures behind primary barriers compared to IMRT. In such cases, a 10 cm x 10 cm field can be used for more accurate shielding evaluation.

A simple theoretical verification of monitor unit calculation for intensity modulated beams using dynamic mini-multileaf collimation

A spreadsheet based program is presented to perform an independent Monitor Unit (MU) calculation verification for the Quality Assurance (QA) of Intensity Modulated Radiation Therapy (IMRT) using Dynamic MultiLeaf Collimation (DMLC). The computed dose value is compared to the planned dose by calculating the percent dose difference per Intensity Modulated Beam (IMB) and absolute dose difference per IMB. The proposed acceptability levels are +/-5.0% or +/-2.0 cGy for the percent dose difference per IMB and the absolute dose difference per IMB, respectively. For percent dose difference per treatment, an acceptability level of +/-2.0% is proposed. The presented program is considered adequate for checking the treatment plans calculated for IMRT treatments using DMLC as a part of the QA procedure.

Intensity-modulated radiation therapy: overlapping co-axial modulated fields

The Varian multi-leaf collimator has a 14.5 cm leaf extension limit from each carriage. This means the target volumes in the head and neck region are sometimes too wide for standard width-modulated fields to provide adequate dose coverage. A solution is to set up asymmetric co-axial overlapping fields. This protects the MLC carriage while in return the MLC provides modulated dose blending in the field overlap region. Planar dose maps for coincident fields from the Pinnacle radiotherapy treatment planning system are compared with planar dose maps reconstructed from radiographic film and electronic portal images. The film and portal images show small leaf-jaw matchlines at each field overlap border. Linear profiles taken across each image show that the observed leaf-jaw matchlines from the accelerator images are not accounted for by the treatment planning system. Dose difference between film reconstructed electronic portal images and planning system are about 2.5 cGy in a modulated field at d(max). While the magnitude of the dose differences are small improved round end leaf modelling combined with a finer dose calculation grid may minimize the discrepancy between calculated and delivered dose.

Monitor unit calculation for Monte Carlo treatment planning

In this work, we investigate a formalism for monitor unit (MU) calculation in Monte Carlo based treatment planning. By relating MU to dose measured under reference calibration conditions (central axis, depth of dose maximum in water, 10 cm x 10 cm field defined at 100 cm source-to-surface distance) our formalism determines the MU required for a treatment plan based on the prescription dose and Monte Carlo calculated dose distribution. Detailed descriptions and formulae are given for various clinical situations including conventional treatments and advanced techniques such as intensity-modulated radiotherapy (IMRT) and modulated electron radiotherapy (MERT). Analysis is made of the effects of source modelling, beam modifier simulation and patient dose calculation accuracy, all of which are important factors for absolute dose calculations using Monte Carlo simulations. We have tested the formalism through phantom measurements and the predicted MU values were consistent with measured values to within 2%. The formalism has been used for MU calculation and plan comparison for advanced treatment techniques such as MERT, extracranial stereotactic IMRT, MRI-based treatment planning and intensity-modulated laser-proton therapy studies. It is also used for absolute dose calculations using Monte Carlo simulations for treatment verification, which has become part of our comprehensive IMRT quality assurance programme.

Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

Patient-specific point dose measurement for IMRT monitor unit verification

PURPOSE

To review intensity-modulated radiation therapy (IMRT) monitor unit verification in a phantom for 751 clinical cases.

METHODS AND MATERIALS

A custom water-filled phantom was used to measure the integral dose with an ion chamber for patient-specific quality assurance. The Corvus IMRT planning system was used for all cases reviewed. The 751 clinical cases were classified into 9 treatment sites: central nervous system (27 cases), gastrointestinal (24 cases), genitourinary (447 cases), gynecologic (18 cases), head and neck (200 cases), hematology (12 cases), pediatric (3 cases), sarcoma (8 cases), and thoracic (12 cases). Between December 1998 and January 2002, 1591 measurements were made for these 751 IMRT quality assurance plans.

RESULTS

The mean difference (MD) in percent between the measurements and the calculations was +0.37% (with the measurement being slightly higher). The standard deviation (SD) was 1.7%, and the range of error was from -4.5% to 9.5%. The MD and SD were +0.49% and 1.4% for MIMiC treatments delivered in 2-cm mode (261 cases) and -0.33% and 2.7% for those delivered in 1-cm mode (36 cases). Most treatments (420) were delivered using the step-and-shoot multileaf collimator with a 6-MV photon beam; the MD and SD were +0.31% and 1.8%, respectively. Among the 9 treatment sites, the prostate IMRT (in genitourinary site) was most consistent with the smallest SD (1.5%). There were 23 cases (3.1% of all cases) in which the measurement difference was greater than 3.5%; of those, 6 cases used the MIMiC in 1-cm mode, and 14 of the cases were from the head-and-neck treatment site.

CONCLUSION

IMRT monitor unit calculations from the Corvus planning system agreed within 3.5% with the point-dose ion chamber measurement in 97% of 751 cases representing 9 different treatment sites. A good consistency was observed across sites.

Shielding evaluation for IMRT implementation in an existing accelerator vault

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator.

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.

Independent monitor unit calculation for intensity modulated radiotherapy using the MIMiC multileaf collimator

A self-consistent monitor unit (MU) and isocenter point-dose calculation method has been developed that provides an independent verification of the MU for intensity modulated radiotherapy (IMRT) using the MIMiC (Nomos Corporation) multileaf collimator. The method takes into account two unique features of IMRT using the MIMiC: namely the gantry-dynamic arc delivery of intensity modulated photon beams and the slice-by-slice dose delivery for large tumor volumes. The method converts the nonuniform beam intensity planned at discrete gantry angles of 5 degrees or 10 degrees into conventional nonmodulated beam intensity apertures of elemental arc segments of 1 degree. This approach more closely simulates the actual gantry-dynamic arc delivery by MIMiC. Because each elemental arc segment is of uniform intensity, the MU calculation for an IMRT arc is made equivalent to a conventional arc with gantry-angle dependent beam apertures. The dose to the isocenter from each 1 degree elemental arc segment is calculated by using the Clarkson scatter summation technique based on measured tissue-maximum-ratio and output factors, independent of the dose calculation model used in the IMRT planning system. For treatments requiring multiple treatment slices, the MU for the arc at each treatment slice takes into account the MU, leakage and scatter doses from other slices. This is achieved by solving a set of coupled linear equations for the MUs of all involved treatment slices. All input dosimetry data for the independent MU/isocenter point-dose calculation are measured directly. Comparison of the MU and isocenter point dose calculated by the independent program to those calculated by the Corvus planning system and to direct measurements has shown good agreement with relative difference less than +/-3%. The program can be used as an independent initial MU verification for IMRT plans using the MIMiC multileaf collimators.

Quality assurance of intensity-modulated radiotherapy

Intensity-modulated radiation therapy (IMRT) requires the use of inverse treatment planning and nonuniform fluence beams delivered by a series of complex radiation portals. The quality assurance procedures for conventional three-dimensional conformal radiation therapy (3D-CRT) have been developed and are in worldwide clinical use, but the more complex nature of IMRT limits the application of much of the quality assurance (QA) procedures developed for IMRT. Although consensus has not yet been reached regarding which procedures will eventually become recommended by official organizations, the field is rapidly coming to agreement on a basic set of procedures. This manuscript describes some of the novel techniques recently developed for IMRT QA, both for the validation of the calculated dose distribution and for assuring that the dose distribution reaches its intended targets.

[Dosimetric verification in intensity modulated radiation therapy]

As part of dosimetric verification for IMRT intensity modulated radiation therapy, we examined the selection of a dosimeter in accordance with the purpose of physical measurement and the process of data analysis. Because of the high dose conformation in the target volume and minimum dose in the organs at risk (OAR) in IMRT, dosimetric verification is essential. Because the performance of dosimetric verification in a patient is not allowed, a physical phantom and dosimeter must be used. Dose verification using a physical phantom, from which the beam data optimized for a patient slated for IMRT are transferred, may cause latent error as a result of change in the depth of each beam toward an isocenter. This effect may change the dose distribution and prescription dose. The basic methods of dosimetric verification with physical measurement are point dosimetry, when the reference dose is given at a point by planning software, and volumetric dosimetry, when planning software gives the dose as a volumetric configuration. While the most accurate dosimetry is done using a calibrated ionization chamber, IMRT requires volumetric dosimetry using some kind of portal film or a polymer gel dosimeter, because of the need for dosimetric verification for an irregular dose distribution in IMRT. The importance of indirect dosimetry using these methods is to provide calibration as a dosimeter, absolute dose, and preservation of calibration. In our study, the verification of dose distribution for IMRT using portal film and a RANDO phantom could be performed with an error of less than 2% in all cases. The measurement error for the central dose using a JARP-type ionization chamber and MixDP was less than 3% in all cases except for the case with the maximum error. At the moment, IMRT requires a great deal of effort in the processes of planning, dosimetric verification, and isocenter checking in every fraction to maintain high accuracy. Although the need for a large amount of effort in the service of maintaining accuracy may be reasonable, it could be enough to inhibit the spread of IMRT. It is hoped that an easy method of dosimetric verification that still maintains a high level of accuracy will develop as a result of this great effort.

Quasi-independent monitor unit calculation for intensity modulated sequential tomotherapy

The number of linac monitor units (MU) from intensity modulated sequential tomotherapy (IMST) is substantially larger than the MU delivered in conventional radiation therapy, and the relation between MU and dose is obscure due to complicated variation of the beam intensities. The purpose of this work was to develop a practical method of verifying the MU and dose from IMST so that the MU of each arced beam could be double-checked for accuracy. MU calculations for 41 arced beams from 14 IMST patients were performed using the variables of vane open fraction time, field size, target depth, output factor, TMR, and derived intensity distribution. Discrepancy between planned and checked MU was quantified as 100 (MU(cal)-MU(plan))/MU(plan) percent. All 41 discrepancies were clustered between -5% to +4%, illustrated in a Gaussian-shaped histogram centered at -1.0+/-3.5% standard deviation indicating the present MU calculations are in agreement with the planned expectations. To confirm the correctness of the present calculated MUs of the IMST plans, eight of the calculated IMST plans are performed dose verifications using their hybrid plans, which are created by transporting patient's IMST plan beams onto a spherical polystyrene Phantom for dose distribution within the Phantom. The dose was measured with a 0.07 cc ionization chamber inserted in the spherical Phantom during the hybrid plan irradiation. Average discrepancy between planned and measured doses was found to be 0.6+/-3.4% with single standard deviation uncertainty. The spread of the discrepancies of present calculated MUs relative to their planned ones are attributed to uncertainties of effective field size, effective planned dose corresponding to each arc, and inaccuracy of quantification of scattered dose from adjacent arced beams. Overall, the present calculation of MUs is consistent with what derived from treatment plans. Since the MUs are verified by actual dose measurements, therefore the present MU calculation technique is considered adequate for double-checking planned IMST MUs.

Dosimetric verification of inverse planned step and shoot multileaf collimator fields from a commercial treatment planning system

An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in-house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a 10x10 cm2 non-MLC field, were found to differ by 2-3% from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a "beam's eye view" (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in-house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of -1.2 cGy, and a standard deviation of 2.4 cGy.

Intensity-modulated radiotherapy: current status and issues of interest

PURPOSE

To develop and disseminate a report aimed primarily at practicing radiation oncology physicians and medical physicists that describes the current state-of-the-art of intensity-modulated radiotherapy (IMRT). Those areas needing further research and development are identified by category and recommendations are given, which should also be of interest to IMRT equipment manufacturers and research funding agencies.

METHODS AND MATERIALS

The National Cancer Institute formed a Collaborative Working Group of experts in IMRT to develop consensus guidelines and recommendations for implementation of IMRT and for further research through a critical analysis of the published data supplemented by clinical experience. A glossary of the words and phrases currently used in IMRT is given in the. Recommendations for new terminology are given where clarification is needed.

RESULTS

IMRT, an advanced form of external beam irradiation, is a type of three-dimensional conformal radiotherapy (3D-CRT). It represents one of the most important technical advances in RT since the advent of the medical linear accelerator. 3D-CRT/IMRT is not just an add-on to the current radiation oncology process; it represents a radical change in practice, particularly for the radiation oncologist. For example, 3D-CRT/IMRT requires the use of 3D treatment planning capabilities, such as defining target volumes and organs at risk in three dimensions by drawing contours on cross-sectional images (i.e., CT, MRI) on a slice-by-slice basis as opposed to drawing beam portals on a simulator radiograph. In addition, IMRT requires that the physician clearly and quantitatively define the treatment objectives. Currently, most IMRT approaches will increase the time and effort required by physicians, medical physicists, dosimetrists, and radiation therapists, because IMRT planning and delivery systems are not yet robust enough to provide totally automated solutions for all disease sites. Considerable research is needed to model the clinical outcomes to allow truly automated solutions. Current IMRT delivery systems are essentially first-generation systems, and no single method stands out as the ultimate technique. The instrumentation and methods used for IMRT quality assurance procedures and testing are not yet well established. In addition, many fundamental questions regarding IMRT are still unanswered. For example, the radiobiologic consequences of altered time-dose fractionation are not completely understood. Also, because there may be a much greater ability to trade off dose heterogeneity in the target vs. avoidance of normal critical structures with IMRT compared with traditional RT techniques, conventional radiation oncology planning principles are challenged. All in all, this new process of planning and treatment delivery has significant potential for improving the therapeutic ratio and reducing toxicity. Also, although inefficient currently, it is expected that IMRT, when fully developed, will improve the overall efficiency with which external beam RT can be planned and delivered, and thus will potentially lower costs.

CONCLUSION

Recommendations in the areas pertinent to IMRT, including dose-calculation algorithms, acceptance testing, commissioning and quality assurance, facility planning and radiation safety, and target volume and dose specification, are presented. Several of the areas in which future research and development are needed are also indicated. These broad recommendations are intended to be both technical and advisory in nature, but the ultimate responsibility for clinical decisions pertaining to the implementation and use of IMRT rests with the radiation oncologist and radiation oncology physicist. This is an evolving field, and modifications of these recommendations are expected as new technology and data become available.

Room shielding for intensity-modulated radiation therapy treatment facilities

PURPOSE

The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments.

METHODS AND MATERIALS

We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both "step and shoot" and dynamic MLC delivered treatments.

RESULTS

We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding.

CONCLUSION

When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.

Requirements for radiation oncology physics in Australia and New Zealand

This Position Paper reviews the role, standards of practice, education, training and staffing requirements for radiation oncology physics. The role and standard of practice for an expert in radiation oncology physics, as defined by the ACPSEM, are consistent with the IAEA recommendations. International standards of safe practice recommend that this physics expert be authorised by a Regulatory Authority (in consultation with the professional organization). In order to accommodate the international and AHTAC recommendations or any requirements that may be set by a Regulatory Authority, the ACPSEM has defined the criteria for a physicist-in-training, a base level physicist, an advanced level physicist and an expert radiation oncology physicist. The ACPSEM shall compile separate registers for these different radiation oncology physicist categories. What constitutes a satisfactory means of establishing the number of physicists and support physics staff that is required in radiation oncology continues to be debated. The new ACPSEM workforce formula (Formula 2000) yields similar numbers to other international professional body recommendations. The ACPSEM recommends that Australian and New Zealand radiation oncology centres should aim to employ 223 and 46 radiation oncology physics staff respectively. At least 75% of this workforce should be physicists (168 in Australia and 35 in New Zealand). An additional 41 registrar physicist positions (34 in Australia and 7 in New Zealand) should be specifically created for training purposes. These registrar positions cater for the present physicist shortfall, the future expansion of radiation oncology and the expected attrition of radiation oncology physicists in the workforce. Registrar physicists shall undertake suitable tertiary education in medical physics with an organised in-house training program. The rapid advances in the theory and methodology of the new technologies for radiation oncology also require a stringent approach to maintaining a satisfactory standard of practice in radiation oncology physics. Appropriate on-going education of radiation oncology physicists as well as the educating of registrar physicists is essential. Institutional management and the ACPSEM must both play a key role in providing a means for satisfactory staff tuition on the safe and expert use of existing and new radiotherapy equipment.

[Comparative dosimetry study of two methods of intensity modulation performed on the same accelerator]

Intensity modulated radiation therapy (IMRT) is an advanced method of conformal radiotherapy. It permits optimal dose distribution to the target volume while preserving surrounding normal tissues. IMRT, with a multileaf collimator, can be realised in two different ways: either the segmented mode, which consists of combining small elementary static field, or the dynamic mode, which consists of moving the leaves while irradiating. The purpose of this work was to study these two methods of modulation on a Varian linear accelerator equipped with a collimator consisting of 40 pairs of one-centimetre-wide leaves. The measurements, obtained by using a diode array, showed that the quality of the irradiation in the dynamic mode does not depend on either the dose rate or the duration of the irradiation. In the segmented mode, weak magnitude segments are preferable, but increase the errors in the delivered dose. Comparisons of various profiles showed that the measured profiles are consistent with those programmed. Both modes seem to be equivalent for step-shaped profiles. In the case of profiles with constant slope, the segmentation generated by the segmented method deteriorates the profile. Even though the choice of technique is strongly dependent on the material available, the dynamic mode presents greater flexibility of use and has been chosen in our institution for IMRT.

A monitor unit verification calculation in intensity modulated radiotherapy as a dosimetry quality assurance

In standard teletherapy, a treatment plan is generated with the aid of a treatment planning system, but it is common to perform an independent monitor unit verification calculation (MUVC). In exact analogy, we propose and demonstrate that a simple and accurate MUVC in intensity modulated radiotherapy (IMRT) is possible. We introduce the concept of modified Clarkson integration (MCI). In MCI, we exploit the rotational symmetry of scattering to simplify the dose calculation. For dose calculation along a central axis (CAX), we first replace the incident IMRT fluence by an azimuthally averaged fluence. Second, the Clarkson integration is carried over annular sectors instead of over pie sectors. We wrote a computer code, implementing the MCI technique, in order to perform a MUVC for IMRT purposes. We applied the code to IMRT plans generated by CORVUS. The input to the code consists of CORVUS plan data (e.g., DMLC files, jaw settings, MU for each IMRT field, depth to isocenter for each IMRT field), and the output is dose contribution by individual IMRTs field to the isocenter. The code uses measured beam data for Sc, Sp, TPR, (D/MU)ref and includes effects from multileaf collimator transmission, and radiation field offset. On a 266 MHz desktop computer, the code takes less than 15 to calculate a dose. The doses calculated with the MCI algorithm agreed within +/-3% with the doses calculated by CORVUS, which uses a 1 cm x 1 cm pencil beam in dose calculation. In the present version of MCI, skin contour variations and inhomogeneities were neglected.

Shielding considerations for tomotherapy

Tomotherapy presents an evolutionary modality that holds forth the promise of better dose conformation to tumor volumes with a concomitant reduction in radiation-induced damage to surrounding normal structures. This delivery technique also presents a new set of radiation protection challenges that impact upon the design of the shielding vault required to house such a unit. A formalism is presented to determine the requisite amounts of shielding for both the primary beam and leakage radiation associated with a generic tomotherapy unit. A comparison is made with the shielding requirements for a conventional linear accelerator operated in a standard manner. Substantial differences in the amount of both primary and secondary shielding are indicated. A tomotherapy primary beam shield is both reduced in width by a factor of almost 10 and increased in thickness by more than a tenth value layer in comparison to a conventional accelerator. Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.

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.

Monitor unit calculation for an intensity modulated photon field by a simple scatter-summation algorithm

An important issue in intensity modulated radiation therapy (IMRT) is the verification of the monitor unit (MU) calculation of the planning system using an independent procedure. Because of the intensity modulation and the dynamic nature of the delivery process, the problem becomes much more involved than that in conventional radiation therapy. In this work, a closed formula for MU calculation is derived. The approach is independent of the specific form of leaf sequence algorithms. It is straightforward to implement the procedure using a simple computer program. The approach is illustrated by a simplified example and is demonstrated by a few CORVUS (NOMOS Corporation, Sewickley, PA) treatment plans. The results indicate that it is robust and suitable for IMRT MU verification.

Clinical implementation of a Monte Carlo treatment planning system

The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.