Generation of discrete beam-intensity modulation by dynamic multileaf collimation under minimum leaf separation constraints

Volumetric modulated arc therapy total body irradiation in pediatric and adolescent/young adult patients undergoing stem cell transplantation: Early outcomes and toxicities

INTRODUCTION

Total body irradiation (TBI) is an important component of many conditioning regimens for hematopoietic stem cell transplantation (HSCT), most commonly used in pediatric and adolescent/young adult (AYA) patients. We aimed to evaluate outcomes and toxicities among pediatric and AYA patients treated with TBI utilizing volumetric modulated arc therapy total body irradiation (VMAT-TBI).

METHODS

We reviewed pediatric and AYA patients treated with VMAT-TBI at our institution from 2019 to 2021. Data on patient and disease characteristics, treatment details, outcomes and toxicities were collected. Overall survival (OS) and relapse-free survival (RFS) were analyzed using the Kaplan-Meier method.

RESULTS

Among 38 patients, 16 (42.1%) were treated with myeloablative regimens and 22 (57.9%) with nonmyeloablative regimens. Median age was 7.2 years (range: 1-27) and median follow-up was 8.7 months (range: 1-21). Lungs D_mean was 7.3 ± 0.3 Gy for myeloablative regimens (range: 6.8-7.8). Kidneys were spared to average mean dose of 71.4 ± 4.8% of prescription dose. Gonadal sparing was achieved for patients treated for nonmalignant diseases to D_mean of 0.7 ± 0.1 Gy. No patient experienced primary graft failure; one (2.6%) experienced secondary graft failure. The most common grade 1-2 acute toxicities were nausea (68.4%) and fatigue (55.3%). Mucositis was the most common grade 3-4 acute toxicity, affecting 39.5% of patients. There were no cases of pneumonitis or nephrotoxicity attributable to TBI.

CONCLUSION

VMAT-TBI offers increased ability to spare organs at risk in pediatric and AYA patients undergoing HSCT, with a favorable acute/subacute toxicity profile and excellent disease control.

Technology-driven research for radiotherapy innovation

Technology has a pivotal role in the continuous development of radiotherapy. The long road toward modern ‘high‐tech’ radiation oncology has been studded with discoveries and technological innovations that resulted from the interaction of various disciplines. In the last decades, a dramatic technology‐driven revolution has hugely improved the capability of accurately and safely delivering complex‐shaped dose distributions. This has contributed to many clinical improvements, such as the successful management of lung cancer and oligometastatic disease through stereotactic body radiotherapy. Technology‐driven research is an active and lively field with promising potential in several domains, including image guidance, adaptive radiotherapy, integration of artificial intelligence, heavy‐particle therapy, and ‘flash’ ultra‐high dose‐rate radiotherapy. The evolution toward personalized Oncology will deeply influence technology‐driven research, aiming to integrate predictive models and omics analyses into fast and efficient solutions to deliver the best treatment for each single patient. Personalized radiation oncology will need affordable technological solutions for middle‐/low‐income countries, as these are expected to experience the highest increase of cancer incidence and mortality. Moreover, technology solutions for automation of commissioning, quality assurance, safety tests, image segmentation, and plan optimization will be required. Although a large fraction of cancer patients receive radiotherapy, this is certainly not reflected in the worldwide budget for radiotherapy research. Differently from the pharmaceutical companies‐driven research, resources for research in radiotherapy are highly limited to equipment vendors, who can, in turn, initiate a limited number of collaborations with academic research centers. Thus, enhancement of investments in technology‐driven radiotherapy research via public funds, national governments, and the European Union would have a crucial societal impact. It would allow for radiotherapy to further strengthen its role as a highly effective and cost‐efficient cancer treatment modality, and it could facilitate a rapid and equalitarian large‐scale transfer of technology to clinic, with direct impact on patient care.

Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory optimization model

Flattening filter free (FFF) linear accelerators produce a fluence distribution that is forward peaked. Various dosimetric benefits, such as increased dose rate, reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. The literature has suggested the idea of vendors offering dedicated FFF units where the flattening filter (FF) is removed completely and manipulating the beam to deliver conventional flat radiotherapy treatments. This work aims to develop an effective way to deliver modulated flat beam treatments, rather than utilizing a physical FF. This novel optimization model is an extension of the direct leaf trajectory optimization (DLTO) previously developed for volumetric modulated radiation therapy (VMAT) and is capable of accounting for all machine and multileaf collimator (MLC) dynamic delivery constraints, using a combination of linear constraints and a convex objective function. Furthermore, the tongue and groove (T&G) effect was also incorporated directly into our model without introducing nonlinearity to the constraints, nor nonconvexity to the objective function. The overall beam flatness, machine deliverability, and treatment time efficiency were assessed. Regular square fields, including field sizes of 10 × 10 cm² to 40 × 40 cm² were analyzed, as well as three clinical fields, and three arbitrary contours with "concave" features. Quantitative flatness was measured for all modulated FFF fields, and the results were comparable or better than their open FF counterparts, with the majority having a quantitative flatness of less than 3.0%. The modulated FFF beams, due to the included efficiency constraint, were able to achieve acceptable delivery time compared to their open FF counterpart. The results indicated that the dose uniformity and flatness for the modulated FFF beams optimized with the DLTO model can successfully match the uniformity and flatness of their conventional FF counterparts, and may even provide further benefit by taking advantage of the unique FFF beam characteristics.

The Impact of Dose Rate on the Accuracy of Step-and-Shoot Intensity-modulated Radiation Therapy Quality Assurance Using Varian 2300CD

Introduction

Intensity-modulated radiation therapy (IMRT) delivery using "step-and-shoot" technique on Varian C-Series linear accelerator (linac) is influenced by the communication frequency between the multileaf collimator and linac controllers. Hence, the dose delivery accuracy is affected by the dose rate.

Aim

Our aim was to quantify the impact of using two dose rates on plan quality assurance (QA).

Materials and Methods

Twenty IMRT patients were selected for this study. The plan QA was measured at two different dose rates. A gamma analysis was performed, and the degree of plan modulation on the QA pass rate was also evaluated in terms of average monitor unit per segment (MU/segment) and the total number of segments.

Results

The mean percentage gamma pass rate of 94.9% and 93.5% for 300 MU/min and 600 MU/min dose rate, respectively, was observed. There was a significant (P = 0.001) decrease in percentage gamma pass rate when the dose rate was increased from 300 MU/min to 600 MU/min. There was a weak, but significant association between the percentage pass rate at both dose rate and total number of segments. The total number of MU was significantly correlated to the total number of segments (r = 0.59). We found a positive correlation between the percentage pass rate and mean MU/segment, r = 0.52 and r = 0.57 for 300 MU/min and 600 MU/min, respectively.

Conclusion

IMRT delivery using step-and-shoot technique on Varian 2300CD is impacted by the dose rate and the total amount of segments.

GENERALIZED GEOMETRIC APPROACHES FOR LEAF SEQUENCING PROBLEMS IN RADIATION THERAPY

The 3-D static leaf sequencing (SLS) problem arises in radiation therapy for cancer treatments, aiming to deliver a prescribed radiation dose to a target tumor accurately and efficiently. The treatment time and machine delivery error are two crucial factors to the solution (i.e., a treatment plan) for the SLS problem. In this paper, we prove that the 3-D SLS problem is NP-hard, and present the first ever algorithm for the 3-D SLS problem that can determine a tradeoff between the treatment time and machine delivery error (also called the "tongue-and-groove" error in medical literature). Our new 3-D SLS algorithm with error control gives the users (e.g., physicians) the option of specifying a machine delivery error bound, and subject to the given error bound, the algorithm computes a treatment plan with the minimum treatment time. We formulate the SLS problem with error control as computing a k-weight shortest path in a directed graph and build the graph by computing g-matchings and minimum cost flows. Further, we extend our 3-D SLS algorithm to all the popular radiotherapy machine models with different constraints. In our extensions, we model the SLS problems for some of the radiotherapy systems as computing a minimum g-path cover of a directed acyclic graph. We implemented our new 3-D SLS algorithm suite and conducted an extensive comparison study with commercial planning systems and well-known algorithms in medical literature. Some of our experimental results based on real medical data are presented.

GEOMETRIC ALGORITHMS FOR STATIC LEAF SEQUENCING PROBLEMS IN RADIATION THERAPY

The static leaf sequencing (SLS) problem arises in radiation therapy for cancer treatments, aiming to accomplish the delivery of a radiation prescription to a target tumor in the minimum amount of delivery time. Geometrically, the SLS problem can be formulated as a 3-D partition problem for which the 2-D problem of partitioning a polygonal domain (possibly with holes) into a minimum set of monotone polygons is a special case. In this paper, we present new geometric algorithms for a basic case of the 3-D SLS problem (which is also of clinical value) and for the general 3-D SLS problem. Our basic 3-D SLS algorithm, based on new geometric observations, produces guaranteed optimal quality solutions using O(1) Steiner points in polynomial time; the previously best known basic 3-D SLS algorithm gives optimal outputs only for the case without considering any Steiner points, and its time bound involves a multiplicative factor of a factorial function of the input. Our general 3-D SLS algorithm is based on our basic 3-D SLS algorithm and a polynomial time algorithm for partitioning a polygonal domain (possibly with holes) into a minimum set of x-monotone polygons, and has a fast running time. Experiments of our SLS algorithms and software in clinical settings have shown substantial improvements over the current most popular commercial treatment planning system and the most well-known SLS algorithm in medical literature. The radiotherapy plans produced by our software not only take significantly shorter delivery times, but also have a much better treatment quality. This proves the feasibility of our software and has led to its clinical applications at the Department of Radiation Oncology at the University of Maryland Medical Center. Some of our techniques and geometric procedures (e.g., for partitioning a polygonal domain into a minimum set of x-monotone polygons) are interesting in their own right.

Semiautomatic method to identify the best phase for gated RT in lung region by 4D-PET/CT acquisitions

PURPOSE

Delineating tumor motion by four-dimensional positron emission tomography/computed tomography (4D-PET/CT) is a crucial step for gated radiotherapy (RT). This article quantitatively evaluates semiautomatic algorithms for tumor shift estimation in the lung region due to patient respiration by 4D-PET/CT, in order to support the selection of the best phases for gated RT, by considering the most stable phases of the breathing cycle.

METHODS

Three mobile spheres and ten selected lesions were included in this study. 4D-PET/CT data were reconstructed and classified into six/ten phases. The semiautomatic algorithms required the generation of single sets of images representative of the full target motion, used as masks for segmenting the phases. For 4D-CT, a pre-established HU range was used, whereas three thresholds (100%, 80%, and 40%) were evaluated for 4D-PET. By using these segmentations, the authors estimated the lesion motion from the shifting centroids, and the phases with the least motion were also deduced including the phases with a curve slope less than 2 mm/ delta phase. The proposed algorithms were validated by comparing the results to those generated entirely by manual contouring.

RESULTS

In the phantom study, the mean difference between the manual contour and the semiautomatic technique was 0.1 +/- 0.1 mm for 4D-CT and 0.2 +/- 0.1 mm for the 4D-PET based on 40% threshold. In the patients' series, the mean difference was 0.9 +/- 0.6 mm for 4D-CT and 0.8 +/- 0.2 mm for the 4D-PET based on 40% threshold.

CONCLUSIONS

Estimation of lesion motion by the proposed semiautomatic algorithm can be used to evaluate tumor motion due to breathing.

Contrast enhanced 4D-CT imaging for target volume definition in pancreatic ductal adenocarcinoma

A procedure to improve target volume definition in pancreatic ductal adenocarcinoma by contrast enhanced 4D-CT imaging has been implemented for radiotherapy planning. The procedure allows good quality images to be obtained over the whole patient's breathing cycle in terms of anatomical details, pancreatic enhancement and vessel definition.

Intensity-modulated radiation therapy and image-guided radiation therapy: small clinic implementation

In a small clinic with a small patient base, the implementation of IMRT/IGRT should be slow, measured, and meticulous. Most radiation oncologists in the United States have had no formal training in IMRT/IGRT because the modalities are so new. Proper patient selection and a team effort among the clinician, physicist, dosimetrist, and therapist are thus all the more critical. The clinician in the small clinic can take comfort in remembering that the technologies are new, but the principles of good radiation medicine are not. With patient selection, a team approach, and publication of data and maturation of the literature, IMRT/IGRT will become the new standard of care in academic centers, large private clinics, and small clinics alike.

Intensity-modulated radiotherapy technique for three-field breast treatment

PURPOSE

To develop a simplified intensity-modulated radiotherapy (IMRT) algorithm for three-field breast treatment using a single isocenter setup. The algorithm aims to deliver a uniform dose throughout the breast volume. Special attention was paid to the highly divergent nature of the beam configuration.

METHODS AND MATERIALS

Computed tomography (CT) image setup of the patient was acquired. On each CT slice, the computer automatically generated lines parallel to the posterior edge of the tangent field. The mid-point of each line segment that intersected the breast was determined and the dose from an open field calculated. The intensity of the divergent pencil beam corresponding to the mid-point was set to be inversely proportional to the open field dose to the mid-point. Forward dose calculation was then performed using this intensity distribution.

RESULTS

A total of 15 breast cancer patients undergoing three-field IMRT who underwent planning and treatment with this algorithm were included in this study. Compared with standard wedged pair tangents, the IMRT plan produced statistically significant better dose distributions in terms of target coverage and target dose uniformity, as well as reduced dose to the contralateral breast and reduced hot spots to the ipsilateral lung.

CONCLUSION

Since March 2004, the new IMRT algorithm has been used for planning and treatment of > 20 patients undergoing three-field treatment, as well as >200 patients undergoing regular two-field tangent treatment, all with excellent dose distributions throughout the breast volume.

IMRT sequencing for a six-bank multi-leaf system

In this study, we present a sequencer for delivering step-and-shoot IMRT using a six-bank multi-leaf system. Such a system was proposed earlier and combines a high-resolution field-shaping ability with a large field size. It consists of three layers of two opposing leaf banks with 1 cm leaves. The layers are rotated relative to each other at 60 degrees . A low-resolution mode of sequencing is achieved by using one layer of leaves as primary MLC, while the other two are used to improve back-up collimation. For high-resolution sequencing, an algorithm is presented that creates segments shaped by all six banks. Compared to a hypothetical mini-MLC with 0.4 cm leaves, a similar performance can be achieved, but a trade-off has to be made between accuracy and the number of segments.

Treatment planning considerations of reshapeable automatic intensity modulator for intensity modulated radiation therapy

As compared with multi-leaf collimator based intensity modulated radiation therapy (IMRT) techniques, physical modulators have the major advantage of temporally invariant intensity map delivery which makes it more flexible with monitor unit rate, simpler resolution of interrupted treatment and easier implementation and use with respiratory gating. However, traditional physical modulator techniques require long fabrication time and operator intervention during treatments. It has been previously proposed [Xu et al., Med. Phys. 29, 2222-2229 (2002)] that a reshapeable automatic intensity modulator (RAIM) can automatically produce physical modulators by molding a deformable high x-ray attenuation material using a matrix of computer-controlled pistons. RAIM can potentially eliminate the limitations of traditional physical modulators. The present study addresses the treatment planning considerations of RAIM for IMRT. In this study, a 3D treatment-planning system (PLUNC) was modified to include the capability of providing treatment planning using RAIM. Two clinically representative cases were studied: nasopharyngeal and prostate tumors. First, the RAIM system with two different spatial resolutions at isocenter, 1 x 1 cm2 and 0.5 x 0.5 cm2, were evaluated. The treatment planning results of RAIM were then compared with other IMRT techniques such as smooth modulator with ideal (100%-2%) and limited (100%-13%) intensity modulation ranges, segmental multi-leaf collimator (SMLC) with ten intensity levels, 1 cm leaf width and 0.5 cm step size and serial tomotherapy using the Peacock system. Bringing the spatial resolution of RAIM down to 0.5 x 0.5 cm2 did not show improvement due to the effect of penumbra. The RAIM system with 1 x 1 cm2 proved slightly inferior as compared to the ideal smooth physical modulator but better than the SMLC technique and the smooth modulator with limited modulation range. When compared to serial tomotherapy, RAIM is only inferior in brain stem sparing for the nasopharynx case. Furthermore, the RAIM system with 1 x 1 cm2 resolution required significantly lower monitor units as compared to the other IMRT techniques for the two cases studied.

A comparison of step-and-shoot leaf sequencing algorithms that eliminate tongue-and-groove effects

The performances of three recently published leaf sequencing algorithms for step-and-shoot intensity-modulated radiation therapy delivery that eliminates tongue-and-groove underdosage are evaluated. Proofs are given to show that the algorithm of Que et al (2004 Phys. Med. Biol. 49 399-405) generates leaf sequences free of tongue-and-groove underdosage and interdigitation. However, the total beam-on times could be up to n times those of the sequences generated by the algorithms of Kamath et al (2004 Phys. Med. Biol. 49 N7-N19), which are optimal in beam-on time for unidirectional leaf movement under the same constraints, where n is the total number of involved leaf pairs. Using 19 clinical fluence matrices and 100000 randomly generated 15 x 15 matrices, the average monitor units and number of segments of the leaf sequences generated using the algorithm of Que et al are about two to four times those generated by the algorithm of Kamath et al.

A graph-searching method for MLC leaf sequencing under constraints

A new leaf-sequencing algorithm for step-and-shoot IMRT that is based on a graph-searching technique is described. An iterative process guided by a quantitative measure for the complexity of the initial or residual intensity pattern is used to identify the field segments shaped by a multileaf collimator (MLC). Given a user selected number of intensity levels, the algorithm searches deliverable segment candidates considering all intensity levels and two collimator positions separated by 90 degrees. The candidates for each intensity level are obtained as the least number of segments to cover the areas with equal or higher intensity. The shape of a deliverable segment is adjusted by leaving out certain beam elements for later delivery if this results in a simpler residual intensity pattern and the segment is still deliverable. For a MLC design that does not allow leaf interdigitation, it is initially assumed that a single segment cannot cover two disjoined areas. Among all candidates the segment with the greatest reduction of the complexity of the residual intensity distribution is chosen for the current step of iteration. The iterative process generates a set of deliverable segments of simply connected areas. These segments are combined later under specific MLC constraints. Different orders of segment combination are considered for minimizing the beam-on time. The final segments are sequenced to minimize the leaf travel. This algorithm has been tested using randomly generated intensity distributions and clinical cases for the Varian, Siemens, and Elekta MLC systems. The results show that as the number of intensity levels is increased, the numbers of segments and MUs increase only modestly. Using two collimator angles results in decreases in the required number of segments and the number of monitor units that can be as much as 20%.

Quantitative measurement of MLC leaf displacements using an electronic portal image device

The success of an IMRT treatment relies on the positioning accuracy of the MLC (multileaf collimator) leaves for both step-and-shoot and dynamic deliveries. In practice, however, there exists no effective and quantitative means for routine MLC QA and this has become one of the bottleneck problems in IMRT implementation. In this work we present an electronic portal image device (EPID) based method for fast and accurate measurement of MLC leaf positions at arbitrary locations within the 40 cm x 40 cm radiation field. The new technique utilizes the fact that the integral signal in a small region of interest (ROI) is a sensitive and reliable indicator of the leaf displacement. In this approach, the integral signal at a ROI was expressed as a weighted sum of the contributions from the displacements of the leaf above the point and the adjacent leaves. The weighting factors or linear coefficients of the system equations were determined by fitting the integral signal data for a group of pre-designed MLC leaf sequences to the known leaf displacements that were intentionally introduced during the creation of the leaf sequences. Once the calibration is done, the system can be used for routine MLC leaf positioning QA to detect possible leaf errors. A series of tests was carried out to examine the functionality and accuracy of the technique. Our results show that the proposed technique is potentially superior to the conventional edge-detecting approach in two aspects: (i) it deals with the problem in a systematic approach and allows us to take into account the influence of the adjacent MLC leaves effectively; and (ii) it may improve the signal-to-noise ratio and is thus capable of quantitatively measuring extremely small leaf positional displacements. Our results indicate that the technique can detect a leaf positional error as small as 0.1 mm at an arbitrary point within the field in the absence of EPID set-up error and 0.3 mm when the uncertainty is considered. Given its simplicity, efficiency and accuracy, we believe that the technique is ideally suitable for routine MLC leaf positioning QA.

A new MLC segmentation algorithm/software for step-and-shoot IMRT delivery

We present a new MLC segmentation algorithm/software for step-and-shoot IMRT delivery. Our aim in this work is to shorten the treatment time by minimizing the number of segments. Our new segmentation algorithm, called SLS (an abbreviation for static leaf sequencing), is based on graph algorithmic techniques in computer science. It takes advantage of the geometry of intensity maps. In our SLS approach, intensity maps are viewed as three-dimensional (3-D) "mountains" made of unit-sized "cubes." Such a 3-D "mountain" is first partitioned into special-structured submountains using a new mixed partitioning scheme. Then the optimal leaf sequences for each submountain are computed by either a shortest-path algorithm or a maximum-flow algorithm based on graph models. The computations of SLS take only a few minutes. Our comparison studies of SLS with CORVUS (both the 4.0 and 5.0 versions) and with the Xia and Verhey segmentation methods on Elekta Linac systems showed substantial improvements. For instance, for a pancreatic case, SLS used only one-fifth of the number of segments required by CORVUS 4.0 to create the same intensity maps, and the SLS sequences took only 25 min to deliver on an Elekta SL 20 Linac system in contrast to the 72 min for the CORVUS 4.0 sequences (a three-fold improvement). To verify the accuracy of our new leaf sequences, we conducted film and ion-chamber measurements on phantom. The results showed that both the intensity distributions as well as dose distributions of the SLS delivery match well with those of CORVUS delivery. SLS can also be extended to other types of Linac systems.

Optimal leaf sequencing with elimination of tongue-and-groove underdosage

The individual leaves of a multileaf collimator (MLC) have a tongue-and-groove or stepped-edge design to minimize leakage radiation between adjacent leaves. This design element has a drawback in that it creates areas of underdosages in intensity-modulated photon beams unless a leaf trajectory is specifically designed such that for any two adjacent leaf pairs, the direct exposure under the tongue-and-groove is equal to the lower of the direct exposures of the leaf pairs. In this work, we present a systematic study of the optimization of a leaf sequencing algorithm for segmental multileaf collimator beam delivery that completely eliminates areas of underdosages due to tongue-and-groove or stepped-edge design of the MLC. Simultaneous elimination of tongue-and-groove effect and leaf interdigitation is also studied. This is an extension of our previous work (Kamath et al 2003a Phys. Med. Biol. 48 307) in which we described a leaf sequencing algorithm that is optimal for monitor unit (MU) efficiency under most common leaf movement constraints that include minimum leaf separation. Compared to our previously published algorithm (without constraints), the new algorithms increase the number of sub-fields by approximately 21% and 25%, respectively, but are optimal in MU efficiency for unidirectional schedules.

Clinical implementation of dynamic and step-and-shoot IMRT to treat prostate cancer with high risk of pelvic lymph node involvement

BACKGROUND AND PURPOSE

Two systems have been developed for treating patients with locally advanced prostate cancer using intensity-modulated radiotherapy (IMRT): one using dynamic multi-leaf collimator delivery and the other using step-and-shoot. This paper describes the clinical implementation of these two techniques, and presents results from the first 14 patients treated in a clinical setting (nine dynamic, five step-and-shoot).

PATIENTS AND METHODS

Dynamic treatments were planned using Corvus, and step-and-shoot using Helax-TMS; all were delivered using Elekta accelerators. Prior to the first clinical treatments, validation measurements were carried out for each system, including measurements for a complete IMRT treatment. The reproducibility of dynamic delivery and the characteristics of the accelerator for low-monitor-unit (MU) deliveries were also assessed. An extensive quality assurance (QA) program was performed for each of the patients. Additionally, timing measurements were carried out to assess the practicalities of the technique.

RESULTS

The planning objectives were met in most cases. Absolute doses for complete IMRT treatments were within 2%, on average, with dose distributions generally showing agreement within 3% or 3 mm. Beam modulation measurements made throughout each patient's treatment indicated that both delivery methods were reproducible. The dynamic plans required an average of 765 MU per beam, with a treatment delivery time of 14 min; corresponding results for step-and-shoot plans were 105 MU and 10 min.

CONCLUSIONS

Two IMRT techniques for this group of patients have been successfully implemented in the clinic. The more complex dynamic treatments showed no advantages over the step-and-shoot approach. QA results have shown accurate and reproducible delivery for both techniques, giving increased confidence in the techniques and allowing a reduction in the QA program.

Algorithms for optimal sequencing of dynamic multileaf collimators

Dynamic multileaf collimator (DMLC) intensity modulated radiation therapy (IMRT) is used to deliver intensity modulated beams using a multileaf collimator (MLC), with the leaves in motion. DMLC-IMRT requires the conversion of a radiation intensity map into a leaf sequence file that controls the movement of the MLC while the beam is on. It is imperative that the intensity map delivered using the leaf sequence file be as close as possible to the intensity map generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. Optimization of the leaf-sequencing algorithm has been the subject of several recent investigations. In this work, we present a systematic study of the optimization of leaf-sequencing algorithms for dynamic multileaf collimator beam delivery and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of monitor unit (MU) efficiency under the most common leaf movement constraints that include leaf interdigitation constraint. Our analytical analysis shows that leaf sequencing based on unidirectional movement of the MLC leaves is as MU efficient as bi-directional movement of the MLC leaves.

Segmental IMRT for oropharyngeal cancer in a clinical setting

BACKGROUND AND PURPOSE

To develop a segmental intensity-modulated radiotherapy (IMRT) technique for the treatment of oropharyngeal cancer.

PATIENTS AND METHODS

Eight patients previously treated for oropharyngeal cancer were replanned with segmental IMRT. The dose distribution was optimized using beam geometries consisting of 3, 5, 7 and 9 equiangular beams. The optimization procedure resulted in a theoretical fluence for each beam. In order to vary the number of segments, the optimized fluence was divided into four different equidistant levels. The final dose distribution was calculated using clinically deliverable segments obtained from optimized fluence.

RESULTS

For our segmental IMRT technique the dose homogeneity within the target volumes improved when the total number of segments increased and reached a saturation level at approximately 150 segments. Seven beams were sufficient to achieve the saturation level for dose homogeneity. The mean dose to the parotid glands depended on the beam geometry and tumor location and did not depend on the number of segments. On average the mean dose to the contralateral parotid gland was 35.7 Gy (27.1-39.9 Gy) for all seven beam plans.

CONCLUSIONS

Seven beams are sufficient to achieve an acceptable dose homogeneity within the target volumes and significant parotid sparing. These results will be used to introduce IMRT in routine clinical practice.

Incorporating leaf transmission and head scatter corrections into step-and-shoot leaf sequences for IMRT

PURPOSE

Leaf transmission and head scatter are two important factors that influence intensity-modulated radiation therapy (IMRT) delivery and should be correctly taken into account when generating multileaf collimator (MLC) sequences. Significant discrepancies between the desired and delivered intensity profiles could otherwise result. The purpose of this article is to propose a reliable algorithm to minimize the dosimetric effects caused by the two factors in step-and-shoot mode.

MATERIALS AND METHODS

The goal of the algorithm is to minimize the difference between the desired fluence map and the fluence map actually delivered. For this purpose, an error function, defined as the least-square difference between the desired and the delivered fluence maps, is introduced. The effects of transmission and head scatter are minimized by adjusting the fractional monitor units (MUs) in the initial MLC sequences, created by using the desired fluence map without inclusion of the contributions from the two factors. Computationally, a downhill simplex optimization method is used to minimize the error function with respect to the fractional MUs. A three-source model is used to evaluate the relative head scatter distribution for each segment at the beginning of the calculation. The algorithm has been assessed by comparing the dose distributions delivered by the corrected leaf sequence files and the theoretic predication, calculated by Monte Carlo simulation using the desired fluence maps, for an intuitive test field and several clinical IMRT cases.

RESULTS

The deviations between the desired fluence maps and those calculated using the corrected leaf sequence files are <0.3% of the maximum MU for the test field and <1.0% for the clinical IMRT cases. The experimental data show that both absolute and relative dose distributions delivered by the corrected leaf sequences agree with the desired ones within 2.5% of the maximum dose or 2 mm in high-dose gradient regions. Compared with the results obtained by using the leaf sequences in which only the transmission or none of the two effects is corrected, significant improvements in the fluence and dose distributions have been observed.

CONCLUSIONS

Transmission and head scatter play important roles in the dosimetric behavior of IMRT delivery. A larger error may result if only one factor is considered because of the opposite effects of the two factors. We noted that the influence of the two effects is more pronounced in absolute dose than in the relative dose. The algorithm proposed in this work accurately corrects for these two effects in step-and-shoot delivery and provides a reliable tool for clinical IMRT application.

Leaf sequencing algorithms for segmented multileaf collimation

The delivery of intensity-modulated radiation therapy (IMRT) with a multileaf collimator (MLC) requires the conversion of a radiation fluence map into a leaf sequence file that controls the movement of the MLC during radiation delivery. It is imperative that the fluence map delivered using the leaf sequence file is as close as possible to the fluence map generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. Optimization of the leaf sequencing algorithm has been the subject of several recent investigations. In this work, we present a systematic study of the optimization of leaf sequencing algorithms for segmental multileaf collimator beam delivery and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of monitor unit (MU) efficiency under most common leaf movement constraints that include minimum leaf separation constraint and leaf interdigitation constraint. Our analytical analysis shows that leaf sequencing based on unidirectional movement of the MLC leaves is as MU efficient as bidirectional movement of the MLC leaves.

The use of electronic portal imaging to verify patient position during intensity-modulated radiotherapy delivered by the dynamic MLC technique

PURPOSE

The precise shape of the three-dimensional dose distributions created by intensity-modulated radiotherapy means that the verification of patient position and setup is crucial to the outcome of the treatment. In this paper, we investigate and compare the use of two different image calibration procedures that allow extraction of patient anatomy from measured electronic portal images of intensity-modulated treatment beams.

METHODS AND MATERIALS

Electronic portal images of the intensity-modulated treatment beam delivered using the dynamic multileaf collimator technique were acquired. The images were formed by measuring a series of frames or segments throughout the delivery of the beams. The frames were then summed to produce an integrated portal image of the delivered beam. Two different methods for calibrating the integrated image were investigated with the aim of removing the intensity modulations of the beam. The first involved a simple point-by-point division of the integrated image by a single calibration image of the intensity-modulated beam delivered to a homogeneous polymethyl methacrylate (PMMA) phantom. The second calibration method is known as the quadratic calibration method and required a series of calibration images of the intensity-modulated beam delivered to different thicknesses of homogeneous PMMA blocks. Measurements were made using two different detector systems: a Varian amorphous silicon flat-panel imager and a Theraview camera-based system. The methods were tested first using a contrast phantom before images were acquired of intensity-modulated radiotherapy treatment delivered to the prostate and pelvic nodes of cancer patients at the Royal Marsden Hospital.

RESULTS

The results indicate that the calibration methods can be used to remove the intensity modulations of the beam, making it possible to see the outlines of bony anatomy that could be used for patient position verification. This was shown for both posterior and lateral delivered fields.

CONCLUSIONS

Very little difference between the two calibration methods was observed, so the simpler division method, requiring only the single extra calibration measurement and much simpler computation, was the favored method. This new method could provide a complementary tool to existing position verification methods, and it has the advantage that it is completely passive, requiring no further dose to the patient and using only the treatment fields.

Reshapable physical modulator for intensity modulated radiation therapy

A new method of generating beam intensity modulation filters for intensity modulated radiation therapy (IMRT) is presented. The modulator was based on a reshapable material, which is not compressible but can be deformed under pressure. A two-dimensional (2D) piston array was used to repeatedly shape the attenuating material. The material is a mixture of tungsten powder and a silicon-based binder. The linear attenuation coefficient of the material was measured to be 0.409 cm(-1) for a 6 MV x-ray beam. The maximum thickness of the physical modulator is 10.2 cm, allowing a transmission of 1.5%. A 16 x 16 square piston array was used to generate a depth pattern in the deformable attenuating material. Each piston has a cross section of 6.37 x 6.37 mm2. The modulator was placed 65 cm from the radiation source of the linear accelerator in the position of the shielding tray. At this position, each piston projects to a 1.0 x 1.0 cm2 area at the isocenter, giving a treatment field of 16 x 16 cm2. The percent depth dose curve and output factor measurement show a slight beam hardening and a 1%-4% increase in scatter fraction when 2.2-4.4 cm uniform thickness filters are in the beam. The surface dose was decreased with the filter in the beam. Ion chamber and verification films were used to verify the entrance dose. The measured absolute and relative doses were compared with the calculated dose. The agreement of measurements and calculations is within 3%. In order to verify the spatial modulation of dose, 1-D dose profiles were obtained using dose calculations. Calculated and measured profiles were compared. The 20%-80% penumbra of the modulator was measured to be 5.5-10 mm. The results show that a physical modulator formed using a 16 x 16 piston array and a deformable attenuation material can provide intensity modulation for IMRT comparable with those provided by currently available commercial MLC techniques.

Beyond bixels: generalizing the optimization parameters for intensity modulated radiation therapy

Intensity modulated radiation therapy (IMRT) treatment planning systems optimize fluence distributions by subdividing the fluence distribution into rectangular bixels. The algorithms typically optimize the fluence intensity directly, often leading to fluence distributions with sharp discontinuities. These discontinuities may yield difficulties in delivery of the fluence distribution, leading to inaccurate dose delivery. We have developed a method for decoupling the bixel intensities from the optimization parameters; either by introducing optimization control points from which the bixel intensities are interpolated or by parametrizing the fluence distribution using basis functions. In either case, the number of optimization search parameters is reduced from the direct bixel optimization method. To illustrate the concept, the technique is applied to two-dimensional idealized head and neck treatment plans. The interpolation algorithms investigated were nearest-neighbor, linear and cubic spline, and radial basis functions serve as the basis function test. The interpolation and basis function optimization techniques were compared against the direct bixel calculation. The number of optimization parameters were significantly reduced relative to the bixel optimization, and this was evident in the reduction of computation time of as much as 58% from the full bixel optimization. The dose distributions obtained using the reduced optimization parameter sets were very similar to the full bixel optimization when examined by dose distributions, statistics, and dose-volume histograms. To evaluate the sensitivity of the fluence calculations to spatial misalignment caused either by delivery errors or patient motion, the doses were recomputed with a 1 mm shift in each beam and compared to the unshifted distributions. Except for the nearest-neighbor algorithm, the reduced optimization parameter dose distributions were generally less sensitive to spatial shifts than the bixel optimization. These results indicate that significant reductions in optimization parameter sets can be accomplished with a negligible reduction in dose distribution quality. The decreased parameters can result in a reduced optimization time, or can be used to allow an improved and consequently more computation-intensive dose calculation for more accurate dose calculations during the optimization process. The basis functions may be generalized to model the accelerator motion for direct computation of the accelerator motion sequence, removing the need for developing an independent leaf sequence step.

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.

Dose verification for patients undergoing IMRT

At Emory Clinic intensity-modulated radiation therapy (IMRT) was started by using dynamic multileaf collimators (dMLC) as electronic tissue compensators in August 1998. Our IMRT program evolved with the inclusion of a commercially available inverse treatment planning system in September 1999. While the introduction of electronic tissue compensators into clinical use did not affect the customary radiation oncology practice, inverse treatment planning does alter our basic routines. Basic concepts of radiation therapy port designs for inverse treatment planning are different from conventional or 3D conformal treatments. With inverse treatment planning, clinicians are required to outline a gross tumor volume (GTV), a clinical target volume (CTV), critical normal structures, and to design a planning target volume (PTV). Clinicians do not designate the volume to be shielded. Because each IMRT radiation portal is composed of many beamlets with varying intensities, methods and practice used to verify delivered dose from IMRT portals are also different from conventional treatment portals. Often, the validity of measured data is in doubt. Therefore, checking treatment planning computer output with measurements are confusing and fruitless, at times. Commissioning an IMRT program and routine patient dose verification of IMRT require films and ionization chamber measurements in phantom. Additional specialized physics instrumentation is not required other than those available in a typical radiation oncology facility. At this time, we consider that routine quality assurance prior to patient treatments is necessary.

Incorporating multi-leaf collimator leaf sequencing into iterative IMRT optimization

Intensity modulated radiation therapy (IMRT) treatment planning typically considers beam optimization and beam delivery as separate tasks. Following optimization, a multi-leaf collimator (MLC) or other beam delivery device is used to generate fluence patterns for patient treatment delivery. Due to limitations and characteristics of the MLC, the deliverable intensity distributions often differ from those produced by the optimizer, leading to differences between the delivered and the optimized doses. Objective function parameters are then adjusted empirically, and the plan is reoptimized to achieve a desired deliverable dose distribution. The resulting plan, though usually acceptable, may not be the best achievable. A method has been developed to incorporate the MLC restrictions into the optimization process. Our in-house IMRT system has been modified to include the calculation of the deliverable intensity into the optimizer. In this process, prior to dose calculation, the MLC leaf sequencer is used to convert intensities to dynamic MLC sequences, from which the deliverable intensities are then determined. All other optimization steps remain the same. To evaluate the effectiveness of deliverable-based optimization, 17 patient cases have been studied. Compared with standard optimization plus conversion to deliverable beams, deliverable-based optimization results show improved isodose coverage and a reduced dose to critical structures. Deliverable-based optimization results are close to the original nondeliverable optimization results, suggesting that IMRT can overcome the MLC limitations by adjusting individual beamlets. The use of deliverable-based optimization may reduce the need for empirical adjustment of objective function parameters and reoptimization of a plan to achieve desired results.

Considerations on treatment efficiency of different conformal radiation therapy techniques for prostate cancer

BACKGROUND AND PURPOSE

To evaluate the treatment efficiency of different conformal radiation therapy techniques in prostate cancer.

MATERIALS AND METHODS

Three major classes of intensity-modulated radiation therapy (IMRT) delivery as well as a conformal rotation technique have been evaluated: sequential tomotherapy, dynamic multileaf collimation (DMLC) with conventional MLC, DMLC with miniMLC and dynamic field shaping arc. Treatment planning for the IMRT techniques has been performed with inverse planning. Forward planning was used for the dynamic arc technique. The four techniques have been compared to treat two different prostate cases with a conservative target dose of 70 Gy: a convex shaped target volume and one containing concavities formed by the bladder and rectum. Cumulative dose volume histograms, tumor control probability and normal tissue complication probability, conformity index and dose heterogeneity, and finally efficiency of treatment delivery have been evaluated.

RESULTS

For the convex shaped target, all treatment modalities met the desired treatment goals, although the conventional MLC delivered more dose to the bladder. Compared to the dynamic arc modality, both tomotherapy and the conventional MLC technique needed a tenfold higher number of monitor units per target dose, and the miniMLC a twofold higher number. The same trend has been observed for the concave target, yet the dynamic arc did not meet the desired dose reduction for the rectum. The miniMLC configuration represented the best compromise for both targets with respect to treatment goals and delivery efficiency. Sequential tomotherapy performed adequately with respect to conformity at the cost of efficiency.

CONCLUSIONS

Together with conformity and delivery efficiency the shape of the target should be considered as an important parameter in the selection of the treatment modality.

Assessment of the acceptability of the Elekta multileaf collimator (MLC) within the Corvus planning system for static and dynamic delivery of intensity modulated beams (IMBs)

The sliding window technique used for static and dynamic segmentation of intensity modulated beams is evaluated. Dynamic delivery is preferred since the resulting distributions correspond better with the calculated distributions, the treatment beam is used more efficiently and the delivery is less sensitive to small variations in the accuracy of the multileaf collimator (MLC).

A simplified intensity modulated radiation therapy technique for the breast

A simplified intensity modulated radiation therapy (sIMRT) technique for the breast is presented. The technique aims to produce a uniform dose distribution in the entire breast volume. Using the standard tangential beam arrangement, we first determine for each pencil beam the midpoint of the segment that intersects the treatment volume. The dose to the midpoint from the open field is then calculated. The intensity of the pencil beam is determined as proportional to the inverse of the open field dose. With this intensity modulated beam, the dose delivered to the midpoint of each pencil beam segment that intersects the treatment volume is now equalized. The dose distribution in the entire treatment volume is nearly as uniform as can be achieved under the given beam arrangement. Fifteen left breast patients were planned with the sIMRT technique. For comparison, the same group of patients was also planned with the standard wedged pair technique and the full-fledged volume-based IMRT (vIMRT) technique. Both the sIMRT and the vIMRT techniques achieved more homogeneous dose in the treatment volume than the standard plan. Doses to the heart, the ipsilateral lung, and the contralateral breast were also reduced. The planning time and the treatment time for the sIMRT technique were comparable to that of the standard technique, and significantly less than that required by the vIMRT technique. The sIMRT technique is practical for large-scale implementation in a busy clinic without requiring significant increase of resources.

Analysis of the effects of the delivery technique on an IMRT plan: comparison for multiple static field, dynamic and NOMOS MIMiC collimation

The process of delivering an IMRT treatment may involve various beam-modifying techniques such as multileaf collimators (MLCs), the NOMOS MIMiC, blocks, wedges, etc. In the case of the MLC, the spatial/temporal variation of the position of the leaves and diaphragms in the beam allows the delivery of modulated beam profiles either by the multiple-static-field (MSF) method or by the dynamic multileaf collimator (DMLC) method. The constraints associated with the IMRT delivery technique are usually neglected in the process of obtaining the 'optimal' inverse treatment plan. Consequently, dose optimization may be significantly reduced when the 'optimal' beam profiles are converted to leaf/diaphragm positions via a leaf-sequencing interpreter. The paper presented here assesses the effects on the optimum treatment plan of the following leaf-sequencing algorithms: MSF, DMLC and NOMOS MIMiC. The results obtained suggest that the delivery of an 'optimum' plan produces an overdosage of the PTV region due to various factors such as leaf/diaphragm transmission effects, head-scatter and phantom-scatter contributions. The overdosage observed for a cohort of ten patients was 2.5, 3.7 and 5.7%, respectively, for the DMLC, MSF and NOMOS MIMiC, after normalizing the delivered fluence to account for IMRT effects (using the method of Convery et al (Convery D J, Cogrove V P and Webb S 2000 Proc. 13th Int. Conf. on Computers in Radiotherapy (Heidelberg, 2000)) such as to obtain 70 Gy at the isocentre. The IMRT techniques DMLC, MIMiC and MSF were compared for the organs at risk: rectum, bladder, and left and right femoral heads.

Improvements in target coverage and reduced spinal cord irradiation using intensity-modulated radiotherapy (IMRT) in patients with carcinoma of the thyroid gland

BACKGROUND AND PURPOSE

External beam radiotherapy for thyroid carcinoma poses a significant technical challenge as the target volume lies close to or surrounds the spinal cord. The potential of intensity-modulated radiotherapy (IMRT) to improve the dose distributions was investigated.

MATERIALS AND METHODS

A planning study was performed on patients with thyroid carcinoma. Plans were generated to irradiate the thyroid bed alone or to treat the thyroid bed and the loco-regional lymph nodes in two phases. Conventional plans with minimal beam shaping were compared to three-dimensional conformal radiotherapy (3DCRT) and inverse-planned IMRT plans to assess target coverage and normal tissue sparing. IMRT techniques were optimized to find the minimum number of equispaced beams required to achieve the clinical benefit and a concomitant boost technique was explored.

RESULTS

For the thyroid bed alone and the thyroid bed plus loco-regional lymph nodes, conventional and conformal techniques produced low minimum doses to the planning target volume (PTV) if spinal cord tolerance was respected. 3DCRT reduced the irradiated volume of normal tissue (P=0.01). IMRT plans achieved the goal dose to the PTV (P<0.01) and also reduced the spinal cord maximum dose (P<0.01). IMRT, using a concomitant boost technique, produced better target coverage than a two-phase technique. For both the two-phase and concomitant boost techniques, IMRT plans with seven and five equispaced fields produced similar dose distributions to nine fields, but three fields were significantly worse.

CONCLUSIONS

3DCRT reduced normal tissue irradiation compared to conventional techniques, but did not improve PTV or spinal cord doses. IMRT improved the PTV coverage and reduced the spinal cord dose. A simultaneous integrated boost technique with five equispaced fields produced the best dose distribution. IMRT should reduce the risk of myelopathy or may allow dose escalation in patients with thyroid cancer.

A simple method to control aspects of fluence modulation in IMRT planning

Many inverse-planning algorithms and commercial systems generate intensity-modulated beam profiles that have considerable structure. This is the desirable outcome of the quest for high dose-space conformality. However, when these profiles are realized experimentally using the dynamic multileaf collimator (DMLC) method of delivery the monitor-unit efficiency can be quite small, with unwanted consequences. Also the interpretation of these fields leads to the generation of small field segments, again with undesirable consequences. In this note it is shown that the features of beam-space can be user-controlled to minimize these problems. There is a tradeoff between obtaining desirable features in beam-space and high conformality in dose-space.

Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing

The frequent blocking of the irradiated volume in intensity modulated radiation therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields. On the other hand. certain collimator configurations block scatter photons directed to a given observation point while allowing primary photons to be transmitted. The 'anomalous blocking' makes the primary field a poor indicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magnify head-scatter uncertainties, it becomes necessary to accurately model both the effective scatter source and the collimator structure that limits the scatter reaching the irradiated volume. First we obtain a dual-source model, using a Taylor series expansion to derive the effective scatter source distribution from the data measured for the Elekta SL20 linac equipped with a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we calculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomalous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurements to an accuracy of 0.002psi10 x 10, where psi10 x 10 is the total (primary + scatter) photon fluence of an open 10 x 10 cm2 field for the same MU delivered. Although the significance of collimator structure in IMRT depends strongly on fields shapes employed for the delivery, potential cumulative errors on the order of a few per cent can be avoided in fluence calculations if the proposed method is used.

A variable fluence step clustering and segmentation algorithm for step and shoot IMRT

A step and shoot sequencer was developed that can be integrated into an IMRT optimization algorithm. The method uses non-uniform fluence steps and is adopted to the constraints of an MLC. It consists of a clustering, a smoothing and a segmentation routine. The performance of the algorithm is demonstrated for eight mathematical profiles of differing complexity and two optimized profiles of a clinical prostate case. The results in terms of stability, flexibility, speed and conformity fulfil the criteria for the integration into the optimization concept. The performance of the clustering routine is compared with another previously published one (Bortfeld et al 1994 Int. J. Radiat. Oncol. Biol. Ph.vs. 28 723-30) and yields slightly better results in terms of mean and maximum deviation between the optimized and the clustered protile. We discuss the specific attributes of the algorithm concerning its integration into the optimization concept.

Leaf sequencing with secondary beam blocking under leaf positioning constraints for continuously modulated radiotherapy beams

The creation of arbitrary photon fluence patterns for intensity modulated radiotherapy is addressed. The proposed method is intended for a class of multileaf collimators with a requirement for minimum leaf separation. Unlike the solution of Convery and Webb in which discrete beam intensity modulation was assumed, the present method deals with continuous modulation or that consisting of infinitely small bixels. The method begins with the time-optimal solution of Spirou-Stein-Svensson disregarding the minimum gap requirement. Subsequently, the gaps are restored by mobilizing the secondary beam blocking devices to prevent overexposure resulting from the leaf separation process. The secondary beam blocking is provided by means of two orthogonal backup diaphragms that are computer controlled. The results indicate that the method can be used to accurately deliver the desired modulation while satisfying the leaf positioning constraints. Furthermore, an example is presented which illustrates the efficacy of using the horizontal backup diaphragms (moving in perpendicular direction of the leaves) in addition to the vertical backup diaphragms (moving in the parallel direction of the leaves) to generate zero fluence regions.

An evaluation of two techniques for beam intensity modulation in patients irradiated for stage III non-small cell lung cancer

In locally advanced lung cancer, the use of high dose radiotherapy (RT) and/or concurrent chemo-RT is associated with significant pulmonary and esophageal toxicity. Despite a 3D conformal RT technique and the omission of elective mediastinal fields, three (of ten) patients with inoperable stage 3 NSCLC who were treated with induction chemotherapy (carboplatin-paclitaxel) followed by RT to 70 Gy, developed symptomatic radiation pneumonitis. In this planning study, the actual treatment plans of all ten patients were compared to plans derived using two beam intensity-modulated (BIM) techniques, for which similar geometrical beam setup parameters were used. In the first technique (BF-BIM), cranial and caudal boost fields were applied in order to allow field length reduction. The second technique (C-BIM) utilised 3-D missing-tissue compensators for all radiation beams. Both BIM techniques resulted in a significant sparing of critical normal tissues and the C-BIM technique was superior in all cases. When compared to the actual RT technique used for treatment, a reduction of 8.1+/-4.7% (1 S.D.) was observed in the mean lung dose for the BF-BIM plan, vs. 20.3+/-5.8% (1 S.D.) for the C-BIM plan. Similar reductions were observed in the percentage of the total lung volume exceeding 20 Gy (V(20)) for these techniques. BIM techniques appear to be a promising tool for enabling radiation dose-escalation and/or intensive concurrent chemo-RT in inoperable lung cancer.

Sampling considerations for intensity modulated radiotherapy verification using electronic portal imaging

A model has been developed to describe the sampling process that occurs when intensity modulated radiotherapy treatments (delivered with a multileaf collimator) are imaged with an electronic portal imaging device that acquires a set of frames with a finite dead-time between them. The effects of the imaging duty cycle and frame rate on the accuracy of dosimetric verification have been studied. A frame interval of 1 s with 25%, 50% and 75% duty cycle, and a 50% duty cycle with frame intervals of 1, 2, 4, 8, and 16 s have been studied for a smoothly varying hemispherical intensity profile, and a 50% duty cycle with frame intervals of 1, 2, 4, and 8 s for a pixellated distribution. In addition an intensity modulated beam for breast radiotherapy has been modeled and imaged for 0.33 s frame time and 1, 2, and 3 s frame separation. The results show that under sparse temporal sampling conditions, errors of the order of 10% may ensue and occur with an oscillatory pattern. For the beams studied, imaging with a 1 or 2 s frame interval resulted in small errors at the 1%-2% level, for all duty cycles shown.

Concepts for shuttling multileaf collimators for intensity-modulated radiation therapy

Shuttling multileaf collimators (SMLCs) can increase the MU efficiency of intensity-modulated radiation therapy compared with the multiple-static-field (MSF-MLC) technique or dynamic MLC (DMLC) technique with conventional MLCs. In a previous paper (Phys. Med. Biol. 45 3343-58) a particular SMLC was shown, for highly modulated intensity distributions, to increase the MU efficiency compared with the MSF-MLC technique. In this companion paper, two new arrangements similar to that described in the earlier paper, but with less mechanical complexity, are shown to be constructionally simpler but less MU efficient. Additionally another new concept of SMLC is shown which also increases the MU efficiency compared with the MSF-MLC technique and often improves the MU efficiency compared with the previously reported SMLC for highly modulated intensity distributions. It also leads to zero tongue-and-groove underdose in the direction orthogonal to that of the shuttling elements (so-called across-the-rows).

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.

A method to incorporate leakage and head scatter corrections into a tomotherapy inverse treatment planning algorithm

A detailed tomotherapy inverse treatment planning method is described which incorporates leakage and head scatter corrections during each iteration of the optimization process, allowing these effects to be directly accounted for in the optimized dose distribution. It is shown that the conventional inverse planning method for optimizing incident intensity can be extended to include a 'concurrent' leaf sequencing operation from which the leakage and head scatter corrections are determined. The method is demonstrated using the steepest-descent optimization technique with constant step size and a least-squared error objective. The method was implemented using the MATLAB scientific programming environment and its feasibility demonstrated for 2D test cases simulating treatment delivery using a single coplanar rotation. The results indicate that this modification does not significantly affect convergence of the intensity optimization method when exposure times of individual leaves are stratified to a large number of levels (>100) during leaf sequencing. In general, the addition of aperture dependent corrections, especially 'head scatter', reduces incident fluence in local regions of the modulated fan beam, resulting in increased exposure times for individual collimator leaves. These local variations can result in 5% or greater local variation in the optimized dose distribution compared to the uncorrected case. The overall efficiency of the modified intensity optimization algorithm is comparable to that of the original unmodified case.

Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation

PURPOSE

We have investigated the potential of applying different doses to the prostate (PTV2) and prostate/seminal vesicles (PTV1) using multileaf collimation (MLC) for intensity modulated radiation therapy (IMRT). Current dose-escalation studies call for treatment of the PTV1 to 54 Gy in 27 fractions followed by 20 Gy minimum to the PTV2. A daily minimum PTV dose of 2 Gy using a 7-field technique (4 obliques, opposed laterals, and an ant-post field) is delivered. This requires monitor unit calculations, paper and electronic chart entry, and quality assurance for a total of 14 fields. The goal of MLC IMRT is to improve efficiency and deliver superior dose distributions. Acceptance testing and commissioning of the dynamic MLC (DMLC) option on a dual-energy accelerator was accomplished. Most of the testing was performed using segmental MLC (SMLC) IMRT with stop-and-shoot sequences built within the dynamic mode of the DMLC.

METHODS AND MATERIALS

The MLC IMRT fields were forward planned using a three-dimensional treatment planning system. The 14 fields were condensed to 7 SMLC IMRT fields with two segments each. In this process, steps were created by moving the leaves to the reduced field positions. No dose (<0.01%) was delivered during this motion. The monitor units were proportioned according to the planned treatment weights. Film and ionization chamber dosimetry were used to analyze leaf positional accuracy and speed, output, and depth-dose characteristics. A geometric phantom was used for absolute and relative measurements. We obtained a volumetric computerized tomography (CT) scan of the phantom, performed 3D planning, and then delivered a single treatment fraction.

RESULTS

The acceptance testing and commissioning demonstrated that the leaves move to programmed positions accurately and in a timely manner. We did find an approximately 1 mm offset of the set leaf position and radiation edge (50%) due to the curved-end nature and calibration limitations. The 7-field SMLC IMRT treatment duplicated the 14-field static plan dose distribution with variations no greater than 1.5%.

CONCLUSIONS

The MLC IMRT approach will improve efficiency because the number of electronic and chart entries has decreased by a factor of 2. Portal images are able to capture the initial and final MLC segments. The question of differential daily dose to the prostate and seminal vesicles remains.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields

BACKGROUND AND PURPOSE

To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs).

MATERIALS AND METHODS

An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented.

RESULTS

The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume.

CONCLUSIONS

This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.