Frame-based radiosurgery of multiple metastases using single-isocenter volumetric modulated arc therapy technique.

Single-isocenter volumetric modulated arc therapy (VMAT) technique can provide stereotactic radiosurgery (SRS) treatment with improved delivery efficiency for treating multiple metastases. Nevertheless, planning is time consuming and verification of frame-based SRS setup, especially at noncoplanar angles, can be challenging. We report on a single-isocenter VMAT technique with a special focus on improving treatment workflow and delivery verification to exploit the minimized patient motion of the frame-based SRS. We developed protocols for preplanning and verification for VMAT and evaluated them for ten patient cases. Preplans based on MRI were used to generate comparable treatment plans using CT taken on the day of treatment after frame placement. Target positioning accuracy was evaluated by stereoscopic in-room kV imaging. Dosimetric accuracy of the noncoplanar plan delivery was validated using measurement-guided 3D dose reconstruction as well as film-based end-to-end test with a Rando phantom. Average absolute differences of homogeneity indices, conformity indices, and V12Gy between MR preplans and CT-based plans were within 5%. In-room imaging positioning accuracy of 0.4 mm was verified to be independent of the distance to the isocenter. For treatment verification, average local and global passing rates of the 3D gamma (1 mm, 3%) were 86% and 99%, respectively. D99 values were matched within 5% for individual target structures (>0.5 cc). Additional film analysis confirmed dosimetric accuracy for small targets that had large verification errors in the 3D dose reconstruction. Our results suggest that the advantages of frame-based SRS and noncoplanar single-isocenter VMAT technique can be combined for efficient and accurate treatment of patients with multiple metastases.

Validation of MLC-based linac radiosurgery for trigeminal neuralgia.

This study details a validation process for linear accelerator-based treatment of trigeminal neuralgia using HD-MLC field collimation. Nine trigeminal neuralgia treatment plans utilizing HD-MLC were selected for absolute dose measurement at isocenter using a commercial scintillating detector in an anthropomorphic phantom. Four plans were chosen for film dosimetry measurements in each of the three principal planes to assess spatial dose distribution agreement with the treatment planning system. Additionally, trajectory log analysis for each treatment field in the nine cases was performed to assess mechanical positioning accuracy of the MLC system during delivery. Scintillator and film measurements both revealed mean dose agreement at isocenter of better than 3% while FWHM of the 2D dose distribution in each plane showed agreement between plan and measurement within 0.2 mm. Analysis of log files revealed a maximum MLC leaf positioning error of 0.04 mm across 178 treatment fields. In conjunction with a quality-controlled treatment delivery methodology, an appropriately commissioned treatment planning system can be used for accurate and clinically appropriate design of trigeminal neuralgia treatment plans utilizing HD-MLC.

Stereotactic body radiotherapy for multisite extracranial oligometastases: final report of a dose escalation trial in patients with 1 to 5 sites of metastatic disease.

BACKGROUND: A subset of patients with metastatic cancer in limited organs may benefit from metastasis-directed therapy. The authors investigated whether patients with limited metastases could be safely treated with metastasis-directed radiotherapy. METHODS: Patients with 1 to 5 metastatic cancer sites with a life expectancy of >3 months received escalating stereotactic body radiotherapy (SBRT) doses to all known cancer sites. Patients were followed radiographically with CT scans of the chest, abdomen, and pelvis and metabolically with fluorodeoxyglucose-positron emission tomography, 1 month after treatment, and then every 3 months. Acute toxicities were scored using the National Cancer Institute's Common Terminology Criteria for Adverse Events version 3.0, and late toxicities were scored using the Radiation Therapy Oncology Group late toxicity scoring system. RESULTS: Sixty-one patients with 113 metastases were enrolled from November 2004 to November 2009 on a prospective radiation dose escalation study. Median follow-up was 20.9 months. Patients tolerated treatment well; the maximal tolerated dose was not reached in any cohort. Eleven patients (18.3%) have not progressed. One and 2-year progression-free survival are 33.3% (95% confidence interval [CI], 22.8-46.1) and 22.0% (95% CI, 12.8-34.4); 1-year and 2-year overall survival are 81.5% (95% CI, 71.1-91.1) and 56.7% (95% CI, 43.9-68.9). Seventy-two percent of patients whose tumors progressed did so in limited (1-3) metastatic sites. CONCLUSIONS: Patients with 1 to 5 metastases can be safely treated to multiple body sites and may benefit from SBRT. Further investigation should focus on patient selection.

Stereotactic body radiation therapy: the report of AAPM Task Group 101.

Task Group 101 of the AAPM has prepared this report for medical physicists, clinicians, and therapists in order to outline the best practice guidelines for the external-beam radiation therapy technique referred to as stereotactic body radiation therapy (SBRT). The task group report includes a review of the literature to identify reported clinical findings and expected outcomes for this treatment modality. Information is provided for establishing a SBRT program, including protocols, equipment, resources, and QA procedures. Additionally, suggestions for developing consistent documentation for prescribing, reporting, and recording SBRT treatment delivery is provided.

ACR-ASTRO Practice Parameter for the Performance of Stereotactic Body Radiation Therapy.

AIM/OBJECTIVES/BACKGROUND: To standardize the practice of stereotactic body radiation therapy (SBRT), the American College of Radiology (ACR) and the American Society for Radiation Oncology (ASTRO) cooperatively developed the practice parameter for SBRT. SBRT is a treatment technique that delivers radiation dose to a well-defined extracranial target in 5 fractions or less and usually employs a higher dose per fraction than used in conventional radiation. METHODS: The ACR-ASTRO Practice Parameter for the Performance of Stereotactic Body Radiation Therapy was revised according to the process described on the ACR website ("The Process for Developing ACR Practice Parameters and Technical Standards," www.acr.org/ClinicalResources/Practice-Parameters-and-Technical-Standards) by the Committee on Practice Parameters of the ACR Commission on Radiation Oncology in collaboration with the ASTRO. Both societies then reviewed and approved the document. RESULTS: Given the complexities of SBRT, a separate document was created to develop a technical standard for the medical physics of SBRT (ACR-AAPM Technical Standard for Medical Physics Performance Monitoring of Stereotactic Body Radiation Therapy). Workflow, qualifications and responsibilities of personnel, specifications, documentation, quality control/safety/improvement, simulation/treatment, and follow-up were addressed in this practice parameter. CONCLUSIONS: This practice parameter assists practitioners in providing safe and appropriate SBRT treatment and care for patients when clinically indicated. As technologies and techniques continue to evolve, this document will be reviewed, revised and renewed accordingly to a 5 year or sooner timeline specified by the ACR.

Dosimetric feasibility of brain stereotactic radiosurgery with a 0.35 T MRI-guided linac and comparison vs a C-arm-mounted linac.

PURPOSE: MRI is the gold-standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35 T MRI-guided linear accelerator (MRL) equipped with a double-focused multileaf collimator (MLC). Dosimetric comparisons were made vs a conventional C-arm-mounted linac with a high-definition MLC. METHODS: The quality of MRL single-isocenter brain SRS treatment plans was evaluated as a function of target size for a series of spherical targets with diameters from 0.6 cm to 2.5 cm in an anthropomorphic head phantom and six brain metastases (max linear dimension = 0.7-1.9 cm) previously treated at our clinic on a conventional linac. Each target was prescribed 20 Gy to 99% of the target volume. Step-and-shoot IMRT plans were generated for the MRL using 11 static coplanar beams equally spaced over 360° about an isocenter placed at the center of the target. Couch and collimator angles are fixed for the MRL. Two MRL planning strategies (VR1 and VR2) were investigated. VR1 minimized the 12 Gy isodose volume while constraining the maximum point dose to be within ±1 Gy of 25 Gy which corresponded to normalization to an 80% isodose volume. VR2 minimized the 12 Gy isodose volume without the maximum dose constraint. For the conventional linac, the TB1 method followed the same strategy as VR1 while TB2 used five noncoplanar dynamic conformal arcs. Plan quality was evaluated in terms of conformity index (CI), conformity/gradient index (CGI), homogeneity index (HI), and volume of normal brain receiving ≥12 Gy (V12Gy ). Quality assurance measurements were performed with Gafchromic EBT-XD film following an absolute dose calibration protocol. RESULTS: For the phantom study, the CI of MRL plans was not significantly different compared to a conventional linac (P > 0.05). The use of dynamic conformal arcs and noncoplanar beams with a conventional linac spared significantly more normal brain (P = 0.027) and maximized the CGI, as expected. The mean CGI was 95.9 ± 4.5 for TB2 vs 86.6 ± 3.7 (VR1), 88.2 ± 4.8 (VR2), and 88.5 ± 5.9 (TB1). Each method satisfied a normal brain V12Gy  ≤ 10.0 cm3 planning goal for targets with diameter ≤2.25 cm. The mean V12Gy was 3.1 cm3 for TB2 vs 5.5 cm3 , 5.0 cm3 and 4.3 cm3 , for VR1, VR2, and TB1, respectively. For a 2.5-cm diameter target, only TB2 met the V12Gy planning objective. The MRL clinical brain plans were deemed acceptable for patient treatment. The normal brain V12Gy was ≤6.0 cm3 for all clinical targets (maximum target volume = 3.51 cm3 ). CI and CGI ranged from 1.12-1.65 and 81.2-88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured vs computed absolute dose difference was -0.1%. CONCLUSIONS: The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤2.25 cm. Large lesions (>2.25 cm) should be treated with a linac capable of delivering noncoplanar beams.

An initial report of a radiation dose-escalation trial in patients with one to five sites of metastatic disease.

PURPOSE: Previous investigations have suggested that a subset of patients with metastatic cancer in a limited number of organs may benefit from local treatment. We investigated whether cancer patients with limited sites of metastatic disease (oligometastasis) who failed standard therapies could be identified and safely treated at one to five known sites of low-volume disease with radiotherapy. EXPERIMENTAL DESIGN: Patients with one to five sites of metastatic cancer with a life expectancy of >3 months and good performance status received escalating doses of radiation to all known sites of cancer with hypofractionated radiation therapy. Patients were followed radiographically with computed tomography scans of the chest, abdomen, and pelvis and metabolically with [18F]fluorodeoxyglucose-positron emission tomography 1 month following treatment and then every 3 months. Acute toxicities were scored using the National Cancer Institute Common Terminology Criteria for Adverse Events version 3.0 and late toxicities were scored using the Radiation Therapy Oncology Group late toxicity scoring system. RESULTS: Twenty-nine patients with 56 metastatic lesions were enrolled from November 2004 to March 2007, with a median follow-up of 14.9 months. Two patients experienced acute (radiation pneumonitis and nausea) and one experienced chronic (gastrointestinal hemorrhage) grade > or =3 toxicity. Fifty-nine percent of patients responded to protocol therapy. Twenty-one percent of patients have not progressed following protocol treatment. Fifty-seven percent of treated lesions have not progressed at last follow-up. Progression was amenable to further local therapy in 48% of patients. CONCLUSIONS: Patients with low-volume metastatic cancer can be identified, safely treated, and may benefit from radiotherapy.

Anniversary Paper: the role of medical physicists in developing stereotactic radiosurgery.

This article is a tribute to the pioneering medical physicists over the last 50 years who have participated in the research, development, and commercialization of stereotactic radiosurgery (SRS) and stereotactic radiotherapy utilizing a wide range of technology. The authors have described the evolution of SRS through the eyes of physicists from its beginnings with the Gamma Knife in 1951 to proton and charged particle therapy; modification of commercial linacs to accommodate high precision SRS setups; the multitude of accessories that have enabled fine tuning patients for relocalization, immobilization, and repositioning with submillimeter accuracy; and finally the emerging technology of SBRT. A major theme of the article is the expanding role of the medical physicist from that of advisor to the neurosurgeon to the current role as a primary driver of new technology that has already led to an adaptation of cranial SRS to other sites in the body, including, spine, liver, and lung. SRS continues to be at the forefront of the impetus to provide technological precision for radiation therapy and has demonstrated a host of downstream benefits in improving delivery strategies for conventional therapy as well. While this is not intended to be a comprehensive history, and the authors could not delineate every contribution by all of those working in the pursuit of SRS development, including physicians, engineers, radiobiologists, and the rest of the therapy and dosimetry staff in this important and dynamic radiation therapy modality, it is clear that physicists have had a substantial role in the development of SRS and theyincreasingly play a leading role in furthering SRS technology.

Accuracy and feasibility of cone-beam computed tomography for stereotactic radiosurgery setup.

Image fusion, target localization, and setup accuracy of cone-beam computed tomography (CBCT) for stereotactic radiosurgery (SRS) were investigated in this study. A Rando head phantom rigidly attached to a stereotactic Brown-Roberts-Wells (BRW) frame was utilized to study the geometric accuracy of CBCT. Measurements of distances and angular separations between selected pairs of multiple radio-opaque targets embedded in the head phantom from a conventional simulation CT provided comparative data for geometric accuracy analysis. Localization accuracy of the CBCT scan was investigated from an analysis of BRW localization of four cylindrical objects (9 mm in diameter and 25 mm in length) independently computed from CBCT and conventional CT scans. Image fusion accuracy was quantitatively evaluated from BRW localization of multiple simulated targets from the CBCT and conventional CT scan. Finally, a CBCT setup procedure for stereotactic radiosurgery treatments was proposed and its accuracy was assessed using orthogonal target verification imaging. Our study showed that CBCT did not present any significant geometric distortions. Stereotactic coordinates of the four cylindrical objects as determined from the CBCT differed from those determined from the conventional CT on average by 0.30 mm with a standard deviation (SD) of 0.09 mm. The mean image registration accuracy of CBCT with conventional CT was 0.28 mm (SD = 0.10 mm). Setup uncertainty of our proposed CBCT setup procedure was on the same order as the conventional framed-based stereotactic systems reported in the literature (mean = 1.34 mm, SD = 0.33 mm). In conclusion, CBCT can be used to guide SRS treatment setup with accuracy comparable to the currently used frame-based stereotactic radiosurgery systems provided that intra-treatment patient motion is prevented.

Multifractionated image-guided and stereotactic intensity-modulated radiotherapy of paraspinal tumors: a preliminary report.

PURPOSE: The use of image-guided and stereotactic intensity-modulated radiotherapy (IMRT) techniques have made the delivery of high-dose radiation to lesions within close proximity to the spinal cord feasible. This report presents clinical and physical data regarding the use of IMRT coupled with noninvasive body frames (stereotactic and image-guided) for multifractionated radiotherapy. METHODS AND MATERIALS: The Memorial Sloan-Kettering Cancer Center (Memorial) stereotactic body frame (MSBF) and Memorial body cradle (MBC) have been developed as noninvasive immobilizing devices for paraspinal IMRT using stereotactic (MSBF) and image-guided (MBC) techniques. Patients were either previously irradiated or prescribed doses beyond spinal cord tolerance (54 Gy in standard fractionation) and had unresectable gross disease involving the spinal canal. The planning target volume (PTV) was the gross tumor volume with a 1 cm margin. The PTV was not allowed to include the spinal cord contour. All treatment planning was performed using software developed within the institution. Isocenter verification was performed with an in-room computed tomography scan (MSBF) or electronic portal imaging devices, or both. Patients were followed up with serial magnetic resonance imaging every 3-4 months, and no patients were lost to follow-up. Kaplan-Meier statistics were used for analysis of clinical data. RESULTS: Both the MSBF and MBC were able to provide setup accuracy within 2 mm. With a median follow-up of 11 months, 35 patients (14 primary and 21 secondary malignancies) underwent treatment. The median dose previously received was 3000 cGy in 10 fractions. The median dose prescribed for these patients was 2000 cGy/5 fractions (2000-3000 cGy), which provided a median PTV V100 of 88%. In previously unirradiated patients, the median prescribed dose was 7000 cGy (5940-7000 cGy) with a median PTV V100 of 90%. The median Dmax to the cord was 34% and 68% for previously irradiated and never irradiated patients, respectively. More than 90% of patients experienced palliation from pain, weakness, or paresthesia; 75% and 81% of secondary and primary lesions, respectively, exhibited local control at the time of last follow-up. No cases of radiation-induced myelopathy or radiculopathy have thus far been encountered. CONCLUSIONS: Precision stereotactic and image-guided paraspinal IMRT allows the delivery of high doses of radiation in multiple fractions to tumors within close proximity to the spinal cord while respecting cord tolerance. Although preliminary, the clinical results are encouraging.

Image-guided radiation therapy for prostate cancer: A computed tomography-based assessment of fiducial marker migration between placement and 7 days.

PURPOSE: This study was conducted to determine whether clinically significant fiducial marker migration occurs immediately after prostatic implantation. METHODS AND MATERIALS: One hundred patients with transperineal (n = 39) or transrectal (n = 61) placement of 3 gold fiducial markers underwent computed tomography scans on day 0 (after placement) and day 7 (at radiation planning). Each marker was marked as a point of interest in a treatment planning system. An automated point-based algorithm was then used to coregister the day 0 and day 7 images by matching the markers through rigid translations and rotations. The mean distance between fiducial pairs (d¯) was recorded to assess the degree of seed migration. Prostate contours were delineated, and the day 0 prostate volumes were uniformly expanded by 1, 3, and 5 mm. The percentage of the day 7 prostate volume covered by each day 0 prostate with expansion was calculated to assess whether prostate contours, if performed on day 0, would adequately cover the prostate on day 7. RESULTS: The average d¯ for all patients was 0.78 ± 0.45 mm; only 1 patient had d¯ > 2 mm. Placement technique, hormonal therapy, prostate size, and marker distance from the capsule were not associated with d¯ (P > .05). The mean percentages of day 7 prostate volumes covered by the day 0 prostate plus 1, 3, and 5 mm were 98.3%, 99.8%, and 100%, respectively. With an expansion of 3 mm, 98% of men had >95% of day 0 volume covered; with an expansion of 5 mm, 100% of men had 100% of the day 0 volume covered. CONCLUSIONS: There is minimal change in the relative positions of fiducial markers (average d¯ < 1.0 mm) 1 week after placement. A 1- to 3-mm expansion would account for the variation in seed position for the vast majority of cases. These results suggest that planning could be performed on the day of implantation without adverse consequence.

CT image-guided intensity-modulated therapy for paraspinal tumors using stereotactic immobilization.

PURPOSE: To design and implement a noninvasive stereotactic immobilization technique with daily CT image-guided positioning to treat patients with paraspinal lesions accurately and to quantify the systematic and random patient setup errors occurring with this method. METHODS AND MATERIALS: A stereotactic body frame (SBF) was developed for "rigid" immobilization of paraspinal patients. The inherent accuracy of this system for stereotactic CT-guided treatment was evaluated with phantom studies. Seven patients with thoracic and lumbar spine lesions were immobilized with the SBF and positioned for 33 treatment fractions using daily CT scans. For all 7 patients, the daily setup errors, as assessed from the daily CT scans, were corrected at each treatment fraction. A retrospective analysis was also performed to assess what the impact on patient treatment would have been without the CT-based corrections (i.e., if patient setup had been performed only with the SBF). RESULTS: The average magnitude of systematic and random errors from uncorrected patient setups using the SBF was approximately 2 mm and 1.5 mm (1 SD), respectively. For fixed phantom targets, the system accuracy for the SBF localization and treatment was shown to be within 1 mm (1 SD) in any direction. Dose-volume histograms incorporating these uncertainties for an intensity-modulated radiotherapy plan for lumbar spine lesions were generated, and the effects on the dose-volume histograms were studied. CONCLUSION: We demonstrated a very accurate and precise method of patient immobilization and treatment delivery based on a noninvasive SBF and daily image guidance for paraspinal lesions. The SBF provides excellent immobilization for paraspinal targets, with setup accuracy better than 2 mm (1 SD). However, for highly conformal paraspinal treatments, uncorrected systematic and random errors of 2 mm in magnitude can result in a significantly greater (>100%) dose to the spinal cord than planned, even though the planned target coverage may not change substantially. With daily CT guidance using the SBF, we showed that the maximal spinal cord dose is ensured to be within 10-15% of the planned value.

Intensity-modulated stereotactic radiotherapy of paraspinal tumors: a preliminary report.

OBJECTIVE: Radioresistant paraspinal tumors may benefit from conformal treatment techniques such as intensity-modulated radiotherapy (IMRT). Local tumor control and long-term palliation for both primary and metastatic tumors may be achieved with IMRT while reducing the risk of spinal cord toxicity associated with conventional radiotherapy techniques. In this article, we report our initial clinical experience in treating 16 paraspinal tumors with IMRT in which the planning target volume was 2 mm or greater from the spinal cord. METHODS: IMRT was administered by using a linear accelerator mounted with a multileaf collimator. Two immobilization body frames developed at Memorial Sloan-Kettering Cancer Center were used for patients with and without spinal implants. During a 30-month period, 16 patients underwent IMRT for metastatic and primary tumors. Eleven patients were treated for symptomatic recurrences after undergoing surgery and prior external beam radiotherapy, and one patient was treated after undergoing radiotherapy for a metastatic pancreatic gastrinoma with overlapping ports to the spine. Four patients with primary tumors were treated after primary resection that resulted in positive histological margins. Twelve patients were symptomatic with pain, functional radiculopathy, or both. Tumoral doses were determined on the basis of the relative radiosensitivity of tumors. Patients with metastatic tumors were administered a median tumoral dose of 20 Gy in four to five fractions and a spinal cord maximum dose of 6.0 Gy in addition to the full tolerance dose administered in previous radiation treatments. The primary tumors were delivered a median dose of 70 Gy in 33 to 37 fractions and a spinal cord maximum dose of 16 Gy. The median tumoral volume was 7.8 cm(3). RESULTS: Of the 15 patients who underwent radiographic follow-up, 13 demonstrated either no interval growth or a reduction in tumor size in a median follow-up period of 12 months (range, 2-23 mo). Two patients, one with a thoracic chondrosarcoma and one with a chordoma, showed tumor progression 1 year after undergoing IMRT. Pain symptoms improved in 11 of 11 patients, and 4 of 4 patients had significant improvement in their functionally significant radiculopathy and/or plexopathy. Pain relief was durable in all patients except the two with tumor progression. No patient showed signs or symptoms of radiation-induced myelopathy, radiculopathy, or plexopathy, including 12 patients with a median follow-up of 18 months. CONCLUSION: IMRT was effective for treating pain and improving functional radiculopathy in patients with metastatic and primary tumors. Although long-term tumor control is not established in this study, high-dose tumoral irradiation can be performed without causing radiation myelopathy in more than 1 year of follow-up.

Image-guided radiotherapy using surgical clips as fiducial markers after prostatectomy: a report of total setup error, required PTV expansion, and dosimetric implications.

PURPOSE: To determine the total setup error and the required planning target volume (PTV) margin for prostate bed without image guided radiotherapy (IGRT), and to demonstrate the feasibility and dosimetric benefit of IGRT post prostatectomy using surgical clips. MATERIALS AND METHODS: Seventeen patients were treated with intensity modulated radiotherapy (IMRT) to the prostate bed with a 1cm PTV margin. Three-dimensional shifts of the surgical clips inside the prostate bed were measured with respect to the isocenter from 364 orthogonal kV image pairs, and the total setup error was calculated to determine the required PTV margin. Alternative IMRT plans using 5mm or 1cm PTV expansion were generated and compared for rectal and bladder sparing. RESULTS: Surgical clips were reproducibly and reliably identified. The mean (standard deviation) shifts in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP), axes were: -0.1 mm (1.7 mm), 0.6 mm (2.4 mm), and -2.1 mm (2.6 mm), respectively. The required PTV margins were calculated to be 6, 8, and 9 mm in the LR, AP, and SI axis, respectively. A PTV expansion of 5mm, compared to 1cm, significantly reduced V65 Gy to the rectum by 10%. CONCLUSIONS: In the absence of IGRT, a non-uniform PTV margin of 6mm LR, 8mm AP, and 9 mm SI should be considered. Use of clips as fiducial markers can decrease the total setup error, enable a smaller PTV margin, and improve rectal sparing.

The utility of FDG-PET for assessing outcomes in oligometastatic cancer patients treated with stereotactic body radiotherapy: a cohort study.

BACKGROUND: Studies suggest that patients with metastases limited in number and destination organ benefit from metastasis-directed therapy. Stereotactic body radiotherapy (SBRT) is commonly used for metastasis directed therapy in this group. However, the characterization of PET response following SBRT is unknown in this population. We analyzed our cohort of patients to describe the PET response following SBRT. METHODS: Patients enrolled on a prospective dose escalation trial of SBRT to all known sites of metastatic disease were reviewed to select patients with pre- and post-therapy PET scans. Response to SBRT was characterized on PET imaging based on standard PET response criteria and compared to CT based RECIST criteria for each treated lesion. RESULTS: 31 patients had PET and CT data available before and after treatment for analysis in this study. In total, 58 lesions were treated (19 lung, 11 osseous, 11 nodal, 9 liver, 6 adrenal and 2 soft tissue metastases). Median follow-up was 14 months (range: 3-41). Median time to first post-therapy PET was 1.2 months (range; 0.5-4.1). On initial post-therapy PET evaluation, 96% (56/58) of treated metastases responded to therapy. 60% (35/58) had a complete response (CR) on PET and 36% (21/58) had a partial response (PR). Of 22 patients with stable disease (SD) on initial CT scan, 13 had CR on PET, 8 had PR, and one had SD. Of 21 metastases with PET PR, 38% became CR, 52% remained PR, and 10% had progressive disease on follow-up PET. 10/35 lesions (29%) with an initial PET CR progressed on follow-up PET scan with median time to progression of 4.11 months (range: 2.75-9.56). Higher radiation dose correlated with long-term PET response. CONCLUSIONS: PET response to SBRT enables characterization of metastatic response in tumors non-measurable by CT. Increasing radiation dose is associated with prolonged complete response on PET.

Development of a frameless stereotactic radiosurgery system based on real-time 6D position monitoring and adaptive head motion compensation.

Stereotactic radiosurgery delivers radiation with great spatial accuracy. To achieve sub-millimeter accuracy for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, the invasiveness of the ring can be highly uncomfortable and not well tolerated. In addition, placing and removing the ring requires special expertise from a neurosurgeon, and patient setup time for SRS can often be long. To reduce the invasiveness, hardware limitations and setup time, we are developing a system for performing accurate head positioning without the use of a head ring. The proposed method uses real-time 6D optical position feedback for turning on and off the treatment beam (gating) and guiding a motor-controlled 3D head motion compensation stage. The setup consists of a central control computer, an optical patient motion tracking system and a 3D motion compensation stage attached to the front of the LINAC couch. A styrofoam head cast was custom-built for patient support and was mounted on the compensation stage. The motion feedback of the markers was processed by the control computer, and the resulting motion of the target was calculated using a rigid body model. If the target deviated beyond a preset position of 0.2 mm, an automatic position correction was performed with stepper motors to adjust the head position via the couch mount motion platform. In the event the target deviated more than 1 mm, a safety relay switch was activated and the treatment beam was turned off. The feasibility of the concept was tested using five healthy volunteers. Head motion data were acquired with and without the use of motion compensation over treatment times of 15 min. On average, test subjects exceeded the 0.5 mm tolerance 86% of the time and the 1.0 mm tolerance 45% of the time without motion correction. With correction, this percentage was reduced to 5% and 2% for the 0.5 mm and 1.0 mm tolerances, respectively.

Effect of MLC leaf width and PTV margin on the treatment planning of intensity-modulated stereotactic radiosurgery (IMSRS) or radiotherapy (IMSRT).

We studied the effect of MLC (multileaf collimator) leaf width and PTV (planning target volume) margin on treatment planning of intensity modulated stereotactic radiosurgery (IMSRS) or radiotherapy (IMSRT). Twelve patients previously treated with IMSRS/IMSRT were retrospectively planned with 5- and 3-mm MLC leaf widths and 3- and 2-mm PTV margins using the already contoured clinical target volume and critical structures. The same beam arrangement, planning parameters, and optimization method were used in each of the 4 plans for a given patient. Each plan was normalized so that the prescription dose covered at least 99% of the PTV. Plan indices--D(mean) (mean dose), conformity index (CI), V(70) (volume receiving >or= 70% of the prescription dose), and V(50) (volume receiving >or= 50% of the prescription dose)--were calculated from the dose-volume histograms (DVHs) of the PTV, normal tissue, and organs at risk (OARs). Hypothesis testing was performed on the mean ratios of plan indices to determine the statistical significance of the relative differences. The PTV was well covered for all plans, as no significant differences were observed for D(95), V(95), D(max), D(min), and D(mean) of the PTV. The irradiated volume was approximately 23% smaller when 2-mm instead of 3-mm PTV margin was used, but it was only reduced by approximately 6% when the MLC leaf width was reduced from 5 mm to 3 mm. For normal tissue and brainstem, V(70), V(50), and D(mean) were reduced more effectively by a decrease in MLC width, while D(mean) of optic nerve and chiasm were more sensitive to a change in PTV margin. The DVH statistics for the PTV and normal structures from the treatment plan with 5-mm MLC and 2-mm PTV margin were equal to those with 3-mm MLC and 3-mm PTV margin. PTV margin reduction is more effective in sparing the normal tissue and OARs than a reduction in MLC leaf width. For IMSRS, where highly accurate setup and small PTV margins are routinely employed, the use of 5-mm MLC is therefore less desirable.