Intensity Modulated Radiation Therapy System Research Articles

Commissioning and dosimetric validation of a novel compensator-based Co-60 IMRT system for evaluating suitability to automated treatment planning.

A novel compensator-based system has been proposed which delivers intensity-modulated radiation therapy (IMRT) with cobalt-60 beams. This could improve access to advanced radiotherapy in low- and middle-income countries. For this system to be clinically viable and to be adapted into the Radiation Planning Assistant (RPA), being developed to offer automated planning services in low- and middle-income countries, it is necessary to commission and validate it in a commercial treatment planning system (TPS). The novel treatment device considered here employs a cobalt-60 source and nine compensators. Each compensator is produced by 3-D printing a thin plastic mold which is then filled on-demand within the machine with reusable 2-mm-diameter spherical tungsten balls. This system was commissioned in the Eclipse TPS and validation tests were conducted with Monte Carlo using Geant4 Application for Tomographic Emission for percentage depth dose, in-plane profiles, penumbra, and IMRT dose validation. And the American Association of Physicists in Medicine Task Group 119 benchmarking testing was performed. Additionally, compensator-based cobalt-60 IMRT plans were created for 46 head-and-neck cancer cases and compared to the linac-based volumetric modulated arc therapy (VMAT) plans used clinically, then dosimetric parameters were evaluated. Beam-on time for each field was calculated. In addition, the measurement was also performed in a limited environment and compared with the Monte Carlo simulations. The differences in percent depth doses and in-plane profiles between the Eclipse and Monte Carlo simulations were 0.65%±0.41% and 1.02%±0.99%, respectively, and the 80%-20% penumbra agreed within 0.46±0.27mm. For the Task Group 119 validation plans, all treatment planning goals were met and gamma passing rates were>95% (3%/3mm criteria). In 46 clinical head-and-neck cases, the cobalt-60 compensator-based IMRT plans had planning target volume (PTV) coverages similar to linac-based VMAT plans: all dosimetric values for PTV were within 1.5%. The organs at risk dose parameters were somewhat higher in cobalt-60 compensator-based IMRT plans versus linac-based VMAT plans. The mean dose differences for the spinal cord, brain, and brainstem were 4.43±1.92, 3.39±4.67, and 2.40±3.71Gy, while those for the rest of the organs were<1Gy. The average beam-on time per field was 0.42±0.10min for the 6 MV multi-leaf-collimator plans while those for the cobalt-60 compensator plans were 0.17±0.01 and 0.31±0.01min at the dose rates of 350 and 175 cGy/min. There was a good agreement between in-plane profiles from measurements and Monte Carlo simulations, which differences are 1.34±1.90% and 0.13±2.16% for two different fields. A novel compensator-based IMRT system using cobalt-60 beams was commissioned and validated in a commercial TPS. Plan quality with this system was comparable to that of linac-based plans in all test cases with shorter estimated beam-on times. This system enables reliable, high-quality plans with reduced cost and complexity and may have benefits for underserved regions of the world. This system is being integrated into the RPA, a web-based platform for auto-contouring and auto-planning.

A deep-learning-based dose verification tool utilizing fluence maps for a cobalt-60 compensator-based intensity-modulated radiation therapy system.

A novel cobalt-60 compensator-based intensity-modulated radiation therapy (IMRT) system was developed for a resource-limited environment but lacked an efficient dose verification algorithm. The aim of this study was to develop a deep-learning-based dose verification algorithm for accurate and rapid dose predictions. A deep-learning network was employed to predict the doses from static fields related to beam commissioning. Inputs were a cube-shaped phantom, a beam binary mask, and an intersecting volume of the phantom and beam binary mask, while output was a 3-dimensional (3D) dose. The same network was extended to predict patient-specific doses for head and neck cancers using two different approaches. A field-based method predicted doses for each field and combined all calculated doses into a plan, while the plan-based method combined all nine fluences into a plan to predict doses. Inputs included patient computed tomography (CT) scans, binary beam masks, and fluence maps truncated to the patient's CT in 3D. For static fields, predictions agreed well with ground truths with average deviations of less than 0.5% for percent depth doses and profiles. Even though the field-based method showed excellent prediction performance for each field, the plan-based method showed better agreement between clinical and predicted dose distributions. The distributed dose deviations for all planned target volumes and organs at risk were within 1.3Gy. The calculation speed for each case was within two seconds. A deep-learning-based dose verification tool can accurately and rapidly predict doses for a novel cobalt-60 compensator-based IMRT system.

Application of a Comprehensive Treatment Planning Test for Credentialing Intensity-Modulated Radiotherapy and RapidArc in a TrueBeam Linear Accelerator Setup.

An extended version of task group report (TG)-119 dosimetric tests was introduced and tested on the TrueBeam linear accelerator setup. Treatment plan results and quality assurance (QA) results of RapidArc (RA) and intensity-modulated radiotherapy (IMRT) were compared to understand the limitation and efficacy of the RA and IMRT system of the linear accelerator. Test structure sets were drawn on OCTAVIUS four-dimensional (4D) phantom computed tomography scan data for this study. We generated treatment plans based on the specified goal in the Eclipse™ treatment planning system using RA and IMRT in the study phantom. We used the same planning objectives for RA and IMRT techniques. Planar dose verification was performed using electronic portal imaging device and OCTAVIUS 4D phantom. The treatment log file was further analyzed using Pylinac (V2.4.0 (Open Source Code library available on Github, runs under Python programming language)) to compare the dosimetric outcome of RA and IMRT. Dose to the planning target volume (PTV) 1-5 and organ at risk (OAR) were analyzed in this study for the efficiency comparison of RA and IMRT. The primary objective was accomplished by adhering to the dose constraints associated with PTV 2 and the OAR. RA and IMRT also met the secondary objective. The tertiary goal of dose delivery to PTV 4 was met with RA but not IMRT. This study can be utilized to compare different institutions' planning and patient-specific QA (PSQA) procedures. The findings of this study were in line with the published works of the literature. A multi-institutional planning and delivery accuracy audit can be built using this structure and set of planning objectives having similar PSQA phantom. The TG-119 report incorporated test challenges that were combined in a single study set and a single plan. This reduces the complexity of performing the original TG-119 tests, whereas keeping the challenges as introduced in the TG-119 report. This study's planning and dosimetric results could be further utilized for dosimetry audit with any institute having a linear accelerator and OCTAVIUS 4D phantom for PSQA.

A Novel Compensator Device to Deliver IMRT With Cobalt-60 Units in Low- and Middle-Income Countries: A Comparative Dosimetric Analysis

Radiation therapy is a critical and cost-effective component of cancer care that is vastly underutilized in low-and middle-income countries (LMICs). Technologies developed for industrialized countries are often ill-suited for sudden adoption in LMIC settings. We propose a novel device to efficiently deliver intensity modulated radiation therapy (IMRT) with lower cost and complexity. The purpose of this study is to demonstrate through dosimetric analysis that this device can create safe, clinically acceptable treatments with similar tumor coverage and organ at risk (OAR) sparing as Linac/MLC based IMRT plans. The device employs nine patient-specific physical compensators filled with reusable tungsten pellets arranged in a ring around the patient. The compensators are manufactured at a regional center, involve no moving parts, and can be retrofitted to existing Cobalt-60 or linear accelerator treatment units. Total treatment times are short (approximately 5 minutes with Cobalt-60) because compensators are extremely efficient at using dose and the ring arrangement eliminates the need to access the treatment vault. We commissioned a Cobalt-60 compensator-based machine in the Pinnacle Treatment Planning System (v.9.8, Philips Inc., Madison, WI) and benchmarked with Monte Carlo calculations (EGSnrc). We generated five head and neck and three pelvic plans using the Cobalt-60 compensator IMRT system and compared these to clinical IMRT plans using a MLC linear accelerator (6MV), and calculated median percent differences and used t test for OAR and PTV coverage. In the five head and neck cases the Cobalt-60 compensator plans resulted in a lower mean parotid gland dose (9% lower) and a lower maximum spinal cord dose (2.5% lower) while maintaining the PTV coverage, i.e. PTV V95 within 1% and no statistical difference between the D2 (P = 0.45) and D99 (P = 0.38). In the three pelvic cases the Cobalt-60 compensator plans had similar doses to femoral heads (max dose 7% difference), bladder (D35 1% difference), and rectum (D30 3% difference). The PTV coverage was similar between the two plans, with no statistical difference between the D2 (P = 0.17) and D99 (P = 0.34). The novel physical compensator device allows for efficient delivery of Cobalt-60 based IMRT without the use of an MLC. Our results suggest that tumor coverage and OAR sparing are similar to treatments using a MLC linear accelerator. A prototype system will be developed with commercial and clinical partners at an LMIC-site and NCI support.

Three-Dimensional Dosimetric Validation of a Magnetic Resonance Guided Intensity Modulated Radiation Therapy System

To validate the dosimetric accuracy of a commercially available magnetic resonance guided intensity modulated radiation therapy (MRgIMRT) system using a hybrid approach: 3-dimensional (3D) measurements and Monte Carlo calculations. We used PRESAGE radiochromic plastic dosimeters with remote optical computed tomography readout to perform 3D high-resolution measurements, following a novel remote dosimetry protocol. We followed the intensity modulated radiation therapy commissioning recommendations of American Association of Physicists in Medicine Task Group 119, adapted to incorporate 3D data.Preliminary tests ("AP" and "3D-Bands") were delivered to 9.5-cm usable diameter cylindrical PRESAGE dosimeters to validate the treatment planning system (TPS) for nonmodulated deliveries; assess the sensitivity, uniformity, and rotational symmetry of the PRESAGE dosimeters; and test the robustness of the remote dosimetry protocol. Following this, 4 clinical MRgIMRT plans ("MultiTarget," "Prostate," "Head/Neck," and "C-Shape") were measured using 13-cm usable diameter PRESAGE dosimeters. For all plans, 3D-γ (3% or 3mm global, 10% threshold) passing rates were calculated and 3D-γ maps were examined.Point doses were measured with an IBA-CC01 ionization chamber for validation of absolute dose. Finally, by use of an in-house-developed, GPU-accelerated Monte Carlo algorithm (gPENELOPE), we independently calculated dose for all 6 Task Group 119 plans and compared against the TPS. For PRESAGE measurements, 3D-γ analysis yielded passing rates of 98.7%, 99.2%, 98.5%, 98.0%, 99.2%, and 90.7% for AP, 3D-Bands, MultiTarget, Prostate, Head/Neck, and C-Shape, respectively. Ion chamber measurements were within an average of 0.5% (±1.1%) from the TPS dose. Monte Carlo calculations demonstrated good agreement with the TPS, with a mean 3D-γ passing rate of 98.5%±1.9% using a stricter 2%/2-mm criterion. We have validated the dosimetric accuracy of a commercial MRgIMRT system using high-resolution 3D techniques. We have demonstrated for the first time that hybrid 3D remote dosimetry is a comprehensive and feasible approach to commissioning MRgIMRT. This may provide better sensitivity in error detection compared with standard 2-dimensional measurements and could be used when implementing complex new magnetic resonance guided radiation therapy technologies.

Evaluation of Intensity Modulated Radiation Therapy Delivery System using a Volumetric Phantom on the Basis of the Task Group 119 Report of American Association of Physicists in Medicine.

The current work describes the implementation of the American Association of Physicists in Medicine (AAPM)'s Task Group 119 report on a volumetric phantom (Delta4, Scandidos, Uppsala, Sweden) following the stated dose goals, to evaluate the step-and-shoot intensity modulated radiation therapy (IMRT) system. Delta4 consists of diode detectors, lying on two crossed planes, measuring the delivered dose, and providing two-dimensional dosimetric information. Seven plans of different goals and complexity were performed, with individual structure sets. TG199 structure sets and plans were transferred and implemented on the Delta4 phantom taking into account its cylindrical geometry. All plans were delivered with a 6 MV linear accelerator equipped with multileaf collimator of 1 cm thickness. Plan results for each test met the recommended dose goals. The evaluation was performed in terms of dose deviation, distance to agreement, and gamma index passing rate. In all test cases, the gamma index passing rate was measured >90%. Delta4 phantom has proven to be fast, applicable, and reliable for the step-and-shoot IMRT commissioning following TG119's recommended tests. Although AAPM's TG119 report is referring to the implementation of test plans that do not correspond to patient plans, it could be used as an evaluation tool of various IMRT systems, considering the local treatment planning system and the delivery system.

Estimation of local confidence limit for 6 MV photon beam IMRT system using AAPM TG 119 test protocol

Purpose: The aim of this study was to estimate local confidence limit for 6 MV photon beam based intensity modulated radiation therapy (IMRT) using TG119 test protocol. Methods: The American Association of Physicists in Medicine (AAPM) Task Group 119 (TG119) prescribed a protocol to evaluate overall accuracy of IMRT system rather than independent uncertainty in dose calculation, dose delivery and measurement system. Two preliminary and five clinical test cases were created based on dose prescriptions and planning objectives given by TG119 report. Verification plans were created in a planning slab phantom, 2D Matrix dosimetry system (I’MatriXX) with multicube phantom and aS-1000 electronic portal imaging device (EPID). Radiation absorbed doses to high dose points in the planning target volume (PTV) region and low dose points in avoidance structures were measured using CC13 ionization chamber having sensitive volume of 0.13 cm 3 . The measured and planned doses were normalized with respect to their prescription doses and intercompared. The gamma analysis was carried out for both I’MatriXX and EPID, adopting the acceptance criteria of 3% DD (dose difference) and 3 mm DTA (distance to agreement) with 10% threshold dose. Results: For the point dose measurements with ion chamber, the average dose difference ratio in high dose low gradient PTV region was -0.0133 ± 0.012 corresponding to a confidence limit of 0.037. The average dose difference in low dose region (avoidance structure) was -0.00004 ± 0.010 corresponding to a confidence limit of 0.021. The average percentage of points passing the gamma criteria of 3% DD and 3 mm DTA for composite planar dose distribution measured by I’MatriXX was 99.47 ± 0.43 which corresponds to a confidence limit of 1.38 (i.e. 98.62% passing). Similarly, the average percentage of points passing the gamma criteria of 3% DD and 3 mm DTA for per-field dose distribution measured by EPID was 98.00 ± 2.49 which corresponds to a confidence limit of 6.87. Conclusion: Our results were well within action level given by AAPM TG119 report through multi-institutional study providing us adequate confidence in delivering IMRT treatment.

Feasibility Study of a Custom-made Film for End-to-End Quality Assurance Test of Robotic Intensity Modulated Radiation Therapy System

This paper aims to verify the clinical feasibility of a custom-made film created by a laser cutting tool for End-to-End (E2E) quality assurance in robotic intensity modulated radiation therapy system. The custom-made film was fabricated from the Gafchromic EBT3 film with the size of $8^{{\prime}{\p...

Comparison of Planning Quality and Efficiency Between Conventional and Knowledge-Based Algorithms in Nasopharyngeal Cancer Patients Using Intensity Modulated Radiation Therapy

Intensity modulated radiation therapy (IMRT) is being widely used for its capability of achieving highly conformal dose and improving locoregional control. However, treatment plan quality and planning time are institute- and planner-dependent, and there is no standardized tool to recognize an optimal plan. A knowledge-based algorithm (KBA) can generate automated constraints to assist optimization and produce planner-independent, high-quality plans within a short time. This report compares the efficiency and quality of IMRT plans generated with and without KBA-assisted IMRT (KBA_IMRT) for nasopharyngeal carcinoma (NPC) patients. Seventy-nine NPC patients treated with radical IMRT at our institution between the years of 2013 and 2014 were used to configure the KBA_IMRT system. Ten consecutive NPC cases indicated for radical radiation therapy were then recruited to evaluate its performance. For these 10 cases, both the IMRT plan quality and planning time with and without using KBA_IMRT were compared using paired t test. Ten consecutive NPC patients treated with radical intent between October 2014 and January 2015 were recruited into the study. Their stage distributions were stage I (1), II (0), III (7), IVA (1), and IVB (1). The treatment gave 70 Gy to PTV1 (for gross tumor volume [GTV]), 63 Gy to PTV2 (for adjacent high-risk structures and bilateral retropharyngeal, levels IB, II, III, VA cervical lymph nodes), and 52.5 Gy to PTV3 (for prophylactic nodal irradiation), over 35 fractions. The results were shown in Table 1. There were no significant differences in dose coverage to GTVP (primary tumor), GTVN (involved nodes), and PTV1. The conformity index to PTV1 was comparable but the homogeneity index with KBA_IMRT was slightly worse. KBA_IMRT achieved a better sparing to brainstem (54.0 Gy vs 56.4 Gy, P=.027) and parotid glands (left: 36.2 Gy vs 43.4 Gy; right: 40.3 Gy vs 45.0 Gy, P<.05), but gave a higher dose to spinal cord (45.6 Gy vs 43.9 Gy, P=.037). The planning time was reduced almost by half using KBA_IMRT (144.5 minutes vs 261.3 minutes, P<.001). This study demonstrates that KBA_IMRT can significantly improve planning efficiency and produce quality IMRT plans for NPC patients. Further fine-tuning of KBA_IMRT parameters and other clinical applications including adaptive radiation therapy are being tested in our center.Poster Viewing Abstracts 2795; Table 1Manual planKBA_IMRTP valueGTVP (min dose)67.6 Gy66.7 Gy.071GTVN (min dose)70.2 Gy70.0 Gy.556% PTV1 < 100% dose2.7%5.4%.192Conformity index PTV11.1501.134.079Homogeneity index PTV10.1220.196.005*Brainstem (max dose)56.4 Gy54.0 Gy.027*Spinal cord (max dose)43.9 Gy45.6 Gy.037*Chiasm (max dose)54.3 Gy54.2 Gy.65Lt Parotid Gland (mean dose)43.4 Gy36.2 Gy.001*Rt Parotid Gland (mean dose)45.0 Gy40.3 Gy.039*Planning time261.3 minutes144.5 minutes<.001**P<.05 was considered statistically significant Open table in a new tab

SU‐E‐T‐661: Nonlinear Continuous Method for Non‐Negatively Constrained Inverse Problem of IMRT Planning

Purpose: The intensity‐modulated radiation therapy (IMRT) is an effective therapeutic technique for the treatment of tumors in the human body without inflicting serious damage to normal tissues. IMRT plans include an optimization strategy for minimizing an objective function of radiation beam weights. We present a novel approach for solving non‐negatively constrained inverse problems in IMRT treatment planning, based on the idea of continuous‐time dynamical methods using nonlinear differential equations. Our method can compute a feasible solution without the second derivative of an objective function, which is required for gradient‐based optimization algorithms such as conjugate gradient and steepest descent methods. Methods: We first consider the stability of an equilibrium, which corresponds to the desired planning in the IMRT system, by the use of Lyapunov's stability theorem. Then we perform some phantom experiments using phantom data simulating a clinical setup. Results: We prove theoretically that a Kullback‐Leibler (KL) divergence can be a Lyapunov function for the IMRT planning system, in consistent case. It means that the KL‐divergence measure decreases along the solution to the differential equation. Additionally, we show that the intensity of any radiation beam is not negative. Because the planning system can be created as an analog electronic circuit, its implementation in actual hardware yields dramatically faster planning as a physical phenomenon with real parallel computing, than software‐based iterative methods. Conclusion: The proposed method provides not only the reduction of a computational cost but also no production of a solution with an unphysical negative radiation beam coefficient in solving IMRT planning inverse problems. Moreover the theoretical results on convergence properties of solutions are confirmed by numerical experiments. Based on the results, we found that we can obtain a feasible solution violating the physical dose constraints minimally, even if the system is inconsistent.

Commissioning of intensity - modulated radiation therapy system with independent collimator

Objective To perform an acceptance test for the IMRT system with independent collimator. Methods An ion chamber dosimeter were used to measure the startup characteristics of the accelerator and the absolute dose at isocenter and given characteristic points for three clinical cases ( a lower nasopharyngeal carcinoma, a lung cancer and a cervical cancer). The characteristic points represented the organs at risk or the target. A Mapeheck2 was used to measure dose maps of basic test fields and the treatment fields for the clinical cases. The basic test fields were as follows: 1 ). Symmetric fields in size of 2 cm ×2 cm, 5 cm ×5 cm, 10 cm× 10 cm, 20 cm ×20 cm, 2 cm × 10 cm, 10 cm ×2 cm, 5 cm ×20 cm and 20 cm ×5 cm;2). Asymmetric fields in size of 2 cm ×2 cm (x1 =4 cm, y1 = 10 cm;x2 = -2 cm, y2 = -8cm) and 5 cm ×5 cm (x1 = -2 cm, y1 = -5 cm;x2 =7 cm, y2 = 10 cm) ;3) A 20 cm ×20 cm composite field composed of five 20 cm× 4 cm narrow bar fields side by side. Gamma Index was used to compare calculated and corresponding measured dose distributions. When the criterion was 3% dose difference or 3 mm distance-to-agreement, the pass rate was required to be more than 90%. Results The accuracy of machine output was better than 2% when machine monitor units increased to 4. Among all basic test fields and all the treatment fields of three clinical cases, the maximal absolute dose error was -3.67%, and only the composite test field and two treatment fields of the lower nasopharyngeal carcinoma case had a pass rate slightly less than 90%, which were 83.6%, 88. 3% and 89. 7% ,respectively. For the three clinical cases the treatment delivery times were 15, 14, and 27 minutes, respectively. Conclusions The overall commissioning results are acceptable, and the system can be used in clinic. Key words: Intensity-modulated radiotherapy system; Independent collimator; Commissioning

Enhanced efficiency in helical tomotherapy quality assurance using a custom-designed water-equivalent phantom

Tomotherapy is an image-guided, intensity-modulated radiation therapy system that delivers highly conformal dose distributions in a helical fashion. This system is also capable of acquiring megavoltage computed-tomography images and registering them to the planning kVCT images for accurate target localization. Quality assurance (QA) of this device is time intensive, but can be expedited by improved QA tools and procedures. A custom-designed phantom was fabricated to improve the efficiency of daily QA of our Tomotherapy machine. The phantom incorporates ionization chamber measurement points, plugs of different densities and slide-out film cartridges. The QA procedure was designed to verify in less than 30 min the vital components of the tomotherapy system: static beam quality and output, image quality, correctness of image registration and energy of the helical dose delivery. Machine output, percent depth dose and off-axis factors are simultaneously evaluated using a static 5 × 40 cm2 open field. A single phantom scan is used to evaluate image quality and registration accuracy. The phantom can also be used for patient plan-specific QA. The QA results over a period of 6 months are reported in this paper. The QA process was found to be simple, efficient and capable of simultaneously verifying several important parameters.

Poster — Wed Eve—34: Design of a Primary Collimator for Cone Beam CT Imaging

Intensity Modulated Radiation Therapy (IMRT) is becoming the standard of care for a number of cancer sites. Our research work has focused on developing Co‐60 based IMRT as an alternative for those areas in the world with limited infrastructure for supporting LINAC based systems. We have, to date, considered Co‐60 IMRT based on the tomotherapy approach because the full rotational delivery properties of tomotherapy helps to overcome the problems associated with the limited penetration of Co‐60 beams. However, with the recent introduction of arc based broad beam IMRT technology such as RapidArc (Varian Medical Systems, Palo Alto, CA), there is a potential for Co‐60 to be used in this approach. One potential advantage of broad beam versus fan beam Co‐60 IMRT delivery would be a reduction in the beam‐on time; a feature particularly important for a source that decays. Cone beam CT (CBCT) imaging is widely used for image guidance of broad‐beam IMRT and could likely play a role in a Co‐60 based broad beam IMRT system. The simplest and least complex design for such a system would consist of a single Co‐60 source for therapy and CBCT imaging. In this report we describe and analyze a new multileaf variable aperture primary collimator that permits a single Co‐60 source to be “switched” from therapy to a lower dose rate imaging mode. Monte carlo simulations of this design show that with a combination of altered aperture shape and an attenuator, an acceptable imaging dose rate without loss of spatial resolution is achieved.

BIFURCATIONS IN AN ITERATIVE OPTIMIZATION PROCESS FOR INTENSITY MODULATED RADIATION THERAPY

We investigate bifurcations in an iterative method for optimizing an objective function derived from the intensity modulated radiation therapy (IMRT). Through the bifurcation analysis of discrete dynamical systems for small IMRT plans, we obtained bifurcation diagrams showing suitable parameter values for the convergence of fixed points corresponding to global or local minima of the objectives, and we found that rich nonlinear phenomena including a chaotic state can occur. We also illustrate that a similar bifurcation phenomenon occurs in a large IMRT system. The results on bifurcation structure are useful for suggesting the parameter design of IMRT plans with normal operation.

Technical and Dosimetric Considerations in Multi-Isocenter Intensity Modulated Radiotherapy for Nasopharyngeal carcinoma with Small Multileaf Collimator

Multileaf collimator (MLC)-assisted intensity modulated radiotherapy (IMRT) has greatly improved the target coverage and avoidance of organs at risk (OAR) for the treatment of nasopharyngeal carcinoma; however, its implementation is also constrained by the features of the MLC. Nasopharyngeal carcinoma tends to have a large gross target volume (GTV) and clinical target volume (CTV) due to its biological characteristics. More than one isocenter may be needed when small MLCs ( i.e., BrainLAB M3, whose largest field is 10 × 10 cm 2) are used to treat the nasopharyngeal carcinoma. The BrainLAB IMRT system was used to evaluate the effectiveness of a multi-isocenter IMRT plan for treating nasopharyngeal cancers. Dose coverage of GTVs and CTVs were compared among IMRT plans with 1, 2 and 3 isocenters, as were dose objectives for OARs including brainstem, cord, and parotids. The dosimetric variation and the delivery time were also measured with a phantom. IMRT plans with more than 1 isocenter achieved a better dose coverage, homogeneity, and conformity on GTVs and CTVs; however, with risk of higher doses given to OARs. In most cases, one can generate satisfactory IMRT plans using the 2-isocenter IMRT planning strategy. Two-isocenter planning strategy may be a suitable compromise when more isocenters are needed.

Simultaneous Multitarget Irradiation Using Helical Tomotherapy for Advanced Hepatocellular Carcinoma With Multiple Extrahepatic Metastases

The prognosis of hepatocellular carcinoma (HCC) patients with extrahepatic metastases is extremely poor. Helical tomotherapy, an image-guided, intensity-modulated radiotherapy system, can allow for simultaneous and precise targeting of multiple cancerous lesions, while sparing normal tissues. This study evaluated the feasibility and outcome of tomotherapy for advanced HCC with metastases. A total of 42 consecutive HCC patients with metastases were treated with tomotherapy using the Hi-Art system. A total of 152 intra- and extrahepatic lesions (3.5 lesions/patient) were treated simultaneously, with a dose of 51.03 Gy (range, 30-57.61) in 10 fractions. Transarterial chemolipiodolization using epirubicin (50 mg) and cisplatin (60 mg) was repeated in patients with intrahepatic HCC (mean size, 9.0 cm) after tomotherapy. An objective response (complete response and partial response) was achieved in 45.2% of patients with intrahepatic tumors, 68.4% of patients with pulmonary lesions, 60.0% of patients with lymph node/adrenal lesions, and 66.7% of patients with soft-tissue metastases. The complete response rate for those with pulmonary and lymph node/adrenal metastases was 26.3% and 5.0%, respectively. The overall survival rate at 1 and 2 years was 50.1% and 14.9%, respectively, with a median survival of 12.3 months. The actuarial in-field tumor control rate for < or =1 year was 79.0%. No cases of Grade 4-5 acute toxicity occurred. The results of this study have shown that helical tomotherapy is safe and feasible without major toxicities for the treatment of advanced HCC and results in excellent tumor control and a potential survival benefit. This approach is expected to be a useful palliative option for selected HCC patients with metastases.

TH-A-M100F-02: Quality Assurance for IMRT and IGRT

Quality assurance (QA) for Intensity Modulated Radiation Therapy(IMRT) and Image Guided Radiation therapy(IGRT) is an essential component of the clinical treatment process. The goal of QA is to ensure that the prescribed doses are delivered to the target volume, and that the surrounding critical structures are spared within an acceptable tolerance. There are many sources of error that can arise during IMRT planning and delivery. However, it is ultimately the responsibility of the Medical Physicist to ensure that the prescribed doses agree with the delivered doses. Differences between planned doses and delivered doses in IMRT and IGRT can originate from errors in the treatment planningmodel, errors in the treatment delivery system, patient setup errors, anatomical changes in the patient, and/or physiological changes in the patient. The first step in the IMRT QA process is to evaluate the treatment‐planningmodel as part of the initial IMRT commissioning, and again after major software upgrades. The AAPM Task Group 119 (Writing group on IMRT QA) is in the process of developing site‐specific acceptance and commissioning tests for treatment planning and delivery systems. These tests evaluate the IMRT system using standardized test plans for multiple geometric and anatomical targets. Using these tests, calculated and measured doses could be compared by medical physicists using their planning and delivery system. Thus providing a standardized mechanism for testing the entire IMRT process from planning to delivery. An additional source of error in IMRT is the uncertainty in the position and shape of the target volume. The use of image‐guided radiation therapy can help minimize, but will not eliminate these errors. Changes in patient setup, normal physiological changes, tumor response, normal tissue response, and weight loss can cause changes in the position of the target volume relative to the surrounding critical structures. It is important that the Medical Physicist know how to identify these changes and determine when a new treatment plan is needed to adapt for these changes. This continuing education course will discuss commercially available dose measurement tools, phantoms, and techniques for performing acceptance testing, commissioning, and routine quality assurance for the IMRT and IGRT process. Types of commonly encountered errors will be discussed, with a special emphasis on the lessons learned and manpower requirements. Educational Objectives: 1. To understand the issues surrounding IMRT and IGRTquality assurance. 2. To identify potential sources of errors in IMRT and IGRTquality assurance. 4. To review the tools used in IMRT and IGRTquality assurance. 4. To understand the impact of target localization and patient positioning.

A survey on planar IMRT QA analysis

Quality assurance (QA) systems for intensity‐modulated radiation therapy (IMRT) have become standard tools in modern clinical medical physics departments. However, because formalized industry standards or recommendations from professional societies have yet to be defined, methods of IMRT QA analysis vary from institution to institution. Understanding where matters stand today is an important step toward improving the effectiveness of IMRT QA and developing standards.We therefore conducted an IMRT QA survey. This particular survey was limited to users of an electronic two‐dimensional diode array device, but we took care to keep the questions as general and useful as possible. The online survey polled institutions (one survey per institution) on a collection of questions about methods of IMRT QA. The topics were general to the IMRT QA analysis methods common to all IMRT systems; none of the questions was vendor‐ or product‐specific.Survey results showed that a significant proportion of responding institutions (32.8%) use the single‐gantry‐angle composite method for IMRT QA analysis instead of field‐by‐field analysis. Most institutions perform absolute dose comparisons rather than relative dose comparisons, with the 3% criterion being used most often for the percentage difference analysis, and the 3 mm criterion for distance‐to‐agreement analysis. The most prevalent standard for acceptance testing is the combined 3% and 3 mm criteria. A significant percentage of responding institutions report not yet having standard benchmarks for acceptance testing—specifically, 26.6%, 35.3%, and 67.6% had not yet established standard acceptance criteria for prostate, head and neck, and breast IMRT respectively.This survey helps in understanding how institutions perform IMRT QA analysis today. This understanding will help to move institutions toward more standardized acceptance testing. But before standards are defined, it would be useful to connect the conventional planar QA analyses to their resulting impact on the overall plan, using clinically relevant metrics (such as estimated deviations in dose–volume histograms).PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

2082: Stereotactic Radiosurgery Using the TomoTherapy HI-ART II

2082: Stereotactic Radiosurgery Using the TomoTherapy HI-ART II

SU‐FF‐J‐01: 3‐D Computed Rotational Angiography for Radiotherapy Planning for Cerebral Arteriovenous Malformations: Comparison of Tomotherapy and Non‐Coplanar Dynamic Arcs

Purpose: Three‐dimensional Computed Rotational Angiography (CRA) provides high quality images of the complex anatomical structure of cerebral arteriovenous malformations (AVMs). The objective of this research is to combine CRA, as an alternative to bi‐plane angiography for target definition, with innovative radiotherapy techniques for stereotactic radiation therapy. This preliminary treatment planning study investigates the utility of Helical Tomotherapy, as compared to multiple non‐coplanar dynamic arcs using a conventional linear accelerator (linac). Materials and Methods: A Siemens Axiom scanner was used to acquire CRA images for this treatment planning study. Non‐coplanar dynamic arc treatments were planned using Theraplan Plus. These plans were designed for delivery on a conventional linac (Varian 2100 EX) equipped with a multileaf collimator (5 mm leaf width at isocentre). The same images and regions of interest were used to generate plans for helical tomotherapy delivery (TomoTherapy Inc., Hi‐Art II). Tomotherapy is a dedicated intensity modulated radiation therapy system that delivers a narrow fan‐beam, modulated by binary multileaf collimators (6.25 mm leaf width at isocentre). Results: The CRA images have an isotropic voxel spacing of less than 0.38 mm, with a signal‐difference‐to‐noise‐ratio greater than 20:1. These high quality images facilitated the delineation of the complex target volume for treatment planning. Each of the treatment planning techniques offered specific advantages. Multiple non‐coplanar arc plans generated a lower integral dose to the surrounding healthy tissue (a 12 Gy isodose volume of 40 vs. 100 cm3 for presented sample patient). Tomotherapy inverse‐planning provided a more homogeneous dose distribution over the target volume (homogeneity index of 1.006 vs. 1.087), as well as better avoidance of critical structures (maximum brain stem dose of 8.2 vs. 13.2 Gy). Conclusion: Tomotherapy presents an alternative to forward planning with non‐coplanar arcs. Moreover, the megavoltage CT imaging capabilities of Tomotherapy could provide frameless, stereotactic localization for AVM radiotherapy.