Image-Guided Radiosurgery for the Spine and Pancreas

Multi-parametric MRI for radiotherapy simulation

Magnetic resonance imaging (MRI) has become an important imaging modality in the field of radiotherapy (RT) in the past decade, especially with the development of various novel MRI and image-guidance techniques. In this review article, we will describe recent developments and discuss the applications of multi-parametric MRI (mpMRI) in RT simulation. In this review, mpMRI refers to a general and loose definition which includes various multi-contrast MRI techniques. Specifically, we will focus on the implementation, challenges, and future directions of mpMRI techniques for RT simulation.

A level-wise spine registration framework to account for large pose changes

PURPOSES

Accurate and efficient spine registration is crucial to success of spine image guidance. However, changes in spine pose cause intervertebral motion that can lead to significant registration errors. In this study, we develop a geometrical rectification technique via nonlinear principal component analysis (NLPCA) to achieve level-wise vertebral registration that is robust to large changes in spine pose.

METHODS

We used explanted porcine spines and live pigs to develop and test our technique. Each sample was scanned with preoperative CT (pCT) in an initial pose and rescanned with intraoperative stereovision (iSV) in a different surgical posture. Patient registration rectified arbitrary spinal postures in pCT and iSV into a common, neutral pose through a parameterized moving-frame approach. Topologically encoded depth projection 2D images were then generated to establish invertible point-to-pixel correspondences. Level-wise point correspondences between pCT and iSV vertebral surfaces were generated via 2D image registration. Finally, closed-form vertebral level-wise rigid registration was obtained by directly mapping 3D surface point pairs. Implanted mini-screws were used as fiducial markers to measure registration accuracy.

RESULTS

In seven explanted porcine spines and two live animal surgeries (maximum in-spine pose change of 87.5 mm and 32.7 degrees averaged from all spines), average target registration errors (TRE) of 1.70  ±  0.15 mm and 1.85 ± 0.16 mm were achieved, respectively. The automated spine rectification took 3-5 min, followed by an additional 30 secs for depth image projection and level-wise registration.

CONCLUSIONS

Accuracy and efficiency of the proposed level-wise spine registration support its application in human open spine surgeries. The registration framework, itself, may also be applicable to other intraoperative imaging modalities such as ultrasound and MRI, which may expand utility of the approach in spine registration in general.

Real-time intrafraction motion monitoring in external beam radiotherapy

Radiotherapy (RT) aims to deliver a spatially conformal dose of radiation to tumours while maximizing the dose sparing to healthy tissues. However, the internal patient anatomy is constantly moving due to respiratory, cardiac, gastrointestinal and urinary activity. The long term goal of the RT community to 'see what we treat, as we treat' and to act on this information instantaneously has resulted in rapid technological innovation. Specialized treatment machines, such as robotic or gimbal-steered linear accelerators (linac) with in-room imaging suites, have been developed specifically for real-time treatment adaptation. Additional equipment, such as stereoscopic kilovoltage (kV) imaging, ultrasound transducers and electromagnetic transponders, has been developed for intrafraction motion monitoring on conventional linacs. Magnetic resonance imaging (MRI) has been integrated with cobalt treatment units and more recently with linacs. In addition to hardware innovation, software development has played a substantial role in the development of motion monitoring methods based on respiratory motion surrogates and planar kV or Megavoltage (MV) imaging that is available on standard equipped linacs. In this paper, we review and compare the different intrafraction motion monitoring methods proposed in the literature and demonstrated in real-time on clinical data as well as their possible future developments. We then discuss general considerations on validation and quality assurance for clinical implementation. Besides photon RT, particle therapy is increasingly used to treat moving targets. However, transferring motion monitoring technologies from linacs to particle beam lines presents substantial challenges. Lessons learned from the implementation of real-time intrafraction monitoring for photon RT will be used as a basis to discuss the implementation of these methods for particle RT.

SBRT targets that move with respiration

The technology of treating SBRT targets that move with respiration has undergone profound changes over the last 20 years. This review article summarizes modern image guidance to localize the target in real-time to account for intra-fraction motion. The state-of-the art respiratory motion compensation techniques will be discussed, including the determination and application of appropriate margins. This includes compression, gating and breath-hold, including the use of audiovisual feedback to manage motion. Approaches to real-time tracking include the use of hybrid external-internal imaging to build a skin-to-tumor correlation, which can then be tracked with a mobile robot (CyberKnife Synchrony, clinical since 2003) as well as the use of non-ionizing electromagnetic tumor surrogate localization followed by real-time tracking with a moving MLC (in clinical trials in Europe and Australia). Lastly, the clinical application of real-time MRI soft-tissue imaging to deliver adaptive, iso-toxic treatments will be presented.

Ultra-efficient, widely tunable gold nanoparticle-based fiducial markers for X-ray imaging

We show the development of a new class of highly efficient, biocompatible fiducial markers for X-ray imaging and radiosurgery, based on polymer shells encapsulating engineered gold nanoparticle (AuNP) suspensions. Our smart fabrication strategy enables wide tunability of the fiducial size, shape, and X-ray attenuation performance, up to record values >20 000 Hounsfield units (HU), i.e. comparable to or even higher than bulk gold. We show that the NP fiducials allow for superior imaging both in vitro and in vivo (yet requiring 2 orders of magnitude less material), with strong stability over time and the absence of classical "streak artifacts" of standard bulk fiducials. NP fiducials were probed in vivo, showing exceptional contrast efficiency, even after 2 weeks post-implant in mice.

Characterization and evaluation of 2.5 MV electronic portal imaging for accurate localization of intra- and extracranial stereotactic radiosurgery

2.5 MV electronic portal imaging, available on Varian TrueBeam machines, was characterized using various phantoms in this study. Its low-contrast detectability, spatial resolution, and contrast-to-noise ratio (CNR) were compared with those of conventional 6 MV and kV planar imaging. Scatter effect in large patient body was simulated by adding solid water slabs along the beam path. The 2.5 MV imaging mode was also evaluated using clinically acquired images from 24 patients for the sites of brain, head and neck, lung, and abdomen. With respect to 6 MV, the 2.5 MV achieved higher contrast and preserved sharpness on bony structures with only half of the imaging dose. The quality of 2.5 MV imaging was comparable to that of kV imaging when the lateral separation of patient was greater than 38 cm, while the kV image quality degraded rapidly as patient separation increased. Based on the results of patient images, 2.5 MV imaging was better for cranial and extracranial SRS than the 6 MV imaging.

Reducing Set-up Uncertainty in the Elekta Stereotactic Body Frame Using Stealthstation Software

The Elekta Stereotactic Body Frame (SBF) is a device which allows extracranial targets to be localized and irradiated in a stereotactic coordinate system. Errors of positioning of the body relative to the frame are indirectly estimated by image fusion of multiple CT scans. A novel repositioning methodology, based on neurosurgical Stealth technology, is presented whereby accurate patient repositioning is directly confirmed before treatment delivery. Repositioning was performed on four extracranial stereotactic radiosurgery patients and a radiotherapy simulation phantom. The setup error was quantitatively measured by fiducial localization. A confirmatory CT scan was performed and the resulting image set registered to the initial scan to quantify shifts in the GTV isocenter. Alignment confirmation using Stealth took between 5 and 10 minutes. For the phantom studies, a reproducibly of 0.6 mm accuracy of phantom-to-SBF alignment was measured. The results on four actual patients showed setup errors of 1.5 mm or less. Using the Stealth Station process, rapid confirmation of alignment on the treatment table is possible.

Automatic tracking of arbitrarily shaped implanted markers in kilovoltage projection images: a feasibility study

PURPOSE

Certain types of commonly used fiducial markers take on irregular shapes upon implantation in soft tissue. This poses a challenge for methods that assume a predefined shape of markers when automatically tracking such markers in kilovoltage (kV) radiographs. The authors have developed a method of automatically tracking regularly and irregularly shaped markers using kV projection images and assessed its potential for detecting intrafractional target motion during rotational treatment.

METHODS

Template-based matching used a normalized cross-correlation with simplex minimization. Templates were created from computed tomography (CT) images for phantom studies and from end-expiration breath-hold planning CT for patient studies. The kV images were processed using a Sobel filter to enhance marker visibility. To correct for changes in intermarker relative positions between simulation and treatment that can introduce errors in automatic matching, marker offsets in three dimensions were manually determined from an approximately orthogonal pair of kV images. Two studies in anthropomorphic phantom were carried out, one using a gold cylindrical marker representing regular shape, another using a Visicoil marker representing irregular shape. Automatic matching of templates to cone beam CT (CBCT) projection images was performed to known marker positions in phantom. In patient data, automatic matching was compared to manual matching as an approximate ground truth. Positional discrepancy between automatic and manual matching of less than 2 mm was assumed as the criterion for successful tracking. Tracking success rates were examined in kV projection images from 22 CBCT scans of four pancreas, six gastroesophageal junction, and one lung cancer patients. Each patient had at least one irregularly shaped radiopaque marker implanted in or near the tumor. In addition, automatic tracking was tested in intrafraction kV images of three lung cancer patients with irregularly shaped markers during 11 volumetric modulated arc treatments. Purpose-built software developed at our institution was used to create marker templates and track the markers embedded in kV images.

RESULTS

Phantom studies showed mean ± standard deviation measurement uncertainty of automatic registration to be 0.14 ± 0.07 mm and 0.17 ± 0.08 mm for Visicoil and gold cylindrical markers, respectively. The mean success rate of automatic tracking with CBCT projections (11 frames per second, fps) of pancreas, gastroesophageal junction, and lung cancer patients was 100%, 99.1% (range 98%-100%), and 100%, respectively. With intrafraction images (approx. 0.2 fps) of lung cancer patients, the success rate was 98.2% (range 97%-100%), and 94.3% (range 93%-97%) using templates from 1.25 mm and 2.5 mm slice spacing CT scans, respectively. Correction of intermarker relative position was found to improve the success rate in two out of eight patients analyzed.

CONCLUSIONS

The proposed method can track arbitrary marker shapes in kV images using templates generated from a breath-hold CT acquired at simulation. The studies indicate its feasibility for tracking tumor motion during rotational treatment. Investigation of the causes of misregistration suggests that its rate of incidence can be reduced with higher frequency of image acquisition, templates made from smaller CT slice spacing, and correction of changes in intermarker relative positions when they occur.

Injectable colloidal gold in a sucrose acetate isobutyrate gelating matrix with potential use in radiation therapy

External beam radiation therapy relies on the ability to deliver high radiation doses to tumor cells with minimal exposure to surrounding healthy tissue. Advanced irradiation techniques, including image-guided radiation therapy (IGRT), rely on the ability to locate tumors to optimize the therapeutic benefit of these techniques. Today, radiopaque fiducial tissue markers are placed in or around tumors, for example, in prostate cancer patients to enhance the precision of daily and/or real-time IGRT. A liquid injectable fiducial marker (nanogel) is developed based on PEGylated gold nanoparticles and sucrose acetate isobutyrate (SAIB) with improved properties compared to current solid fiducial markers. The developed nanogel is investigated in vitro and subsequently evaluated in vivo in immunocompetent NMRI mice. The nanogel shows high CT-contrast and excellent stability in vivo over a period of 12 weeks. The nanogel is found to be biocompatible and well tolerated. No induction of the inflammatory cytokines INF-γ, IL-6, or TNF-α is observed throughout the study period. The developed nanogel seems to be a safe injectable fiducial marker ideally suited for IGRT that may further enhance the effect of radiation.

Feasibility study of dual energy radiographic imaging for target localization in radiotherapy for lung tumors

Purpose

Dual-energy (DE) radiographic imaging improves tissue discrimination by separating soft from hard tissues in the acquired images. This study was to establish a mathematic model of DE imaging based on intrinsic properties of tissues and quantitatively evaluate the feasibility of applying the DE imaging technique to tumor localization in radiotherapy.

Methods

We investigated the dependence of DE image quality on the radiological equivalent path length (EPL) of tissues with two phantoms using a stereoscopic x-ray imaging unit. 10 lung cancer patients who underwent radiotherapy each with gold markers implanted in the tumor were enrolled in the study approved by the hospital's Ethics Committee. The displacements of the centroids of the delineated gross tumor volumes (GTVs) in the digitally reconstructed radiograph (DRR) and in the bone-canceled DE image were compared with the averaged displacements of the centroids of gold markers to evaluate the feasibility of using DE imaging for tumor localization.

Results

The results of the phantom study indicated that the contrast-to-noise ratio (CNR) was linearly dependent on the difference of EPL and a mathematical model was established. The objects and backgrounds corresponding to ΔEPL less than 0.08 are visually indistinguishable in the bone-canceled DE image. The analysis of patient data showed that the tumor contrast in the bone-canceled images was improved significantly as compared with that in the original radiographic images and the accuracy of tumor localization using the DE imaging technique was comparable with that of using fiducial makers.

Conclusion

It is feasible to apply the technique for tumor localization in radiotherapy.

Intradural extramedullary benign spinal lesions radiosurgery. Medium- to long-term results from a single institution experience

BACKGROUND

Surgery represents the first-choice treatment for spinal intradural tumours. On the other hand, whether it is most appropriate in the setting of recurrences, residual or multiple lesions remains an open question. Moreover, some patients are less than ideal candidates for surgery. In this study we report about our own radiosurgery experience in the treatment of benign intradural extramedullary tumours of the spine.

METHODS

In our study we analyzed the outcomes for 18 patients (21 lesions) treated for benign intradural extramedullary lesions, with a minimum follow-up period of 32 months. The lesions included 11 meningiomas, 9 schwannomas and 1 neurofibroma.

RESULTS

The mean follow-up was 43 months (32-73 months). The median tumour volume was 2 cc (0.2-17.7 cc). Eleven lesions underwent single-fraction treatment (mean prescribed dose ranging from 10 to 13 Gy). The others received a multisession radiosurgery treatment (4-6 fractions) with a mean prescription dose ranging from 18.5 to 25 Gy. The maximum dose to the spinal cord ranged from 9.2 to 26 Gy. During the follow-up period, none of the lesions showed radiological evidence of progression. Neurological status was preserved or improved and no permanent sequelae were observed. Significant and durable pain relief was observed.

CONCLUSIONS

Although surgical excision remains the primary treatment option for most intradural tumours, radiosurgery offers a real alternative therapeutic modality, especially in case of recurrent and residual lesions or when surgery is contraindicated.

Assessing Feasibility of Real-Time Ultrasound Monitoring in Stereotactic Body Radiotherapy of Liver Tumors

To monitor tumor motion during stereotactic body radiotherapy (SBRT) for patients with liver cancer, an integrated ultrasound and kilo-voltage cone-beam computed tomography (KV-CBCT) system has been proposed. The presence of an ultrasound probe may interfere with the radiation beams. The purpose of this study is to minimize this interference by altering orientations of the ultrasound probe and directions of radiation beams while not compromising the quality of SBRT plans. Ten patients, who received SBRT of liver cancer, were randomly selected for this study. To simulate the presence of an ultrasound probe, a virtual probe was oriented either parallel or vertical to the longitudinal axis of the patient's body and was added on the surface of the patient's body at the nearest location to the tumor. For both the parallel and vertical probe orientations, 2 new SBRT (Probe-Para and Probe-Vert) plans that minimize the interference between the probe and radiation beams were created for each patient. These SBRT plans were compared to the original clinically accepted SBRT plans, with a treatment goal of 37.5 Gy to the planning target volume (PTV) in 3 fractions. Specific dosimetric endpoints were evaluated, including doses to 95% (D95), of the PTV plan conformal index (CI), homogeneity index (HI), and relevant endpoint doses to organs at risk. For 2 patients with superficially located tumors, no clinically acceptable SBRT plans could be produced without the interference between the probe and radiation beams. For the remaining 8 patients, the Probe-Para plans allowed 7 patients to be treated with coplanar radiation beams (without moving the treatment couch during treatment) and 1 patient to be treated with non-coplanar beams (by moving the treatment couch during treatment). The Probe-Vert plans allowed 2 patients to be treated with coplanar beams and 6 patients to be treated with non-coplanar beams. The average D95 of the PTV were 38.63 Gy ± 0.14 ( p = 0.65) for Probe-Para plans, 38.48 Gy ± 0.31 ( p = 0.33) for Probe-Vert plans, and 38.72 Gy ± 0.14 for clinical SBRT plans. There were no significant differences ( p > 0.05) in CI and HI of all SBRT plans. The endpoint doses to the liver, heart, esophagus, right kidney, and stomach also had no significant differences ( p > 0.05). Except for superficial lesions, real-time ultrasound monitoring during liver SBRT is clinically feasible. Placing the ultrasound probe parallel to the longitudinal axis of the patient allows a greater probability of utilizing preferred coplanar beams.

Respiratory Gating for Radiotherapy: Main Technical Aspects and Clinical Benefits

Respiratory-gated radiotherapy offers a significant potential for improvement in the irradiation of tumor sites affected by respiratory motion such as lung, breast, and liver tumors. An increased conformality of irradiation fields leading to decreased complication rates of organs at risk is expected. Five main strategies are used to reduce respiratory motion effects: integration of respiratory movements into treatment planning, forced shallow breathing with abdominal compression, breath-hold techniques, respiratory gating techniques, and tracking techniques. Measurements of respiratory movements can be performed either in a representative sample of the general population, or directly on the patient before irradiation. Reduction of breathing motion can be achieved by using either abdominal compression, breath-hold techniques, or respiratory gating techniques. Abdominal compression can be used to reduce diaphragmatic excursions. Breath-hold can be achieved with active techniques, in which airflow of the patient is temporarily blocked by a valve, or passive techniques, in which the patient voluntarily breath-holds. Respiratory gating techniques use external devices to predict the phase of the breathing cycle while the patient breathes freely. Another approach is tumor-tracking technique, which consists of a real-time localization of a constantly moving tumor. This work describes these different strategies and gives an overview of the literature.

Stereotactic body radiotherapy for pulmonary metastases. Prognostic factors and adverse respiratory events

PURPOSE

The aim of this retrospective study was to evaluate the feasibility, safety, and effectiveness of stereotactic body radiotherapy (SBRT) for pulmonary metastases.

PATIENTS AND METHODS

Between April 2007 and March 2011, 87 patients underwent SBRT for pulmonary metastases using the in-house Air-Bag System(TM) to obtain the four-dimensional image for treatment planning and to reduce intrafractional intrathoracic organ motion with abdominal compression to reduce the risk of radiation pneumonitis. Survival and respiratory adverse events were analyzed.

RESULTS

The 2- and 3-year overall survival (OS) rates were 47 and 32 %, and the corresponding cause-specific survivals were 52 and 36 %. The 2- and 3-year OS rates were 57 and 49 % for patients in group 1, respectively, while the corresponding OS rates were 48 and 21 %, and 40 and 32 % for patients in groups 2 and 3, respectively. The 2- and 3-year local control (LC) rates were 80 and 80 %, respectively. The corresponding intrathoracic progression-free survival rates were 40 and 32 %, respectively. Concerning adverse respiratory events after SBRT for pulmonary metastases, 14 % were grade 0 (G0), 66 % G1, 13 % G2, 6 % G3, and 1 % G4. Concerning the adverse respiratory events (NCI-CTC) by grade scale, 1- and 2-year cumulative probabilities of radiation pneumonitis were 12 and 20 % for G2 and 4 and 10 % for G3/4, respectively. The mean values for cumulative V20 were 11.6 ± 8.5 %, 29.8 ± 18.6 %, and 25.7 ± 12.8 % in G0/1, G2, and G3/4, respectively. The number of pulmonary metastases that could be safely treated with SBRT was 6 PTVs (or seven gross tumor volumes) within a cumulative V20 of 30 % under the restricted intrafractional respiratory tumor motion using the Air-Bag System(TM).

CONCLUSION

We propose that the number of pulmonary metastases that can be safely treated with SBRT is 6 PTVs with a cumulative V20 of 30 % under the restricted respiratory tumor motion using the Air-Bag System(TM). SBRT for pulmonary metastases offers locally effective treatment for recurrent or residual lesions after first line chemotherapy.

Imaging system QA of a medical accelerator, Novalis Tx, for IGRT per TG 142: our 1 year experience

American Association of Physicists in Medicine (AAPM) task group (TG) 142 has recently published a report to update recommendations of the AAPM TG 40 report and add new recommendations concerning medical accelerators in the era of image‐guided radiation therapy (IGRT). The recommendations of AAPM TG 142 on IGRT are timely. In our institute, we established a comprehensive imaging QA program on a medical accelerator based on AAPM TG 142 and implemented it successfully. In this paper, we share our one‐year experience and performance evaluation of an OBI capable linear accelerator, Novalis Tx, per TG 142 guidelines.

PACS numbers: 87.57.‐s, 87.57.C‐, 87.57.uq, 87.59.‐e, 87.59.bd

[An experience of cyberknife treatment in patients with advanced pancreaticobilliary malignancy]

BACKGROUND/AIMS

CyberKnifeTM stereotactic body radiotherapy (SBRT) has been thought as a promising treatment modality for inoperable or recurred pancreaticobiliary malignancies. But, clinical course of CyberKnifeTM treatment have not been established yet, so we report the experience of CyberKnifeTM treatment in 19 patients with recurred or advanced pancreaticobilliary malignancies.

METHODS

Between July 2008 and May 2009, 19 patients (gallbladder cancer 4, common bile duct cancer 5, and pancreatic cancer 10) with recurred (12) and advanced pancreaticobiliary cancer (7) underwent CyberKnifeTM treatment in Soonchunhyang University Hospital. Tumor size was evaluated at 1, 3, 6, 8 and every 3 months after SBRT.

RESULTS

The mean age was 60.2 years, and the mean size of target lesions was 28.1±1.30 mm. After CyberKnifeTM treatment, the average size of target lesions was decreased; 2.53±4.18 mm from months 0-1 in 19 patients, 2.47±4.7 mm from months 1-3 in 15 patients, 0.08±5.11 mm from months 3-6 in 12 patients. However, the average size of target lesions was increased 3.67±8.98 mm from months 6-8 in 6 patients. There were 2 cases of massive duodenal ulcer bleeding after CyberKnifeTM treatment, one of them expired due to ulcer bleeding. Also, other minor complications appeared such as 1 case of abdominal pain and 1 case of diarrhea.

CONCLUSIONS

CyberKnifeTM treatment seems to be effective in local control of pancreaticobiliary cancer, but we experienced serious complications. Further prospective studies will be needed for the proper evaluation of role of CyberknifeTM treatment in patients with advanced pancreaticobiliary malignancies.

Imaging in radiation oncology: a perspective

This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

An inherent goal of radiation therapy is to deliver enough dose to the tumor to eradicate all cancer cells or to palliate symptoms, while avoiding normal tissue injury. Imaging for cancer diagnosis, staging, treatment planning, and radiation targeting has been integrated in various ways to improve the chance of this occurring. A large spectrum of imaging strategies and technologies has evolved in parallel to advances in radiation delivery. The types of imaging can be categorized into offline imaging (outside the treatment room) and online imaging (inside the treatment room, conventionally termed image-guided radiation therapy). The direct integration of images in the radiotherapy planning process (physically or computationally) often entails trade-offs in imaging performance. Although such compromises may be acceptable given specific clinical objectives, general requirements for imaging performance are expected to increase as paradigms for radiation delivery evolve to address underlying biology and adapt to radiation responses. This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

A preliminary probe into the movement of pancreatic lesions and factors that influence it

The aim of this study was to investigate the movement, and the factors that influence such movement, of pancreatic lesions and to provide a reference for determination of planning target volume (PTV) during stereotactic radiotherapy. We implanted 19 gold markers into the inner pancreatic tumours of 16 pancreatic carcinoma patients percutaneously under B-ultrasonographic guidance. The marked motion of pancreatic lesions in the x (right-left), y (superoinferior) and z (anteroposterior) directions was measured using an X-ray simulator system. Based on the statistical analysis of the detected movements, we investigated the relevant influencing factors of pancreatic lesions with multinomial linear regression. Data showed that the mean motion amplitudes of pancreatic lesions were 0.16 cm +/- 0.06 (range 0.1-0.3 cm) in the x direction, 0.25 cm +/- 0.12 (range 0.1-0.4 cm) in the y direction and 0.88 cm +/- 0.24 (0.5-1.6 cm) in the z direction. Motion amplitude was not correlated with the height, weight or age of the patients nor with the location or size of the tumour. The motion of pancreatic lesions was mainly influenced by the respiratory motion and has maximal amplitude in the z direction. Therefore, motion in the z direction should be given a priority consideration while determining the PTV.

Gating and tracking, 4D in thoracic tumours

The limited ability to control for a tumour's location compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiation therapy (IGRT), these volumes can be optimized and tumouricidal doses may be delivered, achieving maximum tumour control with minimal complications. Moreover, with the ability of high precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications in radiotherapy such as hypofractionated radiotherapy (or stereotactic body radiotherapy), deliberate inhomogeneous dose distributions coping with tumour heterogeneity (dose painting by numbers and biologically conformal radiation therapy), and adaptive radiotherapy. In short: "individualized radiotherapy". Tumour motion management, especially for thoracic tumours, is a particular problem in this context both for the delineation of tumours and organs at risk as well as during the actual treatment delivery. The latter will be covered in this paper with some examples based on the experience of the UZ Brussel. With the introduction of the NOVALIS system (BrainLAB, Feldkirchen, Germany) in 2000 and consecutive prototypes of the ExacTrac IGRT system, gradually a hypofractionation treatment protocol was introduced for the treatment of lung tumours and liver metastases evolving from motion-encompassing techniques towards respiratory-gated radiation therapy with audio-visual feedback and most recently dynamic tracking using the VERO system (BrainLAB, Feldkirchen, Germany). This evolution will be used to illustrate the recent developments in this particular field of research.

Extracranial stereotactic radioablation: physical principles

Extracranial stereotactic radioablation (ESR) involves treating well-demarcated targeted tissues (e.g. tumor with minimal margin for set-up uncertainties) with very large doses of radiation in single or a few fractions with the intent of causing profound late tissue damage within the targeted volume. In such circumstances, considerable effort must be taken to reduce non-target tissue exposure to the high dose levels in order to prevent late complications to involved organs. Consequently, the following conditions for effective delivery of the ESR techniques have to be satisfied: 1) delivery of a high dose per fraction, i.e. 10-24 Gy; 2) delivery of only a few fractions per course of treatment (e.g. 1-4); 3) shaping of the prescription isodose surface conformally to the target surface; 4) delivery of a non-uniform dose distribution within the target with the highest dose in centrally located regions of hypoxia; 5) rapid fall-off of dose from the target volume to healthy tissue in all directions. In this paper it is shown that high doses per fraction in few fractions can be delivered to a variety of locations with both efficacy and acceptable toxicity (conditions 1 and 2). Conformal shaping of the high isodose surfaces is best accomplished by employing many beams (5-10) each with carefully milled apertures precisely coincident with the target projection (condition 3). Beam intensity modulation creating parabolic beam entrance fluence profiles both concentrates the highest dose in central regions of tumor hypoxia and increases fall-off gradients outside of the target (conditions 4 and 5). It is also shown that isotropic, highly non-coplanar beam arrangements avoiding oppositional fields allow more optimal fall-off gradients to normal tissue as opposed to coplanar treatments (condition 5).

Treatment of spinal tumors using cyberknife fractionated stereotactic radiosurgery: pain and quality-of-life assessment after treatment in 200 patients

OBJECTIVE

Benign and malignant tumors of the spine significantly impair the function and quality of life of many patients. Standard treatment options, including conventional radiotherapy and surgery, are often limited by anatomic constraints and previous treatment. Image-guided stereotactic radiosurgery using the CyberKnife system (Accuray, Inc., Sunnyvale, CA) is a novel approach in the multidisciplinary management of spinal tumors. The aim of this study was to evaluate the effects of CyberKnife stereotactic radiosurgery on pain and quality-of-life outcomes of patients with spinal tumors.

METHODS

We conducted a prospective study of 200 patients with benign or malignant spinal tumors treated at Georgetown University Hospital between March 2002 and September 2006. Patients were treated by means of multisession stereotactic radiosurgery using the CyberKnife as initial treatment, postoperative treatment, or retreatment. Pain scores were assessed by the Visual Analog Scale, quality of life was assessed by the SF-12 survey, and neurological examinations were conducted after treatment.

RESULTS

Mean pain scores decreased significantly from 40.1 to 28.6 after treatment (P < 0.001) and continued to decrease over the entire 4-year follow-up period (P < 0.05). SF-12 Physical Component scores demonstrated no significant change throughout the follow-up period. Mental Component scores were significantly higher after treatment (P < 0.01), representing a quality-of-life improvement. Early side effects of radiosurgery were mild and self-limited, and no late radiation toxicity was observed.

CONCLUSION

CyberKnife stereotactic radiosurgery is a safe and effective modality in the treatment of patients with spinal tumors. CyberKnife offers durable pain relief and maintenance of quality of life with a very favorable side effect profile.

On the accuracy of a moving average algorithm for target tracking during radiation therapy treatment delivery

Real-time tumor targeting involves the continuous realignment of the radiation beam with the tumor. Real-time tumor targeting offers several advantages such as improved accuracy of tumor treatment and reduced dose to surrounding tissue. Current limitations to this technique include mechanical motion constraints. The purpose of this study was to investigate an alternative treatment scenario using a moving average algorithm. The algorithm, using a suitable averaging period, accounts for variations in the average tumor position, but respiratory induced target position variations about this average are ignored during delivery and can be treated as a random error during planning. In order to test the method a comparison between five different treatment techniques was performed: (1) moving average algorithm, (2) real-time motion tracking, (3) respiration motion gating (at both inhale and exhale), (4) moving average gating (at both inhale and exhale) and (5) static beam delivery. Two data sets were used for the purpose of this analysis: (a) external respiratory-motion traces using different coaching techniques included 331 respiration motion traces from 24 lung-cancer patients acquired using three different breathing types [free breathing (FB), audio coaching (A) and audio-visual biofeedback (AV)]; (b) 3D tumor motion included implanted fiducial motion data for over 160 treatment fractions for 46 thoracic and abdominal cancer patients obtained from the Cyberknife Synchrony. The metrics used for comparison were the group systematic error (M), the standard deviation (SD) of the systematic error (sigma) and the root mean square of the random error (sigma). Margins were calculated using the formula by Stroom et al. [Int. J. Radiat. Oncol., Biol., Phys. 43(4), 905-919 (1999)]: 2sigma + 0.7sigma. The resultant calculations for implanted fiducial motion traces (all values in cm) show that M and sigma are negligible for moving average algorithm, moving average gating, and real-time tracking (i.e., M and sigma = 0 cm) compared to static beam (M = 0.02 cm and sigma = 0.16 cm) or gated beam delivery (M = -0.05 and 0.16 cm at both exhale and inhale, respectively, and sigma = 0.17 and 0.26 cm at both exhale and inhale, respectively). Moving average algorithm sigma = 0.22 cm has a slightly lower random error than static beam delivery sigma = 0.24 cm, though gating, moving average gating, and real-time tracking have much lower random error values for implanted fiducial motion. Similar trends were also observed for the results using the external respiratory motion data. Moving average algorithm delivery significantly reduces M and sigma compared with static beam delivery. The moving average algorithm removes the nonstationary part of the respiration motion which is also achieved by AV, and thus the addition of the moving average algorithm shows little improvement with AV. Overall, a moving average algorithm shows margin reduction compared with gating and static beam delivery, and may have some mechanical advantages over real-time tracking when the beam is aligned with the target and patient compliance advantages over real-time tracking when the target is aligned to the beam.

A probabilistic framework based on hidden markov model for fiducial identification in image-guided radiation treatments

Fiducial tracking is a common target tracking method widely used in image-guided procedures such as radiotherapy and radiosurgery. In this paper, we present a multifiducial identification method that incorporates context information in the process. We first convert the problem into a state sequence problem by establishing a probabilistic framework based on a hidden Markov model (HMM), where prior probability represents an individual candidate's resemblance to a fiducial; transition probability quantifies the similarity of a candidate set to the fiducials' geometrical configuration; and the Viterbi algorithm provides an efficient solution. We then discuss the problem of identifying fiducials using stereo projections, and propose a special, higher order HMM, which consists of two parallel HMMs, connected by an association measure that captures the inherent correlation between the two projections. A novel algorithm, the concurrent viterbi with association (CVA) algorithm, is introduced to efficiently identify fiducials in the two projections simultaneously. This probabilistic framework is highly flexible and provides a buffer to accommodate deformations. A simple implementation of the CVA algorithm is presented to evaluate the efficacy of the framework. Experiments were carried out using clinical images acquired during patient treatments, and several examples are presented to illustrate a variety of clinical situations. In the experiments, the algorithm demonstrated a large tracking range, computational efficiency, ease of use, and robustness that meet the requirements for clinical use.

Integration of Cone-Beam CT in Stereotactic Body Radiation Therapy

This report describes the technique and initial experience using cone beam CT (CBCT) for localization of treatment targets in patients undergoing stereotactic body radiation therapy (SBRT). Patients selected for SBRT underwent 3-D or 4-D CT scans in a customized immobilization cradle. GTV, CTV, ITV, and PTV were defined. Intensity-modulated radiation beams, multiple 3-D conformal beams, or dynamic conformal arcs were delivered using a Varian 21EX with 120-leaf MLC. CBCT images were obtained prior to each fraction, and registered to the planning CT by using soft tissue and bony structures to assure accurate isocenter localization. Patients were repositioned for treatment based on the CBCT images. Radiographic images (kV, MV, or CBCT) were taken before and after beam delivery to further assess set-up accuracy. Ten patients with lung, liver, and spine lesions received 29 fractions of treatment using this technique. The prescription doses ranged 1250 ~ 6000 cGy in 1 ~ 5 fractions. Compared to traditional 2-D matching using bony structures, CBCT corrects target deviation from 1 mm to 15 mm, with an average of 5 mm. Comparison of pre-treatment to post-treatment radiographic images demonstrated an average 2 mm deviation (ranging from 0–4 mm). Improved immobilization may enhance positioning accuracy. Typical total “in-room” times for the patients are approximately 1 hour. CBCT-guided SBRT is feasible and enhances setup accuracy using 3-D anatomical information.

Synchrony--cyberknife respiratory compensation technology

Studies of organs in the thorax and abdomen have shown that these organs can move as much as 40 mm due to respiratory motion. Without compensation for this motion during the course of external beam radiation therapy, the dose coverage to target may be compromised. On the other hand, if compensation of this motion is by expansion of the margin around the target, a significant volume of normal tissue may be unnecessarily irradiated. In hypofractionated regimens, the issue of respiratory compensation becomes an important factor and is critical in single-fraction extracranial radiosurgery applications. CyberKnife is an image-guided radiosurgery system that consists of a 6-MV LINAC mounted to a robotic arm coupled through a control loop to a digital diagnostic x-ray imaging system. The robotic arm can point the beam anywhere in space with 6 degrees of freedom, without being constrained to a conventional isocenter. The CyberKnife has been recently upgraded with a real-time respiratory tracking and compensation system called Synchrony. Using external markers in conjunction with diagnostic x-ray images, Synchrony helps guide the robotic arm to move the radiation beam in real time such that the beam always remains aligned with the target. With the aid of Synchrony, the tumor motion can be tracked in three-dimensional space, and the motion-induced dosimetric change to target can be minimized with a limited margin. The working principles, advantages, limitations, and our clinical experience with this new technology will be discussed.

The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Innovations in image-guided radiotherapy

The limited ability to control for the location of a tumour compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiotherapy (IGRT) these volumes can be optimized and tumoricidal doses can be delivered, achieving maximal tumour control with minimal complications. Moreover, with the ability of high-precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications for radiotherapy, some of which were previously considered infeasible.

Evaluation of a stochastic approach to 2D-3D intensity-based registration

this paper assesses a new 2D-3D rigid registration method based on stochastic clustering and an enhanced estimator for mutual information. It combines precision, accuracy and acceptable computation time. Spine datasets (fluoroscopy and computed tomography) with their gold standard transformations are used. Both optimization method and similarity measure are assessed separately using standardized evaluation methodologies. Sub-millimeter accuracy and the high convergence rate obtained within one minute are compared to other quasi-global optimization processes such as particle filtering.

Computational Challenges for Image-Guided Radiation Therapy: Framework and Current Research

It is arguable that the imaging and delivery hardware necessary for delivering real-time adaptive image-guided radiotherapy is available on high-end linear accelerators. Robust and computationally efficient software is the limiting factor in achieving highly accurate and precise radiotherapy to the constantly changing anatomy of a cancer patient. The limitations are not caused by the availability of algorithms but rather issues of reliability, integration, and calculation time. However, each of the software components is an active area of research and development at academic and commercial centers. This article describes the software solutions in 4 broad areas: deformable image registration, adaptive replanning, real-time image guidance, and dose calculation and accumulation. Given the pace of technological advancement, the integration of these software solutions to develop real-time adaptive image-guided radiotherapy and the associated challenges they bring will be implemented to varying degrees by all major manufacturers over the coming years.

Advances in image-guided radiation therapy

Imaging is central to radiation oncology practice, with advances in radiation oncology occurring in parallel to advances in imaging. Targets to be irradiated and normal tissues to be spared are delineated on computed tomography (CT) scans in the planning process. Computer-assisted design of the radiation dose distribution ensures that the objectives for target coverage and avoidance of healthy tissue are achieved. The radiation treatment units are now recognized as state-of-the-art robotics capable of three-dimensional soft tissue imaging immediately before, during, or after radiation delivery, improving the localization of the target at the time of radiation delivery, to ensure that radiation therapy is delivered as planned. Frequent imaging in the treatment room during a course of radiation therapy, with decisions made on the basis of imaging, is referred to as image-guided radiation therapy (IGRT). IGRT allows changes in tumor position, size, and shape to be measured during the course of therapy, with adjustments made to maximize the geometric accuracy and precision of radiation delivery, reducing the volume of healthy tissue irradiated and permitting dose escalation to the tumor. These geometric advantages increase the chance of tumor control, reduce the risk of toxicity after radiotherapy, and facilitate the development of shorter radiotherapy schedules. By reducing the variability in delivered doses across a population of patients, IGRT should also improve interpretation of future clinical trials.

Image-guided robotic radiosurgery for spinal metastases

BACKGROUND AND PURPOSE

To determine the effectiveness and safety of image-guided robotic radiosurgery for spinal metastases.

MATERIALS/METHODS

From 1996 to 2005, 74 patients with 102 spinal metastases were treated using the CyberKnife at Stanford University. Sixty-two (84%) patients were symptomatic. Seventy-four percent (50/68) of previously treated patients had prior radiation. Using the CyberKnife, 16-25 Gy in 1-5 fractions was delivered. Patients were followed clinically and radiographically for at least 3 months or until death.

RESULTS

With mean follow-up of 9 months (range 0-33 months), 36 patients were alive and 38 were dead at last follow-up. No death was treatment related. Eighty-four (84%) percent of symptomatic patients experienced improvement or resolution of symptoms after treatment. Three patients developed treatment-related spinal injury. Analysis of dose-volume parameters and clinical parameters failed to identify predictors of spinal cord injury.

CONCLUSIONS

Robotic radiosurgery is effective and generally safe for spinal metastases even in previously irradiated patients.

Image-guided radiosurgical ablation of intra- and extra-cranial lesions

For decades since its introduction, stereotactic radiosurgery (SRS) was used only to treat intracranial lesions because intracranial targets could be immobilized and located relative to a rigid metal frame affixed to the patient's head. Lesions outside the head were generally not treated with SRS because it is difficult to immobilize extracranial lesions and to attach stereotactic frames elsewhere on the body. Advances in computerized image guidance and robotics allowed the development of systems, such as the CyberKnife SRS System (Accuray, Inc, Sunnyvale, CA), that could target intracranial lesions without the stereotactic frame. Enhancements have resulted in a radiation delivery system that can accurately deliver high-dose, focal radiation to lesions in the spine, chest, and abdomen, even if they move during respiration. In this review we will describe the technical features of frameless SRS systems and briefly review their application to treating intracranial and extracranial lesions, focusing in particular on spinal lesions.