MLC-based Intensity Modulated Radiotherapy Research Articles

A novel software and conceptual design of the hardware platform for intensity modulated radiation therapy.

To deliver high quality intensity modulated radiotherapy (IMRT) using a novel generalized sparse orthogonal collimators (SOCs), the authors introduce a novel direct aperture optimization (DAO) approach based on discrete rectangular representation. A total of seven patients-two glioblastoma multiforme, three head & neck (including one with three prescription doses), and two lung-were included. 20 noncoplanar beams were selected using a column generation and pricing optimization method. The SOC is a generalized conventional orthogonal collimators with N leaves in each collimator bank, where N = 1, 2, or 4. SOC degenerates to conventional jaws when N = 1. For SOC-based IMRT, rectangular aperture optimization (RAO) was performed to optimize the fluence maps using rectangular representation, producing fluence maps that can be directly converted into a set of deliverable rectangular apertures. In order to optimize the dose distribution and minimize the number of apertures used, the overall objective was formulated to incorporate an L2 penalty reflecting the difference between the prescription and the projected doses, and an L1 sparsity regularization term to encourage a low number of nonzero rectangular basis coefficients. The optimization problem was solved using the Chambolle-Pock algorithm, a first-order primal-dual algorithm. Performance of RAO was compared to conventional two-step IMRT optimization including fluence map optimization and direct stratification for multileaf collimator (MLC) segmentation (DMS) using the same number of segments. For the RAO plans, segment travel time for SOC delivery was evaluated for the N = 1, N = 2, and N = 4 SOC designs to characterize the improvement in delivery efficiency as a function of N. Comparable PTV dose homogeneity and coverage were observed between the RAO and the DMS plans. The RAO plans were slightly superior to the DMS plans in sparing critical structures. On average, the maximum and mean critical organ doses were reduced by 1.94% and 1.44% of the prescription dose. The average number of delivery segments was 12.68 segments per beam for both the RAO and DMS plans. The N = 2 and N = 4 SOC designs were, on average, 1.56 and 1.80 times more efficient than the N = 1 SOC design to deliver. The mean aperture size produced by the RAO plans was 3.9 times larger than that of the DMS plans. The DAO and dose domain optimization approach enabled high quality IMRT plans using a low-complexity collimator setup. The dosimetric quality is comparable or slightly superior to conventional MLC-based IMRT plans using the same number of delivery segments. The SOC IMRT delivery efficiency can be significantly improved by increasing the leaf numbers, but the number is still significantly lower than the number of leaves in a typical MLC.

Introducing multiple treatment plan-based comparison to investigate the performance of gantry angle optimisation (GAO) in IMRT for head and neck cancer

Background and purpose. The purpose of this study was to evaluate the performance of gantry angle optimisation (GAO) compared to equidistant beam geometry for two inverse treatment planning systems (TPSs) by utilising the information obtained from a range of treatment plans. Material and methods. The comparison was based on treatment plans generated for four different head and neck (H&N) cancer cases using two inverse treatment planning systems (TPSs); Varian Eclipse™ representing dynamic MLC intensity modulated radiotherapy (IMRT) and Oncentra® Masterplan representing segmented MLC-based IMRT. The patient cases were selected on the criterion of representing different degrees of overlap between the planning target volume (PTV) and the investigated organ at risk, the ipsilateral parotid gland. For each case, a number of ‘Pareto optimal’ plans were generated in order to investigate the trade-off between the under-dosage to the PTV (VPTV,D < 95%) or the decrease in dose homogeneity (D5-D95) to the PTV as a function of the mean absorbed dose to the ipsilateral parotid gland (<D>parotid gland). Results. For the Eclipse system, GAO had a clear advantage for the cases with smallest overlap (Cases 1 and 2). The set of data points, representing the underlying trade-offs, generated with and without using GAO were, however, not as clearly separated for the cases with larger overlap (Cases 3 and 4). With the OMP system, the difference was less pronounced for all cases. The Eclipse GAO displays the most favourable trade-off for all H&N cases. Conclusions. We have found differences in the effectiveness of GAO as compared to equidistant beam geometry, in terms of handling conflicting trade-offs for two commercial inverse TPSs. A comparison, based on a range of treatment plans, as developed in this study, is likely to improve the understanding of conflicting trade-offs and might apply to other thorough comparison techniques.

A Clinical Study of MLC-Based IMRT Lung Dose Calculation Accuracy on Plan Evaluation Parameters

Intensity Modulated Radiation Therapy (IMRT) is widely accepted as an appropriate method to treat tumors at many different anatomic locations including lung. Dose calculation algorithms that have different degrees of accuracy are used to produce clinical IMRT treatment plans. In this study, Monte Carlo (MC) dose calculation was used to evaluate the reliability of plan evaluation parameters compared to a pencil beam (PB) dose calculation for IMRT of the lung.Twenty fi ve lung IMRT cases were randomly selected for analysis. Plan evaluation parameters were calculated using PB and MC methods for the targets and organs at risk (OARs). Comparisons were made using dose-volume histograms, mean dose, and equivalent uniform dose. The following doses-volume histogram points were compared: D98, D95 of the GTV and PTV, V20 and V30 for the lungs, D33 for the heart and esophagus and Dmax for the spinal cord. Mean dose differences were 3.6 ± 2.3% and 4.3 ± 2.8% for the GTV and PTV, respectively. The average EUD differences were 4.1 ± 2.4% for the GTV and 5.7 ± 4.9% for the PTV. Less than 2% differences were observed between the MC and PB algorithms for all OAR plan evaluation parameters. However, minimum and maximum differences for some plan evaluation parameters ranged from about ±20%.There are appreciable differences in plan evaluation parameters between the PB and MC calculations for the targets. The mean dose and EUD have a weak but statistically signifi cant inverse dependence on the number of fi elds, total MU, GTV volume and PTV volume for the targets. There can be large case-to-case differences between PB and MC for both the targets and OARs. Accurate MC calculations can remove those remaining systematic errors from treatment plans compared to PB calculations.

Comparison of the Helical Tomotherapy and MLC-based IMRT Radiation Modalities in Treating Brain and Cranio-spinal Tumors

The investigation of the clinical efficacy and effectiveness of Intensity Modulated Radiotherapy (IMRT) using Multileaf Collimators (MLC) and Helical Tomotherapy (HT) has been an issue of increasing interest over the past few years. In order to assess the suitability of a treatment plan, dosimetric criteria such as dose-volume histograms (DVH), maximum, minimum, mean, and standard deviation of the dose distribution are typically used. Nevertheless, the radiobiological parameters of the different tumors and normal tissues are often not taken into account. The use of the biologically effective uniform dose (D=) together with the complication-free tumor control probability (P(+)) were applied to evaluate the two radiation modalities. Two different clinical cases of brain and cranio-spinal axis cancers have been investigated by developing a linac MLC-based step-and-shoot IMRT plan and a Helical Tomotherapy plan. The treatment plans of the MLC-based IMRT were developed on the Philips treatment planning station using the Pinnacle 7.6 software release while the dedicated Tomotherapy treatment planning station was used for the HT plan. With the use of the P(+) index and the D(=) concept as the common prescription point, the different treatment plans were compared based on radiobiological measures. The tissue response probabilities were plotted against D(=) for a range of prescription doses. The applied plan evaluation method shows that in the brain cancer, the HT treatment gives slightly better results than the MLC-based IMRT in terms of optimum expected clinical outcome (P(+) of 66.1% and 63.5% for a D(=) to the PTV of 63.0 Gy and 62.0 Gy, respectively). In the cranio-spinal axis cancer, the HT plan is significantly better compared to the MLC-based IMRT plan over the clinically useful dose prescription range (P(+) of 84.1% and 28.3% for a D(=) to the PTV of 50.6 Gy and 44.0 Gy, respectively). If a higher than 5% risk for complications could be allowed, the complication-free tumor control could be increased by almost 30% compared to the initial dose prescription. In comparison to MLC based-IMRT, HT can better encompass the often large PTV while minimizing the volume of the OARs receiving high dose. A radiobiological treatment plan evaluation can provide a closer association of the delivered treatment with the clinical outcome by taking into account the dose-response relations of the irradiated tumors and normal tissues. The use of P - (D=) diagrams can complement the traditional tools of evaluation such as DVHs, in order to compare and effectively evaluate different treatment plans.

An Interlaced IMRT Technique for Elongated Tumor Volumes

Treatment of large target volumes with intensity modulated radiotherapy (IMRT) can be restricted by the maximum field size of the multileaf collimator (MLC). In this work, a straightforward technique for MLC-based IMRT is presented, which is generally applicable and does not depend on the capabilities of the linear accelerator's IMRT delivery system. A dual isocenter technique was developed that maximizes beam overlap. The beams at the first isocenter are arranged such that they interlace with the beams at the second isocenter. All beams contribute to the overlap region, whereas only some contribute to the superior and some to the inferior part of the target. The interlaced technique (9 beams) was compared with an alternative more complex approach (14 beams) for a head-and-neck case with simultaneous integrated boost and 3 different dose levels. The plans were compared in terms of complexity, dosimetry, and the effect of inaccurate translation between the isocenters. The interlaced and the more complex IMRT technique resulted in nearly identical dose distributions without clinically relevant differences. The total number of monitor units (MUs) was comparable with more MUs per segment for the interlaced technique. For the interlaced technique, the number of segments ≤5 MU was reduced by 43%. Simulation of isocenter setup errors of ±1, ±2, and ±3 mm revealed maximum dose point errors of 1.8%, 3.8%, and 5.4% in the target volume for the interlaced technique. The interlaced IMRT technique resulted in an equivalent plan to the more complex technique without compromising the dose distribution. The technique is less complex and is robust against inaccurate isocenter translations of up to ±1 mm. Due to the versatility of the technique, it can easily be applied to other anatomical regions and is well suited for clinical routine usage.

IMAT‐SIM: A new method for the clinical dosimetry of intensity‐modulated arc therapy (IMAT)

Dynamic-gantry multi-leaf collimator (MLC)-based, intensity-modulated radiotherapy (IMAT) has been proposed as an alternative to tomotherapy. In contrast to fixed-gantry, MLC-based intensity-modulated radiotherapy (IMRT), where commercial treatment planning systems (TPS) or dosimetric analysis software currently provide many automatic tools enabling two-dimensional (2D) detectors (matrix or electronic portal imaging devices) to be used as measurement systems, for the planning and delivery of IMAT these tools are generally not available. A new dosimetric method is proposed to overcome some of these limitations. By converting the MLC files of IMAT beams from arc to fixed gantry-angle modality, while keeping the leaf trajectories equal, IMAT plans can be both simulated in the TPS and executed as fixed-gantry, sliding-window DMLC treatments. In support of this idea, measurements of six IMAT plans, in their double form of original arcs and converted fixed-gantry DMLC beams (IMAT-SIM), have been compared among themselves and with their corresponding IMAT-SIM TPS calculations. Radiographic films and a 2D matrix ionization chamber detector rigidly attached to the accelerator gantry and set into a cubic plastic phantom have been used for these measurements. Finally, the TPS calculation-algorithm implementations of both conformal dynamic MLC arc (CD-ARC) modalities, used for clinical IMAT calculations, and DMLC modalities (IMAT-SIM), proposed as references for validating IMAT plan dose-distributions, have been compared. The comparisons between IMAT and IMAT-SIM delivered beams have shown very good agreement with similar shapes of the measured dose profiles which can achieve a mean deviation (+/-2sigma) of (0.35+/-0.16) mm and (0.37+/-0.14)%, with maximum deviations of 1.5 mm and 3%. Matching the IMAT measurements with their corresponding IMAT-SIM data calculated by the TPS, these deviations remain in the range of (1.01+/-0.28) mm and (-1.76+/-0.42)%, with maximums of 3 mm and 5%, limits generally accepted for IMRT plan dose validation. Differences in the algorithm implementations have been found, but by correcting CD-ARC calculations for the leaf-end transmission offset (LTO) effect the IMAT and IMAT-SIM simulations agree well in terms of final dose distributions. The differences found between IMAT and the IMAT-SIM beam measurements are due to the different controls of leaf motion (via electron gun delay in the latter) that cannot be used in the former to correct possible speed variations in the rotation of the gantry. As the IMAT delivered beams are identical to what the patient will receive during the treatment, and the IMAT-SIM beam calculations made by the TPS reproduce exactly the treatment plans of that patient, the accuracy of this new dosimetric method is comparable to that which is currently used for static IMRT. This new approach of 2D-detector dosimetry, together with the commissioning, quality-assurance, and preclinical dosimetric procedures currently used for IMRT techniques, can be applied and extended to any kind of dynamic-gantry MLC-based treatment modality either CD-ARC or IMAT.

Treatment plan comparison between helical tomotherapy and MLC-based IMRT using radiobiological measures

The rapid implementation of advanced treatment planning and delivery technologies for radiation therapy has brought new challenges in evaluating the most effective treatment modality. Intensity-modulated radiotherapy (IMRT) using multi-leaf collimators (MLC) and helical tomotherapy (HT) are becoming popular modes of treatment delivery and their application and effectiveness continues to be investigated. Presently, there are several treatment planning systems (TPS) that can generate and optimize IMRT plans based on user-defined objective functions for the internal target volume (ITV) and organs at risk (OAR). However, the radiobiological parameters of the different tumours and normal tissues are typically not taken into account during dose prescription and optimization of a treatment plan or during plan evaluation. The suitability of a treatment plan is typically decided based on dosimetric criteria such as dose–volume histograms (DVH), maximum, minimum, mean and standard deviation of the dose distribution. For a more comprehensive treatment plan evaluation, the biologically effective uniform dose is applied together with the complication-free tumour control probability (P+). Its utilization is demonstrated using three clinical cases that were planned with two different forms of IMRT. In this study, three different cancer types at different anatomical sites were investigated: head and neck, lung and prostate cancers. For each cancer type, a linac MLC-based step-and-shoot IMRT plan and a HT plan were developed. The MLC-based IMRT treatment plans were developed on the Philips treatment-planning platform, using the Pinnacle 7.6 software release. For the tomotherapy HiArt plans, the dedicated tomotherapy treatment planning station was used, running version 2.1.2. By using as the common prescription point of the treatment plans and plotting the tissue response probabilities versus for a range of prescription doses, a number of plan trials can be compared based on radiobiological measures. The applied plan evaluation method shows that in the head and neck cancer case the HT treatment gives better results than MLC-based IMRT in terms of expected clinical outcome (P+ of 62.2% and 46.0%, to the ITV of 72.3 Gy and 70.7 Gy, respectively). In the lung cancer and prostate cancer cases, the MLC-based IMRT plans are better over the clinically useful dose prescription range. For the lung cancer case, the HT and MLC-based IMRT plans give a P+ of 66.9% and 72.9%, to the ITV of 64.0 Gy and 66.9 Gy, respectively. Similarly, for the prostate cancer case, the two radiation modalities give a P+ of 68.7% and 72.2%, to the ITV of 86.0 Gy and 85.9 Gy, respectively. If a higher risk of complications (higher than 5%) could be allowed, the complication-free tumour control could increase by over 40%, 2% and 30% compared to the initial dose prescription for the three cancer cases, respectively. Both MLC-based IMRT and HT can encompass the often-large ITV required while they minimize the volume of the organs at risk receiving high doses. Radiobiological evaluation of treatment plans may provide an improved correlation of the delivered treatment with the clinical outcome by taking into account the dose–response characteristics of the irradiated targets and normal tissues. There may exist clinical cases, which may look dosimetrically similar but in radiobiological terms may be quite different. In such situations, traditional dose-based evaluation tools can be complemented by the use of diagrams to effectively evaluate and compare treatment plans.

SU‐FF‐T‐144: Comparison of MLC and Solid Modulation in Pediatric Patients Receiving IMRT

Purpose: Intensity modulated radiation therapy (IMRT) offers the potential of reducing the volume of normal tissue irradiated, and thus may provide significant benefit in pediatric patients. However, the total number of monitor units (MUs) associated with MLC‐based IMRT (MLC‐IMRT) may be 2–5 times higher than conventional planning, resulting in higher total body doses. Solid modulator‐based IMRT (SM‐IMRT) may provide significantly fewer MUs, and thus lower total body doses. The goal of this study is to compare MLC‐IMRT with SM‐IMRT treatment planning and delivery in the pediatric setting. Methods and Materials: Representative pediatric sites (brain, orbit, neck and abdomen) were used in this analysis. For each case, a 5–9 beam MLC‐IMRT plan was generated with the XiO treatment planning system (CMS Inc., St Louis, MO) using 6 MV photons. Dose volume histograms (DVHs) were calculated and the total number of MUs was recorded. Using the same input parameters, a second plan was then generated for each patient with SM‐IMRT. In both cases, 10 intensity levels were utilized. The total MUs and DVHs for the PTV and surrounding normal tissues were compared for each pair of plans. Results: In general, both the MLC‐IMRT and SM‐IMRT plans were comparable. Only minor differences in the DVHs were noted in the low dose region (20–40% of the prescription dose). However, the total number of MUs was significantly different. The average number of MUs for the MLC‐IMRT plans and the SM‐IMRT plans were 797 and 428, respectively (p=0.05). Measurements of the total body doses in a pediatric phantom will also be presented. Conclusions: SM‐IMRT plans result in significantly fewer MUs compared to MLC‐IMRT plans. Pediatric patients may benefit from the reduced total body doses, and hence reduced risk of secondary malignancies associated with this approach. Disclosure: Supported in part by CMS Inc. and Decimal Inc.

TH-C-T-6E-09: Multi-Leaf-Collimator Quality Assurance Using the Electronic Portal Imaging Device

Purpose: To test the amorphous silicon (aSi) electronic portal imaging device (EPID) for quality assurance (QA) of the multi-leaf collimators (MLC) and to validate its use as a dynamic intensity-modulated radiation therapy (IMRT) QA device. Method and Materials: Established MLC QA for IMRT utilizes dosimetric outputs from a single point in phantom with a chamber, and radiographic film to capture the density-pattern of the MLC. We propose using the aSi EPID 2D-density distribution to replace the chamber/film for MLC QA, and to verify the dosimetry and mechanics of the MLC for IMRT. A protocol was developed to acquire QA data from the aSi EPID to provide tests for high dose gradients, average output doses, mechanical leaf stability, speed and positioning. The effects of various EPID exposed areas (2×2, 4×4, 6×6 cm2) were used for intercomparison measurements with ion chamber and film. Results: Radiographic film output at gantry angle 270° relative to 0° varies by less than 2%, while the film measurements at other gantry/collimator angles agree within 1% with the ionization chamber. For the same conditions, EPID outputs vary by 1%–5% depending upon exposed area. Output variations may be due to mechanical shifts, gantry and EPID sag, or MLC drifts due to gravity. Minimizing these effects is under further investigation. Conclusion: A method has been developed to utilize the aSi EPID for MLC QA. The 2D density distributions of the EPID offer the potential for more quantitative analysis, and this procedure can be integrated into a routine program for comprehensive dynamic MLC-based IMRT and EPID QA. Conflict of Interest: This work supported in-part by Varian Medical Systems.

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

The use of intensity modulated radiation therapy (IMRT) in the treatment of head and neck (H&N) cancer is expanding worldwide. As radiation dose to selected normal structures (ie, parotid glands) is reduced, dose to adjacent regions (ie, anterior oral cavity) may be increased. Ultimate H&N cancer outcome and patient quality of life (QOL) is complex and can be influenced by many aspects of the radiation dose profile. This study was carried out to compare the capacity of helical tomotherapy versus linac-based IMRT to achieve target dose conformality and normal tissue sparing in oropharynx cancer patients. Twenty oropharyngeal cancer patients treated with curative intent using 7 field H&N IMRT between 2001–2003 were selected for study. Treatment planning and delivery was performed using the PinnacleTM planning system. In general, doses of 66–70 Gy at 2.1–2.2 Gy per fraction were delivered to primary planning target volume (PTV) targets, with 50–60 Gy to subclinical PTV targets. Dose constraints were employed for various normal H&N tissue structures such as parotid gland (20 Gy mean dose), spinal cord and anterior oral cavity (35 Gy maximum dose). Comprehensive H&N tomotherapy plans were designed for these 20 patients using identical prescription parameters. For conventional IMRT design, a 7 field arrangement of equally spaced beams was employed, whereas for helical tomotherapy, over 1000 projections per treatment were used (51 projections per rotation with a minimum of 20 rotations). Resultant treatment plans were then rigorously compared and contrasted. All 40 oropharynx cancer treatment plans analyzed (20 conventional IMRT and 20 helical tomotherapy) met primary treatment objectives regarding dose specification. Specifically, the resultant Equivalent Uniform Dose (EUD) calculations for high and low dose PTV were ≥the prescription dose. In general, helical tomotherapy plans offered favorable dose distributions over that achieved with 7 field IMRT plans as manifested by increased EUD to PTV targets, as well as reduced mean dose to normal tissue avoidance structures. The EUD analysis revealed that tomotherapy plans provided up to 4% increase to high dose PTV over that achieved with 7 field IMRT plans. Simultaneously, mean dose delivered to normal tissue avoidance structures such as contralateral parotid gland, spinal cord and oral cavity was reduced up to 50% with tomotherapy plans. The daily patient treatment time with helical tomotherapy is essentially identical to that with conventional IMRT. However, to ensure comparable delivery times, a jaw opening of 2.5 cm was chosen which yields a 2-fold increase (from 5 to 10 mm) in the conventional 80–20% penumbra in the cephalo-caudal dimension. The overall analysis of H&N treatment plans identifies several potential advantages for helical tomotherapy over that achieved with conventional IMRT. The improved dose distributions are largely explained by the fact that tomotherapy offers a significantly larger number of projections than available with conventional MLC-based IMRT. Nevertheless, the ultimate clinical significance of these dose distribution refinements remains ill defined. Intuitively, it would seem that higher conformality of dose to specified targets, and dose reduction to normal tissue avoidance structures may be of value. To further evaluation of this question, a prospective clinical trial is underway to examine normal tissue functional outcome and QOL in H&N cancer patients receiving IMRT with conventional and tomotherapy techniques. In addition to direct measurement of salivary, auditory, voice and swallow function, validated health outcome surveys are obtained to assess patient perception regarding voice, swallow, hearing and overall H&N function before and following treatment. These data should provide further insight regarding the potential value of refining dose distribution in H&N cancer patients receiving highly conformal treatment using either conventional IMRT or helical tomotherapy

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

Lens dose in MLC-based IMRT treatments of the head and neck

Purpose The objectives of this work are: ( 1) to determine typical dose to the lens during step-and-shoot intensity modulated radiotherapy (IMRT) treatments of the head and neck, and ( 2) to report on the dose calculation accuracy of a commercial inverse planning system in predicting lens dose. Methods and materials The Corvus inverse treatment planning system (Nomos, Cranberry Township, PA) was used to plan IMRT treatments for patients with head-and-neck cancers in our clinic. Patients were treated on Varian C-series linacs (Varian, Palo Alto, CA) with 4-MV or 6-MV X-rays. A Rando phantom (Alderson Laboratories, Stamford, CT) was specially modified to accommodate 1 × 1 × 1 mm 3 thermoluminescent dosimeters at the position of the lens. The IMRT treatment plans were then delivered to the modified Rando phantom. The thermoluminescent dosimeter measurements were converted to dose and taken as an estimate of the lens dose. A total of 20 cases were used in this study (15 cases with 4 MV and 5 cases with 6 MV). Results Expressed as a percentage of the prescription dose, the mean dose to the left and right lens for all 4-MV cases was 9.1% (range, 2.0% to 61.3%). For the 6-MV cases, the mean dose to the left and right lens was 12.8% (range, 3.6% to 41.3%). For both the 4-MV and 6-MV cases, the case of maximum dose occurred when the IMRT treatment target included volumes superior to the level of the lens. The field size and number of monitor units did not correlate with the measured lens dose. The only factor of significance affecting lens dose was the inferior-to-superior distance of the target to the lens. For target–lens distance ≥6 mm, the maximum measured lens doses were 5.9% and 9.0% relative to the prescribed dose for the 4-MV and 6-MV beams, respectively. These data are similar to those observed in conventional head-and-neck treatments. For all cases, the difference between the dose measured and that predicted by Corvus was less than 2% and 4% of the dose prescribed to the gross tumor volume for the 4-MV and 6-MV cases, respectively. Conclusion In IMRT, factors such as leaf leakage and number of monitor units play a secondary role and are not more significant than what is observed in conventional head-and-neck treatment when the lens is shielded by the collimator jaws. The target–lens distance is the parameter that affects the lens dose most strongly. For cases where the tumor is at or above the level of the lens, the lens dose can amount to an appreciable fraction of the prescription dose. To keep the lens dose to a minimum, noncoplanar beams that enter or exit into the lens should not be used.

Potential for toxicity reduction using intensity modulated radiation therapy (IMRT) for retroperitoneal sarcoma

Purpose/Objective: Radiation therapy for retroperitoneal sarcoma remains challenging because of proximity to surrounding organs at risk(OAR). We report the use of IMRT in the treatment of retroperitoneal sarcomas to minimize dose to OAR while concurrently optimizing tumor dose coverage. Materials/Methods: From January 2000 to October 2002, 10 patients with retroperitoneal sarcoma and 1 with inguinal sarcoma were treated with radiation at Emory Clinic. Average patient age was 58 years. 9 patients were treated preoperatively and 2 postoperatively. Prescription dose varied from 36 Gy at 1.2 Gy BID for a re-irradiation case to 59.4 Gy at 1.5 Gy QD for a postoperative case. The majority of patients received a preoperative dose of 50.4 at 1.8 Gy qd. CT simulation was used in each patient; 3 patients were treated with three-dimensional conformal radiotherapy (3D-CRT) and 8 received MLC-based IMRT. IMRT treatment fields ranged from 8–11 and average volume treated was 3498 cc. For both techniques, the field encompassed gross tumor plus a 5 cm margin in the superior/inferior dimension and a 2 cm margin in the anterior/posterior and medial/lateral dimension; when a boost was used a field reduction with a 2 cm margin was employed. Optimal 3D-CRT plans were generated and compared with IMRT with respect to tumor coverage and OAR dose toxicity. Dose volume histograms (DVH) were compared for both the 3D-CRT and IMRT plans. Results: With IMRT, average dose to 50% of small bowel was 27 Gy with less than 9% of the bowel volume exceeding 50 Gy. Mean dose to both 50% of the left kidney and the entire left kidney was 23 Gy. 50% of the right kidney received 14.6 Gy and mean dose to the entire right kidney was 14.6 Gy. 50% of the liver received 20 Gy and the entire liver received a mean dose of 23 Gy. With 3-D CRT, average dose to 50% of small bowel was 36 Gy and 24% of the bowel volume exceeded 50 Gy. 50% of the left kidney received a mean dose of 24 Gy and mean dose to the entire left kidney was 28 Gy. 50% of the right kidney received 18 Gy and mean dose to the entire right kidney was 15 Gy. 50% of the liver received 22 Gy and the entire liver received a mean dose of 28 Gy. For the same dose constraints assigned to liver, small bowel, kidney, and PTV, IMRT resulted in improved coverage of the PTV and reduced dose to critical organs at risk. Mean dose to small bowel was decreased from 36 Gy with 3D-CRT to 27 Gy using IMRT and tumor coverage as measured by the V95 was increased from 95.3% with conventional to 98.6% using IMRT. Both the maximum and minimum doses delivered to the PTV were significantly increased by 6% and by 22 % respectively (p=.011, p=.055) resulting in better dose distribution within the tumor volume. The volume of small bowel receiving > 30 Gy was significantly decreased from 63.5% to 43.1% with IMRT compared with conventional treatment (p=.043). The volume of left kidney receiving a dose greater than 25 Gy decreased from 49% to 37% with the use of IMRT. All patients were evaluated for toxicity using the RTOG toxicity scale. The most common symptoms were nausea and vomiting and less frequently diarrhea. 7 patients developed grade 2 nausea, 3 developed grade 2 diarrhea and 1 patient with primary groin involvement experienced grade 2 skin toxicity. One patient, who had extensive liver involvement and received conventional treatment developed Grade 3 liver toxicity six months after his radiation and had to be hospitalized for management of ascites. Other than this patient, there have been no other delayed toxicities related to radiation. No GU or wound toxicities were observed and no treatment breaks were necessary. At a median follow up time of 52 weeks, there were no local recurrences and only one patient developed disease progression with distant metastasis occurring in the liver. Conclusions: IMRT for retroperitoneal sarcoma allows for enhanced tumor coverage and better sparing of dose to critical normal structures such as small bowel, liver and kidney. Escalation of dose has been shown to have a positive impact on local control for retroperitoneal sarcoma and IMRT may be an effective method to achieve this goal.

Enhancement of IMRT delivery through MLC rotation

Multileaf collimator (MLC) based intensity modulated radiation therapy (IMRT) techniques are well established but suffer several physical limitations. Dosimetric spatial resolution is limited by the MLC leaf width; interleaf leakage and tongue-and-groove effects degrade dosimetric accuracy and the range of leaf motion limits the maximum deliverable field size.Collimator rotation is used in standard radiation therapy to improve the conformity of the MLC shape to the target volume. Except for opposed orthogonal fields, collimator rotation has not been exploited in IMRT due to the complexity of deriving the MLC leaf configurations for rotated sub-fields. Here we report on a new way that MLC-based IMRT is delivered which incorporates collimator rotation, providing an extra degree of freedom in deriving leaf sequences for a desired fluence map. Specifically, we have developed a series of unique algorithms that are capable of determining rotated MLC segments. These IMRT fields may be delivered statically (with the collimator rotating to a new position in between sub-fields) or dynamically (with the collimator rotating and leaves moving simultaneously during irradiation).This introductory study provides an analysis of the rotating leaf motion calculation algorithms with focus on radiation efficiency, the range of collimator rotation and number of segments. We then evaluate the technique by characterizing the ability of the algorithms to generate rotating leaf sequences for desired fluence maps. Comparisons are also made between our method and conventional sliding window and step-and-shoot techniques. Results show improvements in spatial resolution, reduced interleaf effects and maximum deliverable field size over conventional techniques. Clinical application of these enhancements can be realized immediately with static rotational delivery although improved dosimetric modelling of the MLC will be required for dynamic delivery.

Clinical implementation of intensity-modulated radiation therapy

The clinical implementation of intensity-modulated radiation therapy (IMRT) is a complex process because of the introduction of new treatment planning algorithms and beam delivery systems compared to conventional 3-dimensional conformal radiation therapy (3D-CRT) and the lack of established national performance protocols. IMRT uses an inverse-planning algorithm to create nonuniform fields that are only deliverable through a newly designed beam-modulating delivery system. The intent of this paper is to describe our experience and to elucidate the new clinical procedures that must be executed to have a successful IMRT program. Patients who undergo IMRT at our institution are immobilized and simulated before proceeding to computed tomography scan for patient data acquisition. Treatment planning involves the use of different prescription dose formats and different planning techniques compared to 3D-CRT. The desired dose goals for the target and sensitive structures must be specified before initiating the planning process, which is computer intensive. After the plan is completed, the delivery instructions are transferred to the delivery system via either a floppy disk for MIMiC-based IMRT or through the network for MLC-based IMRT. Target localizations are carried out using orthogonal radiographs. Ultrasound imaging system (BAT) is used to localize the prostate. Dose validation is performed using films, ion chambers or dose-calculation-based techniques.

Commissioning and quality assurance for Mlc-Based IMRT

The commissioning and quality assurance (QA) associated with the implementation of linear accelerator multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) at the University of Nebraska Medical Center are described. Our MLC-based IMRT is implemented using the PRIMUS linear accelerator interface through the IMPAC record and verification system to the CORVUS treatment planning system. The “step-and-shoot” technique is used for this MLC-based IMRT. Commissioning process requires the verification of predefined parameters available on the CORVUS and the collection of some machine data. The machine data required are output factor in air and output factor in phantom, and percent depth dose for a number of field sizes. In addition, inplane and crossplane dose profiles of 4 × 4 cm and 20 × 20 cm field sizes and diagonal dose profiles of a large field size have to be measured. Validation of connectivity and dose model includes the use of uniform intensity bar strips, triangular-shaped nonuniform intensity bar strip, and N-shaped target. QA procedure follows the recommendation of the AAPM Task Group No. 40 report. In addition, the leaf position accuracy and reproducibility of the MLC should be checked at regular intervals. The dose validation is implemented through the hybrid plan where the patient beam parameters are applied to a flat phantom. Independent dose calculation method is used to confirm the dose delivery plan and data input to the CORVUS.

Delivery systems of intensity-modulated radiotherapy using conventional multileaf collimators

Intensity-modulated radiotherapy (IMRT) using conventional multileaf collimators (MLCs) has gained much attention in the radiation therapy community. To implement IMRT safely and efficiently, it is essential to understand the characteristics of MLCs, the associated delivery systems, and the limitations of each system when applied to IMRT. In this article, 3 major manufactured MLC collimators are reviewed, including the general descriptions such as the location of the MLC, whether or not it is single focused or double focused, the physical characteristics of leaves, the leaf movement restrictions, and the maximum achievable field size for IMRT delivery. MLC-based static and dynamic mechanisms are reviewed, and the delivery systems for these 3 collimators are described. Machine-related and patient plan-related quality assurance issues are discussed. Our conclusions are that MLC-based IMRT delivery is practical; however, extensive efforts are needed when IMRT is implemented in clinics. .