Phase 2 study of preoperative image-guided intensity-modulated radiation therapy to reduce wound and combined modality morbidities in lower extremity soft tissue sarcoma.

BACKGROUND: This study sought to determine if preoperative image-guided intensity-modulated radiotherapy (IG-IMRT) can reduce morbidity, including wound complications, by minimizing dose to uninvolved tissues in adults with lower extremity soft tissue sarcoma. METHODS: The primary endpoint was the development of an acute wound complication (WC). IG-IMRT was used to conform volumes to avoid normal tissues (skin flaps for wound closure, bone, or other uninvolved soft tissues). From July 2005 to June 2009, 70 adults were enrolled; 59 were evaluable for the primary endpoint. Median tumor size was 9.5 cm; 55 tumors (93%) were high-grade and 58 (98%) were deep to fascia. RESULTS: Eighteen (30.5%) patients developed WCs. This was not statistically significantly different from the result of the National Cancer Institute of Canada SR2 trial (P = .2); however, primary closure technique was possible more often (55 of 59 patients [93.2%] versus 50 of 70 patients [71.4%]; P = .002), and secondary operations for WCs were somewhat reduced (6 of 18 patients [33%] versus 13 of 30 patients [43%]; P = .55). Moderate edema, skin, subcutaneous, and joint toxicity was present in 6 (11.1%), 1 (1.9%), 5 (9.3%), and 3 (5.6%) patients, respectively, but there were no bone fractures. Four local recurrences (6.8%, none near the flaps) occurred with median follow-up of 49 months. CONCLUSIONS: The 30.5% incidence of WCs was numerically lower than the 43% risk derived from the National Cancer Institute of Canada SR2 trial, but did not reach statistical significance. Preoperative IG-IMRT significantly diminished the need for tissue transfer. RT chronic morbidities and the need for subsequent secondary operations for WCs were lowered, although not significantly, whereas good limb function was maintained.

Medical physics staffing for radiation oncology: a decade of experience in Ontario, Canada.

The January 2010 articles in The New York Times generated intense focus on patient safety in radiation treatment, with physics staffing identified frequently as a critical factor for consistent quality assurance. The purpose of this work is to review our experience with medical physics staffing, and to propose a transparent and flexible staffing algorithm for general use. Guided by documented times required per routine procedure, we have developed a robust algorithm to estimate physics staffing needs according to center-specific workload for medical physicists and associated support staff, in a manner we believe is adaptable to an evolving radiotherapy practice. We calculate requirements for each staffing type based on caseload, equipment inventory, quality assurance, educational programs, and administration. Average per-case staffing ratios were also determined for larger-scale human resource planning and used to model staffing needs for Ontario, Canada over the next 10 years. The workload specific algorithm was tested through a survey of Canadian cancer centers. For center-specific human resource planning, we propose a grid of coefficients addressing specific workload factors for each staff group. For larger scale forecasting of human resource requirements, values of 260, 700, 300, 600, 1200, and 2000 treated cases per full-time equivalent (FTE) were determined for medical physicists, physics assistants, dosimetrists, electronics technologists, mechanical technologists, and information technology specialists, respectively.

Automated beam model optimization.

PURPOSE: The beam model in a three dimensional treatment planning system (TPS) defines virtually the mechanical and dosimetric characteristics of a treatment unit. The manual optimization of a beam model during commissioning can be a time consuming task due to its iterative nature. Furthermore, the quality of the beam model commissioning depends on the user's ability to manage multiple parameters and assess their impact on the agreement between measured and calculated dose. The objective of this work is to develop and validate the performance of an automated beam model optimization system (ABMOS) based on intensity modulated radiotherapy (IMRT) beam measurements to improve beam model accuracy while streamlining the commissioning process. METHODS: The ABMOS was developed to adjust selected TPS beam model parameters iteratively to maximize the agreement between measured and calculated 2D dose maps obtained for an IMRT beam pattern. A 2D diode array with high spatial resolution detectors was used to sample the entire IMRT beam pattern in a single dose measurement. The use of an IMRT beam pattern with large number of monitor units was selected to highlight the difference between planned and delivered dose and improve the signal to noise ratio in the low dose regions. ABMOS was applied to the optimization of a beam model for an Elekta Synergy S treatment unit. The optimized beam model was validated for two anatomical sites (25 paraspinal and 25 prostate cases) using two independent patient-specific IMRT quality control (QC) methods based on ion chamber and 2D diode array measurements, respectively. The conventional approach of comparing calculated and measured beam profiles and percent-depth dose curves was also used to assess improvement in beam model after ABMOS optimization. Elements of statistical process control were applied to the process of patient-specific QC performed with the ion chamber and the 2D array to complement the model comparison. RESULTS: After beam model optimization with ABMOS, improvement in planned to delivered dose agreement was demonstrated with both patient-specific IMRT QC methods and the calculated to measured profile comparison. In terms of ion chamber measurements, the largest improvement was observed for the paraspinal cases with the mean measured to calculated dose difference at the low dose points decreasing from - 13.8% to 2.0% with the optimized beam model. The 2D diode array patient-specific QC also demonstrated clearly the improvement in beam model for both paraspinal and prostate cases with, on average, more than 96% of the diodes satisfying tolerances of 3% of dose difference or 2 mm of distance to agreement after ABMOS optimization. The capability index (C(pk)) for both patient-specific QC methods also increased with the optimized beam model. CONCLUSIONS: In this work, ABMOS was developed to use 2D diode array measurements of an IMRT beam pattern for the automated multivariable optimization of a TPS beam model. Based on the observed improvements in patient-specific QC results for 25 paraspinal and 25 prostate plans, optimization of the remaining clinical beam models using ABMOS is now ongoing in the institution.

A new metric for assessing IMRT modulation complexity and plan deliverability.

PURPOSE: To evaluate the utility of a new complexity metric, the modulation complexity score (MCS), in the treatment planning and quality assurance processes and to evaluate the relationship of the metric with deliverability. METHODS: A multisite (breast, rectum, prostate, prostate bed, lung, and head and neck) and site-specific (lung) dosimetric evaluation has been completed. The MCS was calculated for each beam and the overall treatment plan. A 2D diode array (MapCHECK, Sun Nuclear, Melbourne, FL) was used to acquire measurements for each beam. The measured and planned dose (PINNACLE3, Phillips, Madison, WI) was evaluated using different percent differences and distance to agreement (DTA) criteria (3%/ 3 mm and 2%/ 1 mm) and the relationship between the dosimetric results and complexity (as measured by the MCS or simple beam parameters) assessed. RESULTS: For the multisite analysis (243 plans total), the mean MCS scores for each treatment site were breast (0.92), rectum (0.858), prostate (0.837), prostate bed (0.652), lung (0.631), and head and neck (0.356). The MCS allowed for compilation of treatment site-specific statistics, which is useful for comparing different techniques, as well as for comparison of individual treatment plans with the typical complexity levels. For the six plans selected for dosimetry, the average diode percent pass rate was 98.7% (minimum of 96%) for 3%/3 mm evaluation criteria. The average difference in absolute dose measurement between the planned and measured dose was 1.7 cGy. The detailed lung analysis also showed excellent agreement between the measured and planned dose, as all beams had a diode percentage pass rate for 3%/3 mm criteria of greater than 95.9%, with an average pass rate of 99.0%. The average absolute maximum dose difference for the lung plans was 0.7 cGy. There was no direct correlation between the MCS and simple beam parameters which could be used as a surrogate for complexity level (i.e., number of segments or MU). An evaluation criterion of 2%/ 1 mm reliably allowed for the identification of beams that are dosimetrically robust. In this study we defined a robust beam or plan as one that maintained a diode percentage pass rate greater than 90% at 2%/ 1 mm, indicating delivery that was deemed accurate when compared to the planned dose, even under stricter evaluation criterion. MCS and MU threshold criteria were determined by defining a required specificity of 1.0. A MCS threshold of 0.8 allowed for identification of robust deliverability with a sensitivity of 0.36. In contrast, MU had a lower sensitivity of 0.23 for a threshold of 50 MU. CONCLUSIONS: The MCS allows for a quantitative assessment of plan complexity, on a fixed scale, that can be applied to all treatment sites and can provide more information related to dose delivery than simple beam parameters. This could prove useful throughout the entire treatment planning and QA process.

Reconstruction of 3D lung models from 2D planning data sets for Hodgkin's lymphoma patients using combined deformable image registration and navigator channels.

PURPOSE: Late complications (cardiac toxicities, secondary lung, and breast cancer) remain a significant concern in the radiation treatment of Hodgkin's lymphoma (HL). To address this issue, predictive dose-risk models could potentially be used to estimate radiotherapy-related late toxicities. This study investigates the use of deformable image registration (DIR) and navigator channels (NCs) to reconstruct 3D lung models from 2D radiographic planning images, in order to retrospectively calculate the treatment dose exposure to HL patients treated with 2D planning, which are now experiencing late effects. METHODS: Three-dimensional planning CT images of 52 current HL patients were acquired. 12 image sets were used to construct a male and a female population lung model. 23 "Reference" images were used to generate lung deformation adaptation templates, constructed by deforming the population model into each patient-specific lung geometry using a biomechanical-based DIR algorithm, MORFEUS. 17 "Test" patients were used to test the accuracy of the reconstruction technique by adapting existing templates using 2D digitally reconstructed radiographs. The adaptation process included three steps. First, a Reference patient was matched to a Test patient by thorax measurements. Second, four NCs (small regions of interest) were placed on the lung boundary to calculate 1D differences in lung edges. Third, the Reference lung model was adapted to the Test patient's lung using the 1D edge differences. The Reference-adapted Test model was then compared to the 3D lung contours of the actual Test patient by computing their percentage volume overlap (POL) and Dice coefficient. RESULTS: The average percentage overlapping volumes and Dice coefficient expressed as a percentage between the adapted and actual Test models were found to be 89.2 +/- 3.9% (Right lung = 88.8%; Left lung = 89.6%) and 89.3 +/- 2.7% (Right = 88.5%; Left = 90.2%), respectively. Paired T-tests demonstrated that the volumetric reconstruction method made a statistically significant improvement to the population lung model shape (p < 0.05). The error in the results were also comparable to the volume overlap difference observed between inhale and exhale lung volumes during free-breathing respiratory motion (POL: p = 0.43; Dice: p = 0.20), which implies that the accuracies of the reconstruction method are within breathing constraints and would not be the confining factor in estimating normal tissue dose exposure. CONCLUSIONS: The result findings show that the DIR-NC technique can achieve a high degree of reconstruction accuracy, and could be useful in approximating 3D dosimetric representations of historical 2D treatment. In turn, this could provide a better understanding of the biophysical relationship between dose-volume exposure and late term radiotherapy effects.

Report of AAPM Therapy Physics Committee Task Group 74: in-air output ratio, Sc, for megavoltage photon beams.

The concept of in-air output ratio (Sc) was introduced to characterize how the incident photon fluence per monitor unit (or unit time for a Co-60 unit) varies with collimator settings. However, there has been much confusion regarding the measurement technique to be used that has prevented the accurate and consistent determination of Sc. The main thrust of the report is to devise a theoretical and measurement formalism that ensures interinstitutional consistency of Sc. The in-air output ratio, Sc, is defined as the ratio of primary collision water kerma in free-space, Kp, per monitor unit between an arbitrary collimator setting and the reference collimator setting at the same location. Miniphantoms with sufficient lateral and longitudinal thicknesses to eliminate electron contamination and maintain transient electron equilibrium are recommended for the measurement of Sc. The authors present a correction formalism to extrapolate the correct Sc from the measured values using high-Z miniphantom. Miniphantoms made of high-Z material are used to measure Sc for small fields (e.g., IMRT or stereotactic radiosurgery). This report presents a review of the components of Sc, including headscatter, source-obscuring, and monitor-backscattering effects. A review of calculation methods (Monte Carlo and empirical) used to calculate Sc for arbitrary shaped fields is presented. The authors discussed the use of Sc in photon dose calculation algorithms, in particular, monitor unit calculation. Finally, a summary of Sc data (from RPC and other institutions) is included for QA purposes.

Quality assurance needs for modern image-based radiotherapy: recommendations from 2007 interorganizational symposium on "quality assurance of radiation therapy: challenges of advanced technology".

This report summarizes the consensus findings and recommendations emerging from 2007 Symposium, "Quality Assurance of Radiation Therapy: Challenges of Advanced Technology." The Symposium was held in Dallas February 20-22, 2007. The 3-day program, which was sponsored jointly by the American Society for Therapeutic Radiology and Oncology (ASTRO), American Association of Physicists in Medicine (AAPM), and National Cancer Institute (NCI), included >40 invited speakers from the radiation oncology and industrial engineering/human factor communities and attracted nearly 350 attendees, mostly medical physicists. A summary of the major findings follows. The current process of developing consensus recommendations for prescriptive quality assurance (QA) tests remains valid for many of the devices and software systems used in modern radiotherapy (RT), although for some technologies, QA guidance is incomplete or out of date. The current approach to QA does not seem feasible for image-based planning, image-guided therapies, or computer-controlled therapy. In these areas, additional scientific investigation and innovative approaches are needed to manage risk and mitigate errors, including a better balance between mitigating the risk of catastrophic error and maintaining treatment quality, complimenting the current device-centered QA perspective by a more process-centered approach, and broadening community participation in QA guidance formulation and implementation. Industrial engineers and human factor experts can make significant contributions toward advancing a broader, more process-oriented, risk-based formulation of RT QA. Healthcare administrators need to appropriately increase personnel and ancillary equipment resources, as well as capital resources, when new advanced technology RT modalities are implemented. The pace of formalizing clinical physics training must rapidly increase to provide an adequately trained physics workforce for advanced technology RT. The specific recommendations of the Symposium included the following. First, the AAPM, in cooperation with other advisory bodies, should undertake a systematic program to update conventional QA guidance using available risk-assessment methods. Second, the AAPM advanced technology RT Task Groups should better balance clinical process vs. device operation aspects--encouraging greater levels of multidisciplinary participation such as industrial engineering consultants and use-risk assessment and process-flow techniques. Third, ASTRO should form a multidisciplinary subcommittee, consisting of physician, physicist, vendor, and industrial engineering representatives, to better address modern RT quality management and QA needs. Finally, government and private entities committed to improved healthcare quality and safety should support research directed toward addressing QA problems in image-guided therapies.

Statistical process control for IMRT dosimetric verification.

Patient-specific measurements are typically used to validate the dosimetry of intensity-modulated radiotherapy (IMRT). To evaluate the dosimetric performance over time of our IMRT process, we have used statistical process control (SPC) concepts to analyze the measurements from 330 head and neck (H&N) treatment plans. The objectives of the present work are to: (i) Review the dosimetric measurements of a large series of consecutive head and neck treatment plans to better understand appropriate dosimetric tolerances; (ii) analyze the results with SPC to develop action levels for measured discrepancies; (iii) develop estimates for the number of measurements that are required to describe IMRT dosimetry in the clinical setting; and (iv) evaluate with SPC a new beam model in our planning system. H&N IMRT cases were planned with the PINNACLE treatment planning system versions 6.2b or 7.6c (Philips Medical Systems, Madison, WI) and treated on Varian (Palo Alto, CA) or Elekta (Crawley, UK) linacs. As part of regular quality assurance, plans were recalculated on a 20-cm-diam cylindrical phantom, and ion chamber measurements were made in high-dose volumes (the PTV with highest dose) and in low-dose volumes (spinal cord organ-at-risk, OR). Differences between the planned and measured doses were recorded as a percentage of the planned dose. Differences were stable over time. Measurements with PINNACLE3 6.2b and Varian linacs showed a mean difference of 0.6% for PTVs (n=149, range, -4.3% to 6.6%), while OR measurements showed a larger systematic discrepancy (mean 4.5%, range -4.5% to 16.3%) that was due to well-known limitations of the MLC model in the earlier version of the planning system. Measurements with PINNACLE3 7.6c and Varian linacs demonstrated a mean difference of 0.2% for PTVs (n=160, range, -3.0%, to 5.0%) and -1.0% for ORs (range -5.8% to 4.4%). The capability index (ratio of specification range to range of the data) was 1.3 for the PTV data, indicating that almost all measurements were within +/-5%. We have used SPC tools to evaluate a new beam model in our planning system to produce a systematic difference of -0.6% for PTVs and 0.4% for ORs, although the number of measurements is smaller (n=25). Analysis of this large series of H&N IMRT measurements demonstrated that our IMRT dosimetry was stable over time and within accepted tolerances. These data provide useful information for assessing alterations to beam models in the planning system. IMRT is enhanced by the addition of statistical process control to traditional quality control procedures.

Inverse optimization of objective function weights for treatment planning using clinical dose-volume histograms.

We developed and evaluated a novel inverse optimization (IO) model to estimate objective function weights from clinical dose-volume histograms (DVHs). These weights were used to solve a treatment planning problem to generate 'inverse plans' that had similar DVHs to the original clinical DVHs. Our methodology was applied to 217 clinical head and neck cancer treatment plans that were previously delivered at Princess Margaret Cancer Centre in Canada. Inverse plan DVHs were compared to the clinical DVHs using objective function values, dose-volume differences, and frequency of clinical planning criteria satisfaction. Median differences between the clinical and inverse DVHs were within 1.1 Gy. For most structures, the difference in clinical planning criteria satisfaction between the clinical and inverse plans was at most 1.4%. For structures where the two plans differed by more than 1.4% in planning criteria satisfaction, the difference in average criterion violation was less than 0.5 Gy. Overall, the inverse plans were very similar to the clinical plans. Compared with a previous inverse optimization method from the literature, our new inverse plans typically satisfied the same or more clinical criteria, and had consistently lower fluence heterogeneity. Overall, this paper demonstrates that DVHs, which are essentially summary statistics, provide sufficient information to estimate objective function weights that result in high quality treatment plans. However, as with any summary statistic that compresses three-dimensional dose information, care must be taken to avoid generating plans with undesirable features such as hotspots; our computational results suggest that such undesirable spatial features were uncommon. Our IO-based approach can be integrated into the current clinical planning paradigm to better initialize the planning process and improve planning efficiency. It could also be embedded in a knowledge-based planning or adaptive radiation therapy framework to automatically generate a new plan given a predicted or updated target DVH, respectively.

A small number of objective function weight vectors is sufficient for automated treatment planning in prostate cancer.

Current practice for treatment planning optimization can be both inefficient and time consuming. In this paper, we propose an automated planning methodology that aims to combine both explorative and prescriptive approaches for improving the efficiency and the quality of the treatment planning process. Given a treatment plan, our explorative approach explores trade-offs between different objectives and finds an acceptable region for objective function weights via inverse optimization. Intuitively, the shape and size of these regions describe how 'sensitive' a patient is to perturbations in objective function weights. We then develop an integer programming-based prescriptive approach that exploits the information encoded by these regions to find a set of five representative objective function weight vectors such that for each patient there exists at least one representative weight vector that can produce a high quality treatment plan. Using 315 patients from Princess Margaret Cancer Centre, we show that the produced treatment plans are comparable and, for [Formula: see text] of cases, improve upon the inversely optimized plans that are generated from the historical clinical treatment plans.

Online planning and delivery technique for radiotherapy of spinal metastases using cone-beam CT: image quality and system performance.

PURPOSE: To assess the feasibility of an online strategy for palliative radiotherapy (RT) of spinal bone metastasis, which integrates imaging, planning, and treatment delivery in a single step at the treatment unit. The technical challenges of this approach include cone-beam CT (CBCT) image quality for target definition, online planning, and efficient process integration. METHODS AND MATERIALS: An integrated imaging, planning, and delivery system was constructed and tested with phantoms. The magnitude of CBCT image artifacts following the use of an antiscatter grid and a nonlinear scatter correction was quantified using phantom data and images of patients receiving conventional palliative RT of the spine. The efficacy of online planning was then assessed using corrected CBCT images. Testing of the complete process was performed on phantoms with assessment of timing and dosimetric accuracy. RESULTS: The use of image corrections reduced the cupping artifact from 30% to 4.5% on CBCT images of a body phantom and improved the accuracy of CBCT numbers (water: +/- 20 Hounsfield unit [HU], and lung and bone: to within +/- 130 HU). Bony anatomy was clearly visible and was deemed sufficient for target definition. The mean total time (n = 5) for application of the online approach was 23.1 min. Image-guided dose placement was assessed using radiochromic film measurements with good agreement (within 5% of dose difference and 2 mm of distance to agreement). CONCLUSIONS: The technical feasibility of CBCT-guided online planning and delivery for palliative single treatment has been demonstrated. The process was performed in one session equivalent to an initial treatment slot (<30 min) with dosimetric accuracy satisfying accepted RT standards.

Cone-beam computed tomography for on-line image guidance of lung stereotactic radiotherapy: localization, verification, and intrafraction tumor position.

PURPOSE: Cone-beam computed tomography (CBCT) in-room imaging allows accurate inter- and intrafraction target localization in stereotactic body radiotherapy of lung tumors. METHODS AND MATERIALS: Image-guided stereotactic body radiotherapy was performed in 28 patients (89 fractions) with medically inoperable Stage T1-T2 non-small-cell lung carcinoma. The targets from the CBCT and planning data set (helical or four-dimensional CT) were matched on-line to determine the couch shift required for target localization. Matching based on the bony anatomy was also performed retrospectively. Verification of target localization was done using either megavoltage portal imaging or CBCT imaging; repeat CBCT imaging was used to assess the intrafraction tumor position. RESULTS: The mean three-dimensional tumor motion for patients with upper lesions (n = 21) and mid-lobe or lower lobe lesions (n = 7) was 4.2 and 6.7 mm, respectively. The mean difference between the target and bony anatomy matching using CBCT was 6.8 mm (SD, 4.9, maximum, 30.3); the difference exceeded 13.9 mm in 10% of the treatment fractions. The mean residual error after target localization using CBCT imaging was 1.9 mm (SD, 1.1, maximum, 4.4). The mean intrafraction tumor deviation was significantly greater (5.3 mm vs. 2.2 mm) when the interval between localization and repeat CBCT imaging (n = 8) exceeded 34 min. CONCLUSION: In-room volumetric imaging, such as CBCT, is essential for target localization accuracy in lung stereotactic body radiotherapy. Imaging that relies on bony anatomy as a surrogate of the target may provide erroneous results in both localization and verification.

Radiation planning comparison for superficial tissue avoidance in radiotherapy for soft tissue sarcoma of the lower extremity.

PURPOSE: Three types of preoperative radiotherapy (RT) plans for extremity soft tissue sarcoma were compared to determine the amount of dose reduction possible to the planned surgical skin flaps required for tumor resection and wound closure, without compromising target coverage. METHODS AND MATERIALS: Twenty-four untreated patients with large, deep, lower extremity STS treated with preoperative RT and limb salvage surgery had their original conventional treatment plans re-created. The same clinical target volume was used for all three plans. The future surgical skin flaps were created virtually through contouring by the treating surgeon and regarded as an organ at risk. The original, conformal, and intensity-modulated RT (IMRT) plans were created to deliver 50 Gy in 25 fractions to the clinical target volume. Clinical target volume and organ-at-risk dose-volume histograms were calculated and the plans compared for conformality, target coverage, and dose sparing. RESULTS: The mean dose to the planned skin flaps was 42.62 Gy (range, 30.24-48.65 Gy) for the original plans compared with 40.12 Gy (range, 24.24-47.26 Gy) for the conformal plans and 26.71 Gy (range, 22.31-31.91 Gy) for the IMRT plans (p = 0.0008). An average of 86.4% (range, 53.2-97.4%) of the planned skin flaps received >or=30 Gy in the original plans compared with 83.4% (range, 36.2-96.2%) in the conformal plans and only 34.0% (range, 22.5-53.3%) in the IMRT plans (p = 0.0001). IMRT improved target conformality compared with the original and conformal plans (1.27, 2.34, and 1.76, respectively, p = 0.0001). CONCLUSION: In a retrospective review, preoperative IMRT substantially lowered the dose to the future surgical skin flaps, sparing a greater percentage of this structure's volume without compromising target (tumor) coverage.

Cone beam computed tomography guidance for setup of patients receiving accelerated partial breast irradiation.

PURPOSE: To evaluate the role of cone-beam CT (CBCT) guidance for setup error reduction and soft tissue visualization in accelerated partial breast irradiation (APBI). METHODS AND MATERIALS: Twenty patients were recruited for the delivery of radiotherapy to the postoperative cavity (3850 cGy in 10 fractions over 5 days) using an APBI technique. Cone-beam CT data sets were acquired after an initial skin-mark setup and before treatment delivery. These were registered online using the ipsilateral lung and external contours. Corrections were executed for translations exceeding 3 mm. The random and systematic errors associated with setup using skin-marks and setup using CBCT guidance were calculated and compared. RESULTS: A total of 315 CBCT data sets were analyzed. The systematic errors for the skin-mark setup were 2.7, 1.7, and 2.4 mm in the right-left, anterior-posterior, and superior-inferior directions, respectively. These were reduced to 0.8, 0.7, and 0.8 mm when CBCT guidance was used. The random errors were reduced from 2.4, 2.2, and 2.9 mm for skin-marks to 1.5, 1.5, and 1.6 mm for CBCT guidance in the right-left, anterior-posterior, and superior-inferior directions, respectively. CONCLUSION: A skin-mark setup for APBI patients is sufficient for current planning target volume margins for the population of patients studied here. Online CBCT guidance minimizes the occurrence of large random deviations, which may have a greater impact for the accelerated fractionation schedule used in APBI. It is also likely to permit a reduction in planning target volume margins and provide skin-line visualization and dosimetric evaluation of cardiac and lung volumes.

Comparison of localization performance with implanted fiducial markers and cone-beam computed tomography for on-line image-guided radiotherapy of the prostate.

PURPOSE: The aim of this work was to assess the accuracy of kilovoltage (kV) cone-beam computed tomography (CBCT)-based setup corrections as compared with orthogonal megavoltage (MV) portal image-based corrections for patients undergoing external-beam radiotherapy of the prostate. METHODS AND MATERIALS: Daily cone-beam CT volumetric images were acquired after setup for patients with three intraprostatic fiducial markers. The estimated couch shifts were compared retrospectively to patient adjustments based on two orthogonal MV portal images (the current clinical standard of care in our institution). The CBCT soft-tissue based shifts were also estimated by digitally removing the gold markers in each projection to suppress the artifacts in the reconstructed volumes. A total of 256 volumetric images for 15 patients were analyzed. RESULTS: The Pearson coefficient of correlation for the patient position shifts using fiducial markers in MV vs. kV was (R2 = 0.95, 0.84, 0.81) in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The correlation using soft-tissue matching was as follows: R2 = 0.90, 0.49, 0.51 in the LR, AP and SI directions. A Bland-Altman analysis showed no significant trends in the data. The percentage of shifts within a +/-3-mm tolerance (the clinical action level) was 99.7%, 95.5%, 91.3% for fiducial marker matching and 99.5%, 70.3%, 78.4% for soft-tissue matching. CONCLUSIONS: Cone-beam CT is an accurate and precise tool for image guidance. It provides an equivalent means of patient setup correction for prostate patients with implanted gold fiducial markers. Use of the additional information provided by the visualization of soft-tissue structures is an active area of research.

Integral test phantom for dosimetric quality assurance of image guided and intensity modulated stereotactic radiotherapy.

The objective of this work is to develop a dosimetric phantom quality assurance (QA) of linear accelerators capable of cone-beam CT (CBCT) image guided and intensity-modulated radiotherapy (IG-IMRT). This phantom is to be used in an integral test to quantify in real-time both the performance of the image guidance and the dose delivery systems in terms of dose localization. The prototype IG-IMRT QA phantom consisted of a cylindrical imaging phantom (CatPhan) combined with an array of 11 radiation diodes mounted on a 10 cm diameter disk, oriented perpendicular to the phantom axis. Basic diode response characterization was performed for 6 and 18 MV photons. The diode response was compared to planning system calculations in the open and penumbrae regions of simple and complex beam arrangements. The clinical use of the QA phantom was illustrated in an integral test of an IG-IMRT treatment designed for a clinical spinal radiosurgery case. The sensitivity of the phantom to multileaf collimator (MLC) calibration and setup errors in the clinical setting was assessed by introducing errors in the IMRT plan or by displacing the phantom. The diodes offered good response linearity and long-term reproducibility for both 6 and 18 MV. Axial dosimetry of coplanar beams (in a plane containing the beam axes) was made possible with the nearly isoplanatic response of the diodes over 360 degrees of gantry (usually within +/-1%). For single beam geometry, errors in phantom placement as small as 0.5 mm could be accurately detected (in gradient > or = 1% /mm). In clinical setting, MLC systematic errors of 1 mm on a single MLC bank introduced in the IMRT plan were easily detectable with the QA phantom. The QA phantom demonstrated also sufficient sensitivity for the detection of setup errors as small as 1 mm for the IMRT delivery. These results demonstrated that the prototype can accurately and efficiently verify the entire IG-IMRT process. This tool, in conjunction with image guidance capabilities has the potential to streamline this QA process and improve the level of performance of image guided and intensity modulated radiotherapy.

Image guidance: treatment target localization systems.

Highly conformal radiation therapy tailors treatment to match the target shape and position, minimizing normal tissue damage to a greater extent than previously possible. Technological advances such as intensity-modulated radiation therapy, introduced a decade ago, have yielded significant gains in tumor control and reduced toxicity. Continuing advances have focused on the characterization and control of patient movement, organ motion, and anatomical deformation, which all introduce geometric uncertainty. These sources of uncertainty limit the effectiveness of high-precision treatment. Target localization, performed using appropriate technologies and frequency, is a critical component of treatment quality assurance. Until recently, the target position with respect to the beams has been inferred from surface marks on the patient's skin or through an immobilization device, and verified using megavoltage radiographs of the treatment portal. Advances in imaging technologies have made it possible to image soft tissue volumes in the treatment setting. Real-time tracking is also possible using a variety of technologies, including fluoroscopic imaging and radiopaque markers implanted in or near the tumor. The capacity to acquire volumetric soft tissue images in the treatment setting can also be used to assess anatomical changes over a course of treatment. Enhancing localization practices reduces treatment errors, and gives the capacity to monitor anatomical changes and reduce uncertainties that could influence clinical outcomes. This review presents the technologies available for target localization, and discusses some of the considerations that should be addressed in the implementation of many new clinical processes in radiation oncology.

Feasibility of a novel deformable image registration technique to facilitate classification, targeting, and monitoring of tumor and normal tissue.

PURPOSE: To investigate the feasibility of a biomechanical-based deformable image registration technique for the integration of multimodality imaging, image guided treatment, and response monitoring. METHODS AND MATERIALS: A multiorgan deformable image registration technique based on finite element modeling (FEM) and surface projection alignment of selected regions of interest with biomechanical material and interface models has been developed. FEM also provides an inherent method for direct tracking specified regions through treatment and follow-up. RESULTS: The technique was demonstrated on 5 liver cancer patients. Differences of up to 1 cm of motion were seen between the diaphragm and the tumor center of mass after deformable image registration of exhale and inhale CT scans. Spatial differences of 5 mm or more were observed for up to 86% of the surface of the defined tumor after deformable image registration of the computed tomography (CT) and magnetic resonance images. Up to 6.8 mm of motion was observed for the tumor after deformable image registration of the CT and cone-beam CT scan after rigid registration of the liver. Deformable registration of the CT to the follow-up CT allowed a more accurate assessment of tumor response. CONCLUSIONS: This biomechanical-based deformable image registration technique incorporates classification, targeting, and monitoring of tumor and normal tissue using one methodology.

The stability of mechanical calibration for a kV cone beam computed tomography system integrated with linear accelerator.

The geometric accuracy and precision of an image-guided treatment system were assessed. Image guidance is performed using an x-ray volume imaging (XVI) system integrated with a linear accelerator and treatment planning system. Using an amorphous silicon detector and x-ray tube, volumetric computed tomography images are reconstructed from kilovoltage radiographs by filtered backprojection. Image fusion and assessment of geometric targeting are supported by the treatment planning system. To assess the limiting accuracy and precision of image-guided treatment delivery, a rigid spherical target embedded in an opaque phantom was subjected to 21 treatment sessions over a three-month period. For each session, a volumetric data set was acquired and loaded directly into an active treatment planning session. Image fusion was used to ascertain the couch correction required to position the target at the prescribed iso-center. Corrections were validated independently using megavoltage electronic portal imaging to record the target position with respect to symmetric treatment beam apertures. An initial calibration cycle followed by repeated image-guidance sessions demonstrated the XVI system could be used to relocate an unambiguous object to within less than 1 mm of the prescribed location. Treatment could then proceed within the mechanical accuracy and precision of the delivery system. The calibration procedure maintained excellent spatial resolution and delivery precision over the duration of this study, while the linear accelerator was in routine clinical use. Based on these results, the mechanical accuracy and precision of the system are ideal for supporting high-precision localization and treatment of soft-tissue targets.