CyberKnife Treatment Research Articles

SU-E-T-216: Intercomparison of CyberKnife and GammaKnife Stereotactic Radiosurgery Treatment Plans for Metastatic Brain Tumors

Purpose: Over 40,000 patients are treated annually for metastatic brain tumors. A common method of treatment for these patients is stereotactic radiosurgery (SRS). Two commercially available options for treatment of SRS are the CyberKnife and GammaKnife. The purpose of this work was to perform a detailed comparison of the quality of the two techniques for the treatment of metastatic brain tumors using quantitative parameters. Methods: Six patients with small metastatic brain tumors (Range: 2.2cc– 14.6cc) that had previously received SRS treatment via the GammaKnife system were planned for equivalent treatment using the CyberKnife treatment planning system. To quantify the quality of the individual plans, the conformity number (CN), homogeneity index (HI) and gradient score index (GSI) were measured and compared. Both plans were created to delivery equivalent tumor coverage (Range: 10–14Gy) to the equivalent prescription volume. This was typically 99% prescription dose coverage to the GTV for both plans. Results: The calculated average HI was 0.497±0.155 versus 0.600±0.049, CI was 0.596±0.159 versus 0.865±0.022, and GSI was 41.4±17.8 versus 65.1±20.09 for CyberKnife vs GammaKnife respectively. Lower HI values imply a higher uniformity of dose throughout the target. CI ideally would be 1.000, and the closest to this value is to one, the better is the conformality to the treated site. High values for GSI implies a steeper dose falloff from the tumor to normal tissue and therefore is preferable. For this study statistical significance was achieved for CI, however due to the small sample size differences in HI and GSI were not statistically significant. Conclusion: This data suggests that SRS treatments on CyberKnife and GammaKnife are of similar quality, with Gamma Knife able to deliver a slightly more conformal plan for the treatment of brain metastases.

SU‐E‐T‐258: Parallel Optimization of Beam Configurations for CyberKnife Treatments

Purpose:The CyberKnife delivers a large number of beams originating at different non‐planar positions and with different orientation. We study how much the quality of treatment plans depends on the beams considered during plan optimization. Particularly, we evaluate a new approach to search for optimal treatment plans in parallel by running optimization steps concurrently.Methods:So far, no deterministic, complete and efficient method to select the optimal beam configuration for robotic SRS/SBRT is known. Considering a large candidate beam set increases the likelihood to achieve a good plan, but the optimization problem becomes large and impractical to solve. We have implemented an approach that parallelizes the search by solving multiple linear programming problems concurrently while iteratively resampling zero weighted beams. Each optimization problem contains the same set of constraints but different variables representing candidate beams. The search is synchronized by sharing the resulting basis variables among the parallel optimizations. We demonstrate the utility of the approach based on an actual spinal case with the objective to improve the coverage.Results:The objective function is falling and reaches a value of 5000 after 49, 31, 25 and 15 iterations for 1, 2, 4, and 8 parallel processes. This corresponds to approximately 97% coverage in 77%, 59%, and 36% of the mean number of iterations with one process for 2, 4, and 8 parallel processes, respectively. Overall, coverage increases from approximately 91.5% to approximately 98.5%.Conclusion:While on our current computer with uniform memory access the reduced number of iterations does not translate into a similar speedup, the approach illustrates how to effectively parallelize the search for the optimal beam configuration. The experimental results also indicate that for complex geometries the beam selection is critical for further plan optimization.

SU‐E‐T‐511: Do Presage 3D Dosimeters Show Dose Fractionation Sensitivity?

Purpose:To determine whether Presage 3D polymer dosimeter dose response is sensitive to dose delivery fractionation. Bang gels have demonstrated a dose fractionation related dependence in which a single 400 cGy irradiation would produce a different detector response than four 100 cGy irradiations even if delivered closely in time to one another. Such a fractional dependent response in Presage would be detrimental for measuring multi‐beam irradiations.Methods:Two separate batches of Presage were poured into cuvettes, and a third batch was molded into cuvette shaped blocks. A total of 37 cuvettes/blocks were irradiated in a Cobalt‐60 irradiator to 400 cGy within solid water phantoms in either one, eight, or sixteen fractions. Another group of 15 cuvettes were also kept unirradiated and used for background subtraction between the pre‐scan and post‐scan results. The times between fractional deliveries were held constant at 30 seconds and the Cobalt irradiator dose rate was 49 cGy/min. Each Presage batch has a separate dose sensitivity and therefore fractionation response comparisons were only performed within the same batch. The cuvettes were first pre‐scanned the day prior to irradiation and post‐scanned the day after irradiation. Other than approximately 3 hours warming time prior to each irradiation and optical density measurement the cuvettes were stored in a refrigerator. All cuvettes were stored in a lightless environment throughout manufacturing and testing. The cuvettes’ optical densities were optically measured at 632 nm with a spectrophotometer.Results:No noticeable dose fractionation dependence was detected for any of the three independent batches of Presage for either the eight or sixteen fraction irradiation schemes.Conclusion:These results indicate using Presage 3D dosimeters to measure multi‐beam photon irradiations common in IMRT, Gamma Knife, and Cyberknife treatment delivery schemes.Presage dosimeters are made by and trademarked by Heuris Inc. Skillman, NJ. Dr. John Adamovics is a founder of Heuris Inc.

Clinical image: rare localization of a giant recurrent malignant meningioma

Meningiomas are the most common benign brain tumors. Malignant meningioma is a rare subtype with a high recurrence rate and poor prognosis. Most meningiomas grow intracranially, whereas extracranial growth of meningiomas has been rarely reported. We herein report a rare case of a giant malignant meningioma with multiple recurrences. A 56-year old man presented with a forehead swelling. He had a history of repeated craniotomy for a frontal sinus invasion. The tumor recurred one year after the last surgery on the forehead and was classified as a giant meningioma at admission to our hospital. Neurosurgeons performed total resection of the giant meningioma and reconstructed the defect. However, six months after the surgery, the tumor recurred at the same site. The patient refused surgery and underwent cyber-knife treatment. An hydroxyurea treatment was initiated after radiotherapy, but the tumor progressed. Temozolomide and cisplatin treatment was started. A valuable clinical improvement was observed after the initiation of the first cycle. After completion of two cycles of systemic chemotherapy, the patient will be assessed clinically and radiologically.

Dose distribution transfer from CyberKnife to Varian treatment planning system

The aim of this paper was to introduce one of the options of the locally developed DDcon.exe which gives the possibility to transfer the dose distribution from CyberKnife (Accuray) treatment planning system (CK TPS) to Varian treatment planning system (Eclipse TPS, Varian). DICOM format is known as a universal format for medical data. The dose distribution is stored as RTdose file in DICOM format, so there should be a possibility to transfer it between different treatment planning systems. Trying to transfer RTdose file from CK TPS to Eclipse TPS the error message occurs. That's because the RTdose file in CK TPS is connected with Structure_Set_Sequence against Eclipse TPS where it's connected with RT_Plan_Sequence. To make it transferable RTdose file from CK TPS have to be 'disconnected' from Structure_Set_Sequence and 'connected' with RT_Plan_Sequence. This is possible thanks DDcon software which creates new RTdose file by changing proper DICOM tags in original RTdose file. New homemade software gives us an opportunity to transfer dose distribution from CyberKnife TPS to TPS Eclipse. This method opens new possibilities to combine or compare different treatment techniques in Varian TPS.

Intrafraction Prostate Translations and Rotations During Hypofractionated Robotic Radiation Surgery: Dosimetric Impact of Correction Strategies and Margins

To investigate the dosimetric impact of intrafraction prostate motion and the effect of robot correction strategies for hypofractionated CyberKnife treatments with a simultaneously integrated boost. A total of 548 real-time prostate motion tracks from 17 patients were available for dosimetric simulations of CyberKnife treatments, in which various correction strategies were included. Fixed time intervals between imaging/correction (15, 60, 180, and 360 seconds) were simulated, as well as adaptive timing (ie, the time interval reduced from 60 to 15 seconds in case prostate motion exceeded 3 mm or 2° in consecutive images). The simulated extent of robot corrections was also varied: no corrections, translational corrections only, and translational corrections combined with rotational corrections up to 5°, 10°, and perfect rotational correction. The correction strategies were evaluated for treatment plans with a 0-mm or 3-mm margin around the clinical target volume (CTV). We recorded CTV coverage (V100%) and dose-volume parameters of the peripheral zone (boost), rectum, bladder, and urethra. Planned dose parameters were increasingly preserved with larger extents of robot corrections. A time interval between corrections of 60 to 180 seconds provided optimal preservation of CTV coverage. To achieve 98% CTV coverage in 98% of the treatments, translational and rotational corrections up to 10° were required for the 0-mm margin plans, whereas translational and rotational corrections up to 5° were required for the 3-mm margin plans. Rectum and bladder were spared considerably better in the 0-mm margin plans. Adaptive timing did not improve delivered dose. Intrafraction prostate motion substantially affected the delivered dose but was compensated for effectively by robot corrections using a time interval of 60 to 180 seconds. A 0-mm margin required larger extents of additional rotational corrections than a 3-mm margin but resulted in lower doses to rectum and bladder.

Image-guided robotic stereotactic radiotherapy for synchronous cancer of maxillary gingiva and lung

The use of stereotactic body radiation therapy is rapidly increasing among patients with lung cancer not amenable to surgery. The authors report their experience using the CyberKnife system (Accuray Inc., Sunnyvale, CA, USA) as a treatment option for synchronous cancer of oral squamous cell carcinoma and a malignant lung tumour. An 88-year-old woman with two cancers (oral and lung masses) underwent CyberKnife treatment, with fiducial gold pins implanted using bronchoscopy. Toxicity was limited, and at the 2-year follow-up the lesions had not recurred.

Establishing a process of irradiating small animal brain using a CyberKnife and a microCT scanner

Establish and validate a process of accurately irradiating small animals using the CyberKnife G4 System (version 8.5) with treatment plans designed to irradiate a hemisphere of a mouse brain based on microCT scanner images. These experiments consisted of four parts: (1) building a mouse phantom for intensity modulated radiotherapy (IMRT) quality assurance (QA), (2) proving usability of a microCT for treatment planning, (3) fabricating a small animal positioning system for use with the CyberKnife's image guided radiotherapy (IGRT) system, and (4)in vivo verification of targeting accuracy. A set of solid water mouse phantoms was designed and fabricated, with radiochromic films (RCF) positioned in selected planes to measure delivered doses. After down-sampling for treatment planning compatibility, a CT image set of a phantom was imported into the CyberKnife treatment planning system--MultiPlan (ver. 3.5.2). A 0.5 cm diameter sphere was contoured within the phantom to represent a hemispherical section of a mouse brain. A nude mouse was scanned in an alpha cradle using a microCT scanner (cone-beam, 157 × 149 pixels slices, 0.2 mm longitudinal slice thickness). Based on the results of our positional accuracy study, a planning treatment volume (PTV) was created. A stereotactic body mold of the mouse was "printed" using a 3D printer laying UV curable acrylic plastic. Printer instructions were based on exported contours of the mouse's skin. Positional reproducibility in the mold was checked by measuring ten CT scans. To verify accurate dose delivery in vivo, six mice were irradiated in the mold with a 4 mm target contour and a 2 mm PTV margin to 3 Gy and sacrificed within 20 min to avoid DNA repair. The brain was sliced and stained for analysis. For the IMRT QA using a set of phantoms, the planned dose (6 Gy to the calculation point) was compared to the delivered dose measured via film and analyzed using Gamma analysis (3% and 3 mm). A passing rate of 99% was measured in areas of above 40% of the prescription dose. The final inverse treatment plan was comprised of 43 beams ranging from 5 to 12.5 mm in diameter (2.5 mm size increments are available up to 15 mm in diameter collimation). Using the Xsight Spine Tracking module, the CyberKnife system could not reliably identify and track the tiny mouse spine; however, the CyberKnife system could identify and track the fiducial markers on the 3D mold.In vivo positional accuracy analysis using the 3D mold generated a mean error of 1.41 mm ± 0.73 mm when fiducial markers were used for position tracking. Analysis of the dissected brain confirmed the ability to target the correct brain volume. With the use of a stereotactic body mold with fiducial markers, microCT imaging, and resolution down-sampling, the CyberKnife system can successfully perform small-animal radiotherapy studies.

The outcome of cyberknife treatment for primary or metastatic malignant lung tumors

Background: To analyze the local control of malignant lung tumors and survival of nonsmall cell lung cancer (NSCLC) patients after stereotactic ablative radiotherapy with CyberKnife. Materials and Methods: Patients with malignant lung tumors treated by CyberKnife between July 2007 and October 2010 at our institute were retrospectively reviewed. A total of 55 patients with 110 malignant lung tumors were included. There were 32 men and 23 women, and the median age was 67 years. There were 11 early-stage NSCLCs, while the other 44 patients with 99 lesions were metastatic lung tumors. The median gross tumor volume was 13.3 ml. Radiotherapy schedules include 40-60 Gy in 4-5 fractions, 45-60 Gy in 3 fractions and 30 Gy in 1 fraction. Results: The median follow-up time for patients alive was 34 months. The local control rates for all tumors were 96% at 1-year and 80% at 2 years. Univariate analysis demonstrated that target volume was important for local control. Biologically equivalent dose (BED) ≥100 Gy provided significantly higher chance to achieve a complete response than BED ＜100 Gy. The disease-specific survival rates for early-stage NSCLC were 80% at 1-year and 60% at 2 years. Treatment related complications were acceptable. No grade 2-5 adverse events were noted. Conclusions: CyberKnife can be used for NSCLC and metastatic lung tumors, either peripheral or central location, with good local control and acceptable side-effects.

Prostate-Specific Antigen Bounce Following Stereotactic Body Radiation Therapy for Prostate Cancer

Introduction: Prostate-specific antigen (PSA) bounce after brachytherapy has been well-documented. This phenomenon has also been identified in patients undergoing stereotactic body radiation therapy (SBRT). While the parameters that predict PSA bounce have been extensively studied in prostate brachytherapy patients, this study is the first to analyze the clinical and pathologic predictors of PSA bounce in prostate SBRT patients.Materials and Methods: Our institution has maintained a prospective database of patients undergoing SBRT for prostate cancer since 2006. Our study population includes patients between May 2006 and November 2011 who have at least 18 months of follow-up. All patients were treated using the CyberKnife treatment system. The prescription dose was 35–36.25 Gy in five fractions.Results: One hundred twenty patients were included in our study. Median PSA follow-up was 24 months (range 18–78 months). Thirty-four (28%) patients had a PSA bounce. The median time to PSA bounce was 9 months, and the median bounce size was 0.50 ng/mL. On univariate analysis, only younger age (p = 0.011) was shown to be associated with an increased incidence of PSA bounce. Other patient factors, including race, prostate size, prior treatment by hormones, and family history of prostate cancer, did not predict PSA bounces. None of the tumor characteristics studied, including Gleason score, pre-treatment PSA, T-stage, or risk classification by NCCN guidelines, were associated with increased incidence of PSA bounces. Younger age was the only statistically significant predictor of PSA bounce on multivariate analysis (OR = 0.937, p = 0.009).Conclusion: PSA bounce, which has been reported after prostate brachytherapy, is also seen in a significant percentage of patients after CyberKnife SBRT. Close observation rather than biopsy can be considered for these patients. Younger age was the only factor that predicted PSA bounce.

Comparing Gamma Knife and CyberKnife in patients with brain metastases

The authors compared the relative dosimetric merits of Gamma Knife (GK) and CyberKnife (CK) in 15 patients with 26 brain metastases. All patients were initially treated with the Leksell GK 4C. The same patients were used to generate comparative CK treatment plans. The tissue volume receiving more than 12 Gy (V12), the difference between V12 and tumor volume (V12net), homogeneity index (HI), and gradient indices (GI25, GI50) were calculated. Peripheral dose falloff and three conformity indices were compared. The median tumor volume was 2.50 cm3 (range, 0. 044‐19.9). A median dose of 18 Gy (range, 15‐22) was prescribed. In GK and CK plans, doses were prescribed to the 40‐50% and 77‐92% isodose lines, respectively. Comparing GK to CK, the respective parametric values (median±standard deviation) were: minimum dose (18.2±3.4 vs. 17.6±2.4 Gy, p=0.395); mean dose (29.6±5.1 vs.20.6±2.8 Gy, p<0.00001); maximum dose (40.3±6.5 vs.22.7±3.3 Gy, p<0.00001); and HI (2.22±0.19 vs. 1.18±0.06, p<0.00001). The median dosimetric indices (GK vs. CK, with range) were: RTOG_CI, 1.76 (1.12‐4.14) vs. 1.53 (1.16‐2.12), p=0.0220; CI, 1.76 (1.15‐4.14) vs. 1.55 (1.18‐2.21), p=0.050; nCI, 1.76 (1.59‐4.14) vs. 1.57 (1.20‐2.30), p=0.082; GI50, 2.91 (2.48‐3.67) vs. 4.90 (3.42‐11.68), p<0.00001; GI25, 6.58 (4.18‐10.20) vs. 14.85 (8.80‐48.37), p<0.00001. Average volume ratio (AVR) differences favored GK at multiple normalized isodose levels (p<0.00001). We concluded that in patients with brain metastases, CK and GK resulted in dosimetrically comparable plans that were nearly equivalent in several metrics, including target coverage and minimum dose within the target. Compared to GK, CK produced more homogenous plans with significantly lower mean and maximum doses, and achieved more conformal plans by RTOG_CI criteria. By GI and AVR analyses, GK plans had sharper peripheral dose falloff in most cases.PACS number: 89.20.‐a

Quality of life in the follow-up of uveal melanoma patients after CyberKnife treatment

To assess quality of life in uveal melanoma patients within the first and second year after CyberKnife radiosurgery. Overall, 91 uveal melanoma patients were evaluated for quality of life through the Short-form (SF-12) Health Survey at baseline and at every follow-up visit over 2 years after CyberKnife radiosurgery. Statistical analysis was carried out using SF Health Outcomes Scoring Software and included subgroup analysis of patients developing secondary glaucoma and of patients maintaining a best corrected visual acuity (BCVA) of the treated eye of 0.5 log(MAR) or better. Analysis of variance, Greenhouse-Geisser correction, Student's t-test, and Fisher's exact test were used to determine statistical significance. Physical Functioning (PF) and Role Physical (RP) showed a significant decrease after CyberKnife radiosurgery, whereas Mental Health (MH) improved (P=0.007, P<0.0001 and P=0.023). MH and Social Functioning (SF) increased significantly (P=0.0003 and 0.026) in the no glaucoma group, MH being higher compared with glaucoma patients (P=0.02). PF and RP were significantly higher in patients with higher BCVA at the second follow-up (P=0.02). RP decreased in patients with BCVA<0.5 log(MAR) (P=0.013). Vitality (VT) increased significantly in patients whose BCVA could be preserved (P=0.031). Neither tumor localization nor size influenced the development of secondary glaucoma or change in BCVA. Although PF and RP decreased over time, MH improved continuously. Prevention of secondary glaucoma has a significant influence on both SF and MH, whereas preservation of BCVA affects VT. Emotional stability throughout follow-up contributes positively toward overall quality of life. CyberKnife radiosurgery may contribute to attenuation of emotional distress in uveal melanoma patients.

Accuracy of out-of-field dose calculation of tomotherapy and cyberknife treatment planning systems: A dosimetric study

PurposeLate toxicities such as second cancer induction become more important as treatment outcome improves. Often the dose distribution calculated with a commercial treatment planning system (TPS) is used to estimate radiation carcinogenesis for the radiotherapy patient. However, for locations beyond the treatment field borders, the accuracy is not well known. The aim of this study was to perform detailed out-of-field-measurements for a typical radiotherapy treatment plan administered with a Cyberknife and a Tomotherapy machine and to compare the measurements to the predictions of the TPS. Materials and methodsIndividually calibrated thermoluminescent dosimeters were used to measure absorbed dose in an anthropomorphic phantom at 184 locations. The measured dose distributions from 6 MV intensity-modulated treatment beams for CyberKnife and TomoTherapy machines were compared to the dose calculations from the TPS. ResultsThe TPS are underestimating the dose far away from the target volume. Quantitatively the Cyberknife underestimates the dose at 40cm from the PTV border by a factor of 60, the Tomotherapy TPS by a factor of two. If a 50% dose uncertainty is accepted, the Cyberknife TPS can predict doses down to approximately 10 mGy/treatment Gy, the Tomotherapy-TPS down to 0.75 mGy/treatment Gy. The Cyberknife TPS can then be used up to 10cm from the PTV border the Tomotherapy up to 35cm. ConclusionsWe determined that the Cyberknife and Tomotherapy TPS underestimate substantially the doses far away from the treated volume. It is recommended not to use out-of-field doses from the Cyberknife TPS for applications like modeling of second cancer induction. The Tomotherapy TPS can be used up to 35cm from the PTV border (for a 390 cm3 large PTV).

Treating Adrenal Tumors in 26 Patients with CyberKnife: A Mono-Institutional Experience

BackgroundCyberKnife (CK) is a novel stereotactic radiosurgery system for treating tumors in any part of the body. It is a non-invasive or minimally invasive tumor treatment modality that can deliver high doses of spatially precise radiation and minimize exposure to neighboring healthy tissues or vital organs. The purpose of this study was to investigate the safety and efficacy of CK in the treatment of adrenal tumors. Methods and ResultsWe performed a retrospective analysis of 26 patients with adrenal tumors who had been treated with CK in the radiotherapy center of our hospital between March 2009 and March 2012. Eight patients had primary adrenal tumors and 18 patients had metastatic adrenal tumors. In addition to CK, 4 patients received chemotherapy and 2 patients received immunotherapy. The average tumor volume was 72.1 cm3 and the prescribed radiation dosage ranged from 30 to 50 Gy and was fractionated 3 to 5 times with a 58% to 80% isodose line. Abdominal CT was performed between 1 to 3 months after the CK treatment to evaluate the short-term efficacy with follow-up examinations once every 3 months. Three patients had complete remission, 12 patients had partial remission, 5 patients had stable disease, and 6 patients had progressive illness. The effective rate of pain relief was 93.8% and the disease control rate was 77% with a median overall survival of 17 months and a median progression-free survival of 14 months. Treatment Related toxicity was well-tolerated, but preventative measure need to be taken for radiation enteritis.Conclusions CK is safe and effective for treating adrenal tumors with few adverse reactions. Nonetheless, its long-term effects requires further follow-up.

Dosimetric effect of CT contrast agent in CyberKnife treatment plans

BackgroundTo investigate the effect of computed tomography (CT) contrast enhancement (CE) on the 3D dose distributions of non-coplanar small field beams in the CyberKnife (CK) treatment planning system (TPS) for the stereotactic ablative radiotherapy (SABR).MethodsTwenty-two pre-CE CT treatment plans were recruited to this retrospective plan study. Their post-CE CT plans were based on the pre-CE CT plan data and calculated using the same MU and beam paths in either Ray-Tracing or Monte Carlo (MC) algorithms. The differences in the doses of the beam path and the reference point between the pre- and post-CE CT plans were compared. The minimum, maximum, and mean doses in dose-volume histograms (DVHs) of target and organs-at-risk (OARs) were also compared.ResultsThe dose differences between the pre- and post-CE plans in a single beam path were less than 1.05% in both calculation algorithms, with respect to the prescription dose. At the center of the target volume, it was 1.9% (maximum 6.2%) in Ray-Tracing and 1.6% (maximum 4.0%) in MC. The CA effect showed on average 1.2% difference in the OAR maximum dose (maximum 7.8% in Ray-Tracing and 7.2% in MC). In the lung cases, the CT CE resulted in a dose difference of 2.4% (from 1.0% to 6.5%) without the calculation algorithm effect (maximum 20.3%).ConclusionsThe CK treatment plan using the post-CE CT generally afforded less than 2% dose differences from the pre-CE CT plan. However, it could be up to 7.8% depending on the target positions in a body and be more than 20% with the calculation algorithms. Thus, the post-CE CT in CK treatment plans should be used with careful consideration for the CA effect, target position, and calculation algorithm factors.

Comparative research on the efficacy of CyberKnife&amp;reg; and surgical excision for Stage I hepatocellular carcinoma

ObjectiveTo retrospectively analyze and compare the outcomes of patients with hepatocellular carcinoma treated with either surgical excision or CyberKnife® from September 2006 to August 2011.Materials and methodsLocal control and toxicity were the primary endpoints, followed by local progression-free survival, progression-free survival, and overall survival as the secondary endpoints. Response Evaluation Criteria In Solid Tumors were the evaluation criteria for efficacy; Common Toxicity Criteria 3.0 were the evaluation criteria for adverse events. Local control was calculated using the direct method (nonactuarial). The survival curves were drawn using the Kaplan–Meier method along with log-rank test analysis.ResultsThe research included 26 patients treated with tumor-free cutting edge (R0) surgical excision and 22 patients treated with CyberKnife treatment. The results showed that the adverse effects of CyberKnife were milder, with 1-, 2-, and 3-year local control rates of 92.9%, 90.0%, and 67.7%, respectively. The overall survival rates of the surgical treatment were 88.5%, 73.1%, and 69.2% for the same periods, while those of CyberKnife treatment were 72.7%, 66.7%, and 57.1%, respectively. In this study, surgical excision appeared to prolong overall survival to a greater extent, but with no statistical significance; no statistical difference was observed in the tumor-specific overall survival and progression-free survival between the two cohorts.ConclusionAccording to this preliminary study, with its mild toxicity, the efficacy of CyberKnife treatment for early hepatocellular carcinoma was on par with that of surgical resection.

Clinical observation on the efficacy of CyberKnife for primary or metastatic retroperitoneal tumours

The aim of the paper is to evaluate the efficacy of CyberKnife® for the treatment of primary or metastatic retroperitoneal tumours. Twenty-eight patients were treated. The prescription isodose line (median, 78%; range, 70-84%) covered the planning target volume at a total dose of 2000-6000 cGy (median, 4500 cGy) and a biologically effective dose of 3750-10,080 cGy (median, 7680 cGy) in 2-10 fractions (median, five fractions). The results showed that the complete response, partial response, stable disease, and progressive disease rates were 43% (12/28), 36% (10/28), 18% (5/28), and 4% (1/28) respectively. The overall response rate was 96%. The 1-, 2- and 3-year local control rates were 92%, 86%, and 86% respectively. The 1-, 2- and 3-year overall survival rates were 60%, 49%, and 49% respectively. No significant difference was found between local progression-free survival and overall survival. Moreover, if a patient only had metastases in the retroperitoneum and local control was effective, there was no significant difference between local progression-free survival and overall progression-free survival. In conclusion, CyberKnife treatment for retroperitoneal tumours resulted in high response rates with minimal side effects. All radiation-induced side effects were well tolerated.

Dosimetric evaluation of four‐dimensional dose distributions of CyberKnife and volumetric‐modulated arc radiotherapy in stereotactic body lung radiotherapy

Advanced image‐guided stereotatic body lung radiotherapy techniques using volumetric‐modulated arc radiotherapy (VMAT) with four‐dimensional cone‐beam computed tomography (4D CBCT) and CyberKnife with real‐time target tracking have been clinically implemented by different authors. However, dosimetric comparisons between these techniques are lacking. In this study, 4D CT scans of 14 patients were used to create VMAT and CyberKnife treatment plans using 4D dose calculations. The GTV and the organs at risk (OARs) were defined on the end‐exhale images for CyberKnife planning and were then deformed to the midventilation images (MidV) for VMAT optimization. Direct 4D Monte Carlo dose optimizations were performed for CyberKnife (4DCK). Four‐dimensional dose calculations were also applied to VMAT plans to generate the 4D dose distributions (4DVMAT) on the exhale images for direct comparisons with the 4DCK plans. 4DCK and 4DVMAT showed comparable target conformity (1.31±0.13 vs. 1.39±0.24,p=0.05). GTV mean doses were significantly higher with 4DCK. Statistical differences of dose volume metrics were not observed in the majority of OARs studied, except for esophagus, with 4DVMAT yielding marginally higher D1% than 4DCK. The normal tissue volumes receiving 80%, 50%, and 30% of the prescription dose (V80%,V50%, and V30%) were higher with 4DVMAT, whereas 4DCK yielded slightly higher V10% in posterior lesions than 4DVMAT. VMAT resulted in much less monitor units and therefore greater delivery efficiency than CyberKnife. In general, it was possible to produce dosimetrically acceptable plans with both techniques. The selection of treatment modality should consider the dosimetric results as well as the patient's tolerance of the treatment process specific to the SBRT technique.PACS numbers: 87.53.Ly, 87.55.km

SU-E-T-286: Radiation Shielding Evaluation Based On Five Years of Data From A Busy CyberKnife Center

Purpose: We examined the adequacy of shielding guidelines using 5‐year data from a busy CyberKnife Center. Methods: Data from 1370 patients, treated from June 2006 through July 2011 with a total of 4900 fractions were analyzed to estimate the workload and monitor unit (MU)/dose ratio. Each of the 680,691 delivered radiation beams was simulated with Matlab software to estimate the use factor. Results: On average, each patient received 3.6 fractions, with average 9.1 Gy per fraction prescribed at the 71.1% isodose line, using 133.7 beams and 6200 MU. Collimator sizes for intracranial patients were usually smaller (7.5 to 20 mm) than for extracranial patients (20 to 40 mm). Eighty‐four percent of the total beams exited through the floor. Due to mechanical limit, about 40% of the surrounding wall received no direct beams. For the rest of the wall, we found hot spots that received above‐average MU. The distribution of the spots was correlated with the distribution to the nodes for extracranial treatments, and there were fewer hot spots for intracranial treatments. The maximum MU a spot received is less than 0.25% of total MU used for intracranial treatments and less than 1.2% of total MU used for extracranial treatments. The combination of workload, MU/dose ratio and use factor in our practice are about 10‐fold less than recommendations in the CyberKnife shielding guidelines. Conclusion: The current guidelines were found to be adequate for shielding, even in a busy center. There may be a potential to reduce shielding in areas with no or few direct beams. The shielding design using the parameters mentioned requires less shielding in term of meeting integrated dose goals, but CyberKnife treatments can Result in high instantaneous dose rates to restricted portions of the surrounding walls. The uneven distribution of use factor may be considered in the vault design.

SU‐E‐T‐422: Evaluation of PTV Dose Margins for Lung SBRT On Cyberknife Treatment

Purpose: The purpose of this study is to retrospectively evaluate the PTV margins used on CyberKnife for lung SBRT treatment. Methods: The CyberKnife system uses an active tracking system that models the motion of the target based on information from orthogonal images and the skin markers. Deviations between the model predicted position and the real target position show up when patient breaths irregularly. This discrepancy can be up to 1 cm for some situations. Some previous research suggested that 5 mm or 6 mm PTV margins are needed to cover the target 96∼98% of the time. The CK lung SBRT treatment usually prescribed at 55–70% isodose line (Monte Carlo dose calculation), that means the region covered by the full prescription dose in real delivery will be larger than the planned PTV region when the tracking uncertainty is accounted. This indicates that the PTV margin could be less than 5∼6 mm when the real dose is considered. 20 previous treated lung SBRT patients were examined. The real target location relatives to the model position at that time were extracted and the dose was recalculated based on the real target position using an in‐house developed Monte Carlo program. Results: Analysis of all the CyberKnife 20 patients (94 treatments) showed that by using 2 mm physical PTV margin, the region received full prescription dose was 5.4 mm +/− 1.6mm from the GTV. The planning criteria are 100% GTV volume receives at least 100% prescription dose. By consider the tracking uncertainty, for 2 mm physical PTV margin, 99.5 +/− 0.4% GTV volume still receives at least 100% prescription dose. The maximum dose reduced by about 12% on average. Conclusion: Conclusions: Image tracking log from the CyberKnife indicate that a 5mm margin may be overkill when motion tracking is used.