The sliding slit test for dynamic IMRT: a useful tool for adjustment of MLC related parameters

For treatments with dynamic intensity modulated radiotherapy (IMRT), the adjustment of multileaf collimator (MLC) parameters affecting both the optimization algorithm and dose distributions is crucial. The main parameters characterizing the MLC are the transmission (T) and the dosimetric leaf separation (DLS). The aim of this study is twofold: a methodology based on the 'sliding slit' test is proposed to determine (T, DLS) combinations inducing the best conformity between calculations and measurements. Secondly, the effects of the MLC adjustment on measured dose and on optimization are presented for different configurations as the chair test and for the patient dosimetric quality control (DQC). Tests were performed with a Varian 23EX linac operated at 20 MV and equipped with a 120 leaf Millenium dynamic collimator. The treatment planning system was CadPlan/Helios (version 6.3.6). Results demonstrated that the sliding width (SW) strongly depends on the (T, DLS) combinations, and the measured dose is a linear function of the SW. Different (T, DLS) combinations induced a good agreement between calculations and measurements. The influence of the MLC calibration was found to be particularly important on the 'sliding slit' test (11.8% for a gap change of 0.8 mm) but not so much on the chair test and on the DQC. To detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients, a daily check called IMRT MU check is proposed.

Intensity-modulated radiation therapy: emerging cancer treatment technology

The use of intensity-modulated radiation therapy (IMRT) is rapidly advancing in the field of radiation oncology. Intensity-modulated radiation therapy allows for improved dose conformality, thereby affording the potential to decrease the spectrum of normal tissue toxicities associated with IMRT. Preliminary results with IMRT are quite promising; however, the clinical data is relatively immature and overall patient numbers remain small. High-quality IMRT requires intensive physics support and detailed knowledge of three-dimensional anatomy and patterns of tumour spread. This review focuses on basic principles, and highlights the clinical implementation of IMRT in head and neck and prostate cancer.

Benefit of using biologic parameters (EUD and NTCP) in IMRT optimization for treatment of intrahepatic tumors

PURPOSE

To investigate whether intensity-modulated radiotherapy (IMRT), optimized using the generalized equivalent uniform dose (gEUD) and normal tissue complication probability (NTCP) models, can increase the safe dose to intrahepatic tumors compared with three-dimensional conformal RT (3D-CRT). A secondary objective was to investigate the optimal beam arrangement for liver IMRT plans.

METHODS AND MATERIALS

Planning CT data of 15 patients with intrahepatic tumors, previously treated with 3D-CRT, were used as input. The dose delivered using 3D-CRT had been limited either by tolerance of adjacent organs, which were close to, or overlapped with, the planning target volume (PTV; overlap cases, n = 8), or liver toxicity (nonoverlap, n = 7). IMRT plans were created using the gEUD to maximize the dose across the PTV and the NTCP to maintain the organ-at-risk toxicity to that of the conformal plan. Increased heterogeneity was allowed across the PTV in overlap cases, without compromising the minimal PTV dose of the conformal plan and restricting the maximal dose to within 110% of the mean. Three different beam arrangements were used for each case: seven-field equidistant axial, six-field noncoplanar (predominantly right-sided beams), and a reproduction of the conformal gantry angles. gEUDs were also computed and used for evaluation of the plans (regardless of planning technique) to reflect the response of both high- and low-grade tumors. The IMRT plan that allowed the greatest gEUD across the PTV was used in the comparison with the 3D-CRT plan.

RESULTS

The use of IMRT significantly increased the maximal gEUD achievable across the PTV compared with the 3D-CRT plans. This was the case for the assumptions of both high- and low-grade tumors, irrespective of the tumor position within the liver. The mean gEUD increase was 11 Gy (high grade) and 18.0 Gy (low grade) for overlap cases (p = 0.001 and p = 0.003, respectively) and 10 Gy for nonoverlap cases (p = 0.020). When comparing the IMRT beam arrangements, gEUDs were considered equivalent if they differed by less than one fraction (1.5 Gy). In overlap cases (n = 8), an equivalent "best" gEUD value was obtained in 3, 5, and 7 cases for the original conformal angle, seven-field axial, and six-field noncoplanar plan, respectively. The corresponding results were 5, 2, and 3 in the cases without an overlap (n = 7).

CONCLUSION

We have successfully used mathematical/biologic models directly as cost functions within the optimizing process to produce IMRT plans that maximize the gEUD while maintaining compliance with a well-defined protocol for the treatment of intrahepatic cancer. For both PTV-organ-at-risk overlap and nonoverlap situations, IMRT has the capacity to improve the maximal dose achievable across the PTV, expressed in terms of the gEUD. The use of multiple noncoplanar beams appears to confer an advantage over fewer beams in cases with PTV-organ-at-risk overlap. When liver toxicity is the dose-limiting factor, high gEUD values are obtained most frequently when the field arrangement is chosen to provide the shortest possible transhepatic path length.

A Monte Carlo derived TG-51 equivalent calibration for helical tomotherapy

Helical tomotherapy (HT) requires a method of accurately determining the absorbed dose under reference conditions. In the AAPM's TG-51 external beam dosimetry protocol, the quality conversion factor, kQ, is presented as a function of the photon component of the percentage depth-dose at 10 cm depth, %dd(10)x, measured under the reference conditions of a 10 x 10 cm2 field size and a source-to-surface distance (SSD) of 100 cm. The value of %dd(10)x from HT cannot be used for the determination of kQ because the design of the HT does not meet the following TG-51 reference conditions: (i) the field size and the practical SSD required by TG-51 are not obtainable and (ii) the absence of the flattening filter changes the beam quality thus affecting some components of kQ. The stopping power ratio is not affected because of its direct relationship to %dd(10)x. We derive a relationship for the Exradin A1SL ion chamber converting the %dd(10)x measured under HT "reference conditions" of SSD=85 cm and a 5 x 10 cm2 field-size [%dd(10)x[HT Ref]], to the dosimetric equivalent value under for TG-51 reference conditions [%dd(10)x[HT TG-51]] for HT. This allows the determination of kQ under the HT reference conditions. The conversion results in changes of 0.1% in the value of kQ for our particular unit. The conversion relationship should also apply to other ion chambers with possible errors on the order of 0.1%.

In vivo measurements with MOSFET detectors in oropharynx and nasopharynx intensity-modulated radiation therapy

PURPOSE

To evaluate the feasibility of in vivo measurements with metal oxide semiconductor field effect transistor (MOSFET) dosimeters for oropharynx and nasopharynx intensity-modulated radiation therapy (IMRT).

METHODS AND MATERIALS

During a 1-year period, in vivo measurements of the dose delivered to one or two points of the oral cavity by IMRT were obtained with MOSFET dosimeters. Measurements were obtained during each session of 48 treatment plans for 21 patients, all of whom were fitted with a custom-made mouth plate. Calculated and measured values were compared.

RESULTS

A total of 344 and 452 measurements were performed for the right and left sides, respectively, of the oral cavity. Seventy percent of the discrepancies between calculated and measured values were within +/-5%. Uncertainties were due to interfraction patient positions, intrafraction patient movements, and interfraction MOSFET positions. Nevertheless, the discrepancies between the measured and calculated means were within +/-5% for 92% and 95% of the right and left sides, respectively. Comparison of these discrepancies and the discrepancies between calculated values and measurements made on a phantom revealed that all differences were within +/-5%.

CONCLUSION

Our experience demonstrates the feasibility of in vivo measurements with MOSFET dosimeters for oropharynx and nasopharynx IMRT.

[Highly quality-controlled radiation therapy]

Advanced radiation therapy for intracranial disease has focused on set-up accuracy for the past 15 years. However, quality control in the prescribed dose is actually as important as the tumor set-up in radiation therapy. Because of the complexity of the three-dimensional radiation treatment planning system in recent years, the highly quality-controlled prescription of the dose has now been reappraised as the mainstream to improve the treatment outcome of radiation therapy for intracranial disease. The Japanese Committee for Quality Control of Radiation Therapy has developed fundamental requirements such as a QC committee in each hospital, a medical physicist, dosimetrists (QC members), and an external audit.

Use of an amorphous silicon electronic portal imaging device for multileaf collimator quality control and calibration

Multileaf collimator (MLC) calibration and quality control is a time-consuming procedure typically involving the processing, scanning and analysis of films to measure leaf and collimator positions. Faster and more reliable calibration procedures are required for these tasks, especially with the introduction of intensity modulated radiotherapy which requires more frequent checking and finer positional leaf tolerances than previously. A routine quality control (QC) technique to measure MLC leaf bank gain and offset, as well as minor offsets (individual leaf position relative to a reference leaf), using an amorphous silicon electronic portal imaging device (EPID) has been developed. The technique also tests the calibration of the primary and back-up collimators. A detailed comparison between film and EPID measurements has been performed for six linear accelerators (linacs) equipped with MLC and amorphous silicon EPIDs. Measurements of field size from 4 to 24 cm with the EPID were systematically smaller than film measurements over all field sizes by 0.4 mm for leaves/back-up collimators and by 0.2 mm for conventional collimators. This effect is due to the gain calibration correction applied by the EPID, resulting in a 'flattening' of primary beam profiles. Linac dependent systematic differences of up to 0.5 mm in individual leaf/collimator positions were also found between EPID and film measurements due to the difference between the mechanical and radiation axes of rotation. When corrections for these systematic differences were applied, the residual random differences between EPID and film were 0.23 mm and 0.26 mm (1 standard deviation) for field size and individual leaf/back-up collimator position, respectively. Measured gains (over a distance of 220 mm) always agreed within 0.4 mm with a standard deviation of 0.17 mm. Minor offset measurements gave a mean agreement between EPID and film of 0.01+/-0.10 mm (1 standard deviation) after correction for the tilt of the EPID and small rotational misalignments between leaf banks and the back-up collimators used as a reference straight edge. Reproducibility of EPID measurements was found to be very high, with a standard deviation of <0.05 mm for field size and <0.1 mm for individual leaf/collimator positions for a 10x10 cm2 field. A standard set of QC images (three field sizes defined both by leaves only and collimators only) can be acquired in less than 20 min and analysed in 5 min.

Virtual micro MLC commissioning

The resolution of multileaf collimators (MLCs) is limited by their finite leaf width. A commercial package (HD-270) uses 3D couch translation and leaf adjustments to emulate smaller leaf widths. In this paper, we report on the commissioning of this feature using software testing, dial gauge measurements, and film dosimetry. We also identify a variety of limitations: software bugs and truncation artifacts, MLC leaf positioning uncertainties (random variations, systematic gantry dependence and backlash), and uncertainties in couch positioning. These reduce the capabilities of this implementation below that achievable theoretically.

Enhanced dynamic wedge and independent monitor unit verification

Some serious radiation accidents have occurred around the world during the delivery of radiotherapy treatment. The regrettable incident in Panama clearly indicated the need for independent monitor unit (MU) verification. Indeed the International Atomic Energy Agency (IAEA), after investigating the incident, made specific recommendations for radiotherapy centres which included an independent monitor unit check for all treatments. Independent monitor unit verification is practiced in many radiotherapy centres in developed countries around the world. It is mandatory in USA but not yet in Australia. This paper describes development of an independent MU program, concentrating on the implementation of the Enhanced Dynamic Wedge (EDW) component. The difficult case of non centre of field (COF) calculation points under the EDW was studied in some detail. Results of a survey of Australasian centres regarding the use of independent MU check systems is also presented. The system was developed with reference to MU calculations made by Pinnacle 3D Radiotherapy Treatment Planning (RTP) system (ADAC - Philips) for 4MV, 6MV and 18MV X-ray beams used at the Newcastle Mater Misericordiae Hospital (NMMH) in the clinical environment. A small systematic error was detected in the equation used for the EDW calculations. Results indicate that COF equations may be used in the non COF situation with similar accuracy to that achieved with profile corrected methods. Further collaborative work with other centres is planned to extend these findings.

Dose calibration of nonconventional treatment systems applied to helical tomotherapy

Current dosimetric protocols based on the absorbed dose (AAPM TG-51 and IAEA TRS-398 protocols) require calibration measurements under reference conditions. For some radiotherapy systems, this requirement cannot be met, and calibration has to be performed under nonreference experimental conditions. In order to solve this problem, both protocols can be extended by inclusion of the measured-to-reference conversion factor, k(mr). In order to determine this factor, basic dosimetric quantities, like stopping power ratios, mass attenuation coefficients and chamber correction factors have to be calculated. If measurements are not feasible, accurate Monte Carlo modeling is required. The extension of the protocols is illustrated using the case of the helical tomotherapy radiation unit, where the typical calibration measurement conditions are the 10 x 5 cm2 field size and the 85 cm surface source distance, limited by the system design. It was calculated that the k(mr) factor for this conditions is close to unity (0.997+/-0.001). In addition, the deviation of the measurement conditions from the reference conditions results in the change of the quality conversion factor (approximately 0.995-0.998, depending on the ionization chamber used). This change is the same regardless of the used calibration protocol. For smaller field sizes the corrections become more significant, resulting in the total correction factor compared to the reference conditions of up to 1.5% for the smallest considered field size of 2 x 2 cm2.

Performance analysis of a film dosimetric quality assurance procedure for IMRT with regard to the employment of quantitative evaluation methods

A system for dosimetric verification of intensity-modulated radiotherapy (IMRT) treatment plans using absolute calibrated radiographic films is presented. At our institution this verification procedure is performed for all IMRT treatment plans prior to patient irradiation. Therefore clinical treatment plans are transferred to a phantom and recalculated. Composite treatment plans are irradiated to a single film. Film density to absolute dose conversion is performed automatically based on a single calibration film. A software application encompassing film calibration, 2D registration of measurement and calculated distributions, image fusion, and a number of visual and quantitative evaluation utilities was developed. The main topic of this paper is a performance analysis for this quality assurance procedure, with regard to the specification of tolerance levels for quantitative evaluations. Spatial and dosimetric precision and accuracy were determined for the entire procedure, comprising all possible sources of error. The overall dosimetric and spatial measurement uncertainties obtained thereby were 1.9% and 0.8 mm respectively. Based on these results, we specified 5% dose difference and 3 mm distance-to-agreement as our tolerance levels for patient-specific quality assurance for IMRT treatments.

An extensive log-file analysis of step-and-shoot intensity modulated radiation therapy segment delivery errors

We present a study to evaluate the monitor unit (MU), dosimetric, and leaf-motion errors found in the delivery of 91 step-and-shoot IMRT treatment plans performed at three nominal dose rates using a dual modality high energy Linac (Varian 2100 C/D, Varian Medical Systems Inc., Palo Alto, CA) equipped with a 120-leaf multileaf collimator (MLC). The analysis was performed by studying log files generated by the MLC controller system. Recent studies by our group have validated that the automatically generated MLC log files accurately record the actual system delivery. A total of 635 beams were delivered at three nominal dose rates: 100, 300, and 600 MU/min. The log files were manually retrieved and analysis software was developed to extract the recorded MU delivery and leaf positions for each segment. Our analysis revealed that the magnitude of segment MU errors were independent of the planned segment MUs. Segment MU errors were found to increase with dose rate having maximum errors per segment of +/-1.8 MU at 600 MU/min, +/-0.8 MU at 300 MU/min, and +/-0.5 MU at 100 MU/min. The total absolute MU error in each plan was observed to increase with the number of plan segments, with the trend increasing more rapidly for higher dose rates. Three dimensional dose distributions were recomputed based on the observed segment MU errors for three plans with large cumulative absolute MU errors. Comparison with the original treatment plans indicated no clinically significant consequences due to these errors. In addition, approximately 80% of the total segment deliveries reported at least one collimator leaf moving at least 1 mm (projected at isocenter) during segment delivery. Such errors occur near the end of segment delivery and have been previously observed by our group using a fast video-based electronic portal imaging device. At 600 MU/min, between 5% and 23% of the plan MUs were delivered during leaf motion that had exceeded a 1 mm position tolerance. These leaf motion errors were not included in the treatment plan recalculations performed in this study.

Technical note: output and energy fluctuations of the tomotherapy Hi-Art helical tomotherapy system

The output and energy calibrations for the first clinical Hi-Art 2.0 helical tomotherapy system have been reviewed. Fixed-gantry/fixed-couch and rotational-gantry/fixed-couch measurements were made on a daily basis over a period of 20 weeks to investigate system stability. Static gantry measurements were taken at 10 cm depth in a rectangular stack of Virtual Water at an SSD distance of 90 cm and a field size of 5 x 40 cm. Rotational gantry measurements were taken in a cylindrical phantom Virtual Water phantom for a field size of 5 x 40 cm. The Hi-Art 2.0 system has maintained its calibration to within +/-2% and energy to within +/- 1.5% over the initial 20 week period.

Intensificación de dosis con radioterapia conformacional 3D en cáncer de próstata. ¿Más dosis es mejor?

PURPOSE

The present study was undertaken to determine the effect of radiation dose on biochemical control and morbidity in prostate cancer patients.

MATERIALS AND METHODS

Between 1995 and 2003, 360 patients with T1-T3b prostate cancer were treated in a sequential radiation dose escalation trial from 66.0 to 82.6 Gy. These patients were prospectively assigned to 1 of 3 prognostic groups according to risk factors: a) low risk patients were treated with 3DCRT alone; b) intermediate risk patients were allocated to receive neoadjuvant AD (NAD) 4-6 months prior and during 3DCRT; and c) high-risk received NAD and adjuvant AD (AAD) 2 years after 3DCRT. RTOG/EORTC toxicity score was used to analyze late complications

RESULTS

Median follow-up was 48 months (12-138). The actuarial biochemical disease free survival (bDFS) at 4 years for low risk, intermediate risk and high risk patients was 88%, 68% and 79% respectively. Stratified and multivariate analysis showed that higher radiation dose (>76 Gy) (p=0.0053) and the use of AAD for high risk patients (p=0.0046) correlated significantly with an improvement of bDFS for all patients. The incidence of late grade 2 rectal and urinary bleeding were 7% and 11% respectively.

CONCLUSION

The present study confirms an independent benefit of high-dose (> 76 Gy) radiation therapy and long-term AAD in high-risk prostate cancerpatients.

High conformality radiotherapy in Europe: thirty-one centres participating in the quality assurance programme of the EORTC prostate trial 22991

Today, conformality in radiotherapy is at the centre of many investments in equipment and staffing. To estimate the current situation within the European Organisation for Research and Treatment of Cancer (EORTC) conformal radiotherapy trial for prostate cancer, a technology questionnaire was designed to assess whether participating centres can comply with the required radiotherapy procedures of EORTC trial 22991, where a high dose is prescribed to the prostate. Questions covered various items of computed tomography, data acquisition, treatment planning, delivery and verification. All centres (n=31) replied to the questionnaire. All generate beam's eye views and dose volume histograms. All, but two, centres use digitally reconstructed radiographs to display images. The vast majority of the centres perform at least weekly treatment verification and half have access to individual in vivo dosimetry. The results of the questionnaire indicate that participating centres have access to the equipment and apply the procedures that are essential for conformal prostate radiotherapy. The technology questionnaire is the first step in the extensive quality assurance programme dedicated to this high-tech radiotherapy trial.

Recommandations pour un protocole d’assurance de qualité de la radiothérapie conformationnelle avec modulation d’intensité des cancers de la tête et du cou

Head and neck tumors represent very interesting targets for IMRT techniques because of the complex shape of the structures and the organs at risk close by. The use of this kind of techniques requires a quality assurance protocol. The physicists of the GORTEC group shared their experience to define some recommendations in order to draw up a QA protocol. The dosimetric verification of the treatment plans (in terms of absolute and relative dose), the control of the reproducibility of the patient positioning and the use of a record and verify system to control the different parameters form the main parts of these recommendations. Each chapter comprises a description of the different methods, recommendations concerning the equipment, the adopted tolerances, the frequency of controls. At the end of each chapter, a table summarizes the main actions to carry out. These recommendations will allow to harmonize our practices whatever the softwares and the accelerator that are being used. They will simplify the task of the teams that wish to implement IMRT for head and neck tumors.

Australian and New Zealand three-dimensional conformal radiation therapy consensus guidelines for prostate cancer

Three-dimensional conformal radiation therapy (3DCRT) has been shown to reduce normal tissue toxicity and allow dose escalation in the curative treatment of prostate cancer. The Faculty of Radiation Oncology Genito-Urinary Group initiated a consensus process to generate evidence-based guidelines for the safe and effective implementation of 3DCRT. All radiation oncology departments in Australia and New Zealand were invited to complete a survey of their prostate practice and to send representatives to a consensus workshop. After a review of the evidence, key issues were identified and debated. If agreement was not reached, working parties were formed to make recommendations. Draft guidelines were circulated to workshop participants for approval prior to publication. Where possible, evidence-based recommendations have been made with regard to patient selection, risk stratification, simulation, planning, treatment delivery and toxicity reporting. This is the first time a group of radiation therapists, physicists and oncologists representing professional radiotherapy practice across Australia and New Zealand have worked together to develop best-practice guidelines. These guidelines should serve as a baseline for prospective clinical trials, outcome research and quality assurance.

Three-dimensional conformal radiotherapy in the treatment of prostate cancer in Australia and New Zealand: Report on a survey of radiotherapy centres and the proceedings of a consensus workshop

There is an increasing use of 3-D conformal radiotherapy (3DCRT) in the radiotherapeutic management of prostate cancer. The Faculty of Radiation Oncology Genito-Urinary Group carried out a survey of Australian and New Zealand radiotherapy centres in the preparation of a consensus workshop. Of the 19 centres that were represented, there were 24 radiation oncologists, 16 radiation therapists and 12 medical physicists. The survey collected demographic information and data on the practices undertaken at those centres when delivering curative radiotherapy in the treatment of prostate cancer. There was much variation in the delivery of treatment in the areas of patient set-up, contouring of target volumes and organs of interest during computer planning, the techniques and the dose constraints used in these techniques, the use of adjuvant androgen deprivation therapy and the quality assurance processes used in monitoring effects of treatment. This variability reflects the range of data in the published literature. Emerging trends of practices were also identified. This is a first report on a multi-disciplinary approach to the development of guidelines in 3DCRT of prostate cancer.

Functional stereotactic radiosurgery involving a dedicated linear accelerator and gamma unit: a comparison study

OBJECT

The purpose of this work was to investigate the targeting and dosimetric characteristics of a linear accelerator (LINAC) system dedicated for stereotactic radiosurgery compared with those of a commercial gamma knife (GK) unit.

METHODS

A phantom was rigidly affixed within a Leksell stereotactic frame and axial computerized tomography scans were obtained using an appropriate stereotactic localization device. Treatment plans were performed, film was inserted into a recessed area, and the phantom was positioned and treated according to each treatment plan. In the case of the LINAC system, four 140 degrees arcs, spanning +/-60 degrees of couch rotation, were used. In the case of the GK unit, all 201 sources were left unplugged. Radiation was delivered using 3- and 8-mm LINAC collimators and 4- and 8-mm collimators of the GK unit. Targeting ability was investigated independently on the dedicated LINAC by using a primate model. Measured 50% spot widths for multisource, single-shot radiation exceeded nominal values in all cases by 38 to 70% for the GK unit and 11 to 33% for the LINAC system. Measured offsets were indicative of submillimeter targeting precision on both devices. In primate studies, the appearance of an magnetic resonance imaging-enhancing lesion coincided with the intended target.

CONCLUSIONS

Radiosurgery performed using the 3-mm collimator of the dedicated LINAC exhibited characteristics that compared favorably with those of a dedicated GK unit. Overall targeting accuracy in the submillimeter range can be achieved, and dose distributions with sharp falloff can be expected for both devices.

Significance of different conformity indices for evaluation of radiosurgery treatment plans for vestibular schwannomas

OBJECT

There are various kinds of conformity parameters currently in use, although several of them are limited and reflect only target volume coverage or normal tissue overdosage. Indices are reviewed with the goal of determining those that are most significant for the evaluation of radiosurgery treatment plans for patients with vestibular schwannoma, based on the authors' experience at the Novalis Shaped Beam Surgery Center.

METHODS

Fifty-five radiosurgery plans for patients with vestibular schwannomas (VSs) have been evaluated. In this paper the conformation number (CN) and dose-related CN (dCN) are evaluated, and a penalty for underdosed target volumes and overdosed normal tissue is incorporated. A strategy is discussed to apply these indices (CN and dCN) to define the optimal prescription isodose (PI). For a given radiosurgery treatment plan, permitting partial target underdosage may offer an improvement of the CN. Variations of different conformation indices have been calculated for varying prescription levels--for example, an isodose plan. The resulting graph for the CN is discussed in detail to illustrate its use in defining the optimal PI level. For the 55 cases of VSs reported on, the median CNmax result was 0.78.

CONCLUSIONS

It is possible to achieve highly conformal dose distributions with Novalis radiosurgical system. The CN is the parameter of choice when evaluating radiosurgery treatment plans and scoring possible treatment plans. It takes into account both target underdosage and normal tissue overdosage and offers a valuable scoring parameter while avoiding false-perfect scores.

Development of methods for beam angle optimization for IMRT using an accelerated exhaustive search strategy

PURPOSE

The purpose of this article is to explore the use of the accelerated exhaustive search strategy for developing and validating methods for optimizing beam orientations for intensity-modulated radiation therapy (IMRT). Combining beam-angle optimization (BAO) with intensity distribution optimization is expected to improve the quality of IMRT treatment plans. However, BAO is one of the most difficult problems to solve adequately because of the huge hyperspace of possible beam configurations (e.g., selecting 7 of 36 uniformly spaced coplanar beams would require the intercomparison of 8,347,680 IMRT plans).

METHODS AND MATERIALS

An "influence vector" (IV) approximation technique for high-speed estimation of IMRT dose distributions was used in combination with a fast gradient search algorithm (Newton's method) for IMRT optimization. In the IV approximation, it is assumed that the change in intensity of a ray (or bixel) proportionately changes dose along the ray. Evidence is presented that the IV approximation is valid for BAO. The scatter contribution at points away from the ray is accounted for fully in IMRT optimization after the optimum beam orientation has been determined. IVs for all candidate beam angles are generated before the start of optimization. For all subsets of beams selected from a given pool of beams (e.g., 5 of 24 uniformly spaced beams), the distribution of planning scores for the best and the worst plans, optimum angle distributions, dose distributions, and dose-volume histograms (DVH) were analyzed for one prostate and two lung cancer cases. The results of the exhaustive search technique were used to develop a "multiresolution" search strategy. In this approach, a smaller number of beams (e.g., three) is first used to explore the hyperspace of solutions to determine the most preferred and the least preferred directions. The results of such exploration are then used as a starting point for determining an optimum configuration comprising a larger number of beams (e.g., seven). This two-step process is considerably faster than full exhaustive search. The question to be answered was whether the two methods lead to the same or similar solutions. The results of exhaustive search and multiresolution approaches were also compared with a previously published approach that used beam's-eye-view dosimetrics (BEVD).

RESULTS

The relative ranks of plans optimized by an accurate dose calculation method were highly correlated with those of the plans optimized by the fast calculation method (i.e., using the IV approximation), which suggests that an approximate dose calculation algorithm can be used effectively for ranking of plans during BAO. We found that dose distributions and DVH of many beam configurations within a specified subset from a given pool of beams (e.g., 5 of 18) may be clinically indistinguishable and acceptable. Their optimized IMRT scores fall in a narrow range, although beam configurations and dose distributions may be different. We used the frequency distributions as a function of beam angles for the best 100 and the worst 100 plans to determine the most and the least preferred beam angles. We found that the most and the least preferred angle distributions for 3 of 18 configurations were very similar to those for 5, 6, 7, or 8 of 18 or 24 configurations, but the size of the search space was much smaller for the 3 of 18 case. Using fewer than three beams was discovered to be inadequate. This information was used to select the most preferred angles and eliminate the least preferred ones before searching for the optimum angles for the remaining beams. For the cases we studies, the multiresolution strategy produced very similar results to the full exhaustive search. Based on the observation that the worst plans had at least one parallel-opposed pair of beams and virtually all of the best plans had none, we were able to further reduce the size of the search space dramatically by using a pool of only nonparallel-opposed equispaced beams (i.e., 7 of 19 instead of 7 of 36). Another observation was that the probability of finding an optimum configuration in a smaller beam pool is substantially lower than in a larger pool (e.g., 5 of 18 vs. 5 of 24). The implication of this BAO is not very important when a large number of beams (nine or more) is used and vice versa. Our results showed that the plans with fewer but optimally placed beams could be as good as or better than plans using a larger number of unoptimized or uniformly placed beams.

CONCLUSION

Exhaustive search with fast IMRT algorithms provides a novel and realistic approach to study the characteristics of IMRT dose distributions as a function of beam angles and to design practical BAO strategies for IMRT planning.

[Conformal radiotherapy by modulation of intensity]

External beam radiation therapy has changed dramatically during the past decade. Intensity Modulated Radiotherapy (IMRT) represents one of the most important technical advances. IMRT allows dose escalation with potential improvement in local control and less toxicity to normal tissues. It's also a radical change in practice for the physicist but particularly for the radiation oncologist. In this article, we have been described the physical and technical principles of IMRT. Indications are reviewed and aiming to summarize the published data. At last, importance of rigorous and reproducible quality assurance procedures should be emphasized and reminded. The potential clinical applications of IMRT are broad and continue to expand but enthusiasm for IMRT does not have to mask reality. Undesired or even potentially dangerous results may occur and no clinical data are currently available to support its use. More long term data and prospective controlled trials are required to confirm the clinical benefits of IMRT.

SIFT: A method to verify the IMRT fluence delivered during patient treatment using an electronic portal imaging device

PURPOSE

Radiotherapy patients are increasingly treated with intensity-modulated radiotherapy (IMRT) and high tumor doses. As part of our quality control program to ensure accurate dose delivery, a new method was investigated that enables the verification of the IMRT fluence delivered during patient treatment using an electronic portal imaging device (EPID), irrespective of changes in patient geometry.

METHODS AND MATERIALS

Each IMRT treatment field is split into a static field and a modulated field, which are delivered in sequence. Images are acquired for both fields using an EPID. The portal dose image obtained for the static field is used to determine changes in patient geometry between the planning CT scan and the time of treatment delivery. With knowledge of these changes, the delivered IMRT fluence can be verified using the portal dose image of the modulated field. This method, called split IMRT field technique (SIFT), was validated first for several phantom geometries, followed by clinical implementation for a number of patients treated with IMRT.

RESULTS

The split IMRT field technique allows for an accurate verification of the delivered IMRT fluence (generally within 1% [standard deviation]), even if large interfraction changes in patient geometry occur. For interfraction radiological path length changes of 10 cm, deliberately introduced errors in the delivered fluence could still be detected to within 1% accuracy. Application of SIFT requires only a minor increase in treatment time relative to the standard IMRT delivery.

CONCLUSIONS

A new technique to verify the delivered IMRT fluence from EPID images, which is independent of changes in the patient geometry, has been developed. SIFT has been clinically implemented for daily verification of IMRT treatment delivery.

A multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators

PURPOSE

The evolution of three-dimensional conformal radiation treatment has led to the use of multileaf collimators (MLCs) in intensity-modulated radiation therapy (IMRT) and other treatment techniques to increase the conformity of the dose distribution. A new quality assurance (QA) phantom has been designed to check the handling of MLC settings in treatment planning and delivery.

METHODS AND MATERIALS

The phantom consists of a Perspex block with stepped edges that can be rotated in all planes. The design allows for the assessment of several MLC and micro-MLC types from various manufacturers, and is therefore applicable to most radiation therapy institutions employing MLCs. The phantom is computed tomography (CT) scanned as is a patient, and QA assessments can be made of field edge display for a variety of shapes and orientations on both radiation treatment planning systems (RTPS) and computed tomography simulators.

RESULTS

The dimensions of the phantom were verified to be physically correct within an uncertainty range of 0-0.7 mm. Errors in leaf position larger than 1 mm were easily identified by multiple observers.

CONCLUSIONS

The MLC geometry phantom is a useful tool in the QA of radiation therapy with application to RTPS, CT simulators, and virtual simulation packages with MLC display capabilities.