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Purpose: The purpose of this paper is to suggest guidelines for target-dose conformity in gamma knife stereotactic radiosurgery (GKSRS) by taking into account factors that have been linked to GKSRS complications. We also suggest an explanation for the failure of previous studies to find a correlation between improved conformity index and reduced risk of GKSRS toxicity, where the conformity index,CS, is defined as the ratio of the prescription volume, VP, to the target volume, VT. Methods: Previous investigations have shown that symptomatic toxicity in GKSRS is correlated with the volume of nontarget tissue receiving the prescription dose,DP. In this study, we formulated the volume of nontarget tissue,VNTD, receiving doseD ≤ DPas a function of the target volume, prescription volume, and prescription dose. We verified the model for D = 12–15 Gy by comparing VNTD calculated from the model versus VNTD calculated directly for 114 tumors in 63 consecutive patients treated at our institution. Once verified, we used this formulation of VNTD to calculate the volume of nontarget tissue receiving doses between 12 and 15 Gy from published data reported for patients experiencing varying degrees of GKSRS toxicity. Next, assuming that the VNTD values calculated for those patients who had either no toxicity or mild neurological symptoms in the published study represented safe levels of normal tissue irradiated to the dose in question, we substituted these VNTD values into an equation expressing CS in terms of VNTD, VT, and DP, and examined how CS varied as a function of VT and DP. Results: TheR2 value for the correlation between VNTD calculated directly or calculated with the proposed formula for VNTD ranged from 0.98 to 0.99, indicating that the formula accurately models the behavior of the nontarget volume receiving doseD. Applying this formulation of VNTD to historical data suggested that the requirements VNT15 ≤ 2.2 cm3, VNT14 ≤ 2.6 cm3, VNT13 ≤ 3.1 cm3, and VNT12 ≤ 3.8 cm3 minimize the risk of severe complications following GKSRS. Imposing these criteria imply that as the target size increases, delivering a given prescription dose requires increasing target-dose conformity. For tumor sizes >5 cm3CS must be ≤1.2 to restrict VNTD to the values listed above. For very small targets, on the other hand, nearly any reasonable conformity index will lead to acceptable values of VNTD. These observations may explain why previous investigations failed to show a correlation between improved conformity and decreased toxicity in GKSRS, because in these earlier studies the range of conformity indices represented was not wide enough, in particular CS values

A new noninvasive monitoring system for fixing the eye has been developed to treat orbital and choroidal tumors with CyberKnife-based radiotherapy. This device monitors the eye during CT/MRI scanning and during treatment. The results of this study demonstrate the feasibility of the fixation light system for CyberKnife-based treatments of orbital and choroidal tumors and supports the idea that larger choroidal melanomas and choroidal metastases could be treated with CyberKnife without implanting fiducial markers.

A quality assurance method was developed to investigate the effects of residual target motion for hypofractionated spine radiosurgery. The residual target motion (target movement between successive image-guided corrections) was measured on-line via dual x-ray imagers for patients treated with CyberKnife (Accuray, Inc., Sunnyvale, CA), a robotic linear accelerator with intrafractional image-tracking capability. The six degree-of-freedom characteristics of the residual target motion were analyzed, the effects of such motion on patient treatment delivery were investigated by incorporating the probability distribution of the residual motion into the treatment planning dose calculations, and deviations of the doses from those originally planned were calculated. Measurements using a programmable motion phantom were also carried out and compared with the static treatment plan calculations. It was found that the residual target motions were patient specific and typically on the order of 2 mm. The measured dose distributions incorporating the residual target motion also exhibited 2.0 mm discrepancy at the prescription isodose level when compared with the static treatment plan calculations. For certain patients, residual errors introduced significant uncertainties (-1 Gy) for the dose delivered to the spinal cord, especially at the high dose levels covering a small volume of the spinal cord (e.g., 0.1 cc). In such cases, stringent cord constraints and frequent monitoring of the target position should be implemented.

The purpose of this work is to measure the dose outside the treatment field for conformal CyberKnife treatments, to compare the results to those obtained for similar treatments delivered with gamma knife or intensity-modulated radiation therapy (IMRT), and to investigate the sources of peripheral dose in CyberKnife radiosurgery. CyberKnife treatment plans were developed for two hypothetical lesions in an anthropomorphic phantom, one in the thorax and another in the brain, and measurements were made with LiF thermoluminescent dosimeters (TLD-100 capsules) placed within the phantom at various depths and distances from the irradiated volume. For the brain lesion, gamma knife and 6-MV IMRT treatment plans were also developed, and peripheral doses were measured at the same locations as for the CyberKnife plan. The relative contribution to the CyberKnife peripheral dose from inferior- or superior-oblique beams entering or exiting through the body, internally scattered radiation, and leakage radiation was assessed through additional experiments using the single-isocenter option of the CyberKnife treatment-planning program with different size collimators. CyberKnife peripheral doses (in cGy) ranged from 0.16 to 0.041 % ({+-}0.003%) of the delivered number of monitor units (MU) at distances between 18 and 71 cm from the field edge. These values are two tomore » five times larger than those measured for the comparable gamma knife brain treatment, and up to a factor of four times larger those measured in the IMRT experiment. Our results indicate that the CyberKnife peripheral dose is due largely to leakage radiation, however at distances less than 40 cm from the field edge, entrance, or exit dose from inferior- or superior-oblique beams can also contribute significantly. For distances larger than 40 cm from the field edge, the CyberKnife peripheral dose is directly related to the number of MU delivered, since leakage radiation is the dominant component.« less

The authors investigated the peripheral dose reduction for CyberKnife radiosurgery treatments after the installation of a linac shielding upgrade. As in a previous investigation, the authors considered two treatment plans, one for a hypothetical target in the brain and another for a target in the thorax, delivered to an anthropomorphic phantom. The results of the prior investigation showed that the CyberKnife delivered significantly higher peripheral doses than comparable model C Gamma Knife or IMRT treatments. Current measurements, after the linac shielding upgrade, demonstrate that the additional shielding decreased the peripheral dose, expressed as a percentage of the delivered monitor units (MU), by a maximum of 59%. The dose reduction was greatest for cranial-caudal distances from the field edge less than 30 cm, and at these distances, the CyberKnife peripheral dose, expressed as a percentage of the delivered MU, is now comparable to that measured for the other treatment modalities in our previous investigation. For distances between 30 and 70 cm from the field edge, the additional shielding reduced the peripheral dose by between 20% and 55%. At these distances, the CyberKnife peripheral dose remains higher than doses measured in our previous study for the model C Gamma Knife and IMRT.

The authors investigated the peripheral dose reduction for CyberKnife radiosurgery treatments after the installation of a linac shielding upgrade. As in a previous investigation, the authors considered two treatment plans, one for a hypothetical target in the brain and another for a target in the thorax, delivered to an anthropomorphic phantom. The results of the prior investigation showed that the CyberKnife delivered significantly higher peripheral doses than comparable model C Gamma Knife or IMRT treatments. Current measurements, after the linac shielding upgrade, demonstrate that the additional shielding decreased the peripheral dose, expressed as a percentage of the delivered monitor units (MU), by a maximum of 59%. The dose reduction was greatest for cranial-caudal distances from the field edge less than 30 cm, and at these distances, the CyberKnife peripheral dose, expressed as a percentage of the delivered MU, is now comparable to that measured for the other treatment modalities in our previous investigation. For distances between 30 and 70 cm from the field edge, the additional shielding reduced the peripheral dose by between 20% and 55%. At these distances, the CyberKnife peripheral dose remains higher than doses measured in our previous study for the model C Gamma Knife and IMRT.

Purpose: Target contouring for high-dose treatments such as radiosurgery of brain metastases is highly critical in eliminating marginal failure and reducing complications as shown by recent clinical studies. In order to improve contouring accuracy and practice consistency for the procedure, we introduced a self-assessed physics lab practice for the physicians-in-training. Methods: A set of commercially acquired high-precision PMMA plastic spheres were randomly embedded in a Styrofoam block and then scanned with the CT/MR via the clinical procedural imaging protocol. A group of first-year physicians-in-training (n=6) from either neurosurgery or radiation oncology department were asked to contour the scanned objects (diameter ranged from 0.4 cm to 3.8 cm). These user-defined contours were then compared with the ideal contour sets of object shape for self assessments to determine the maximum areas of the observed discrepancies and method of improvements. Results: The largest discrepancies from initial practice were consistently found to be located near the extreme longitudinal portions of the target for all the residents. Discrepancy was especially prominent when contouring small objects 0.23more » suggesting agreement cannot be established). However, when incorporating a secondary imaging scan such as reconstructed coronal or sagittal images in a repeat practice, the agreement was dramatically improved yielding p

The purpose of this work is to measure the dose outside the treatment field for conformal CyberKnife treatments, to compare the results to those obtained for similar treatments delivered with gamma knife or intensity-modulated radiation therapy (IMRT), and to investigate the sources of peripheral dose in CyberKnife radiosurgery. CyberKnife treatment plans were developed for two hypothetical lesions in an anthropomorphic phantom, one in the thorax and another in the brain, and measurements were made with LiF thermoluminescent dosimeters (TLD-100 capsules) placed within the phantom at various depths and distances from the irradiated volume. For the brain lesion, gamma knife and 6-MV IMRT treatment plans were also developed, and peripheral doses were measured at the same locations as for the CyberKnife plan. The relative contribution to the CyberKnife peripheral dose from inferior- or superior-oblique beams entering or exiting through the body, internally scattered radiation, and leakage radiation was assessed through additional experiments using the single-isocenter option of the CyberKnife treatment-planning program with different size collimators. CyberKnife peripheral doses (in cGy) ranged from 0.16 to 0.041% (+/- 0.003%) of the delivered number of monitor units (MU) at distances between 18 and 71 cm from the field edge. These values are two to five times larger than those measured for the comparable gamma knife brain treatment, and up to a factor of four times larger those measured in the IMRT experiment. Our results indicate that the CyberKnife peripheral dose is due largely to leakage radiation, however at distances less than 40 cm from the field edge, entrance, or exit dose from inferior- or superior-oblique beams can also contribute significantly. For distances larger than 40 cm from the field edge, the CyberKnife peripheral dose is directly related to the number of MU delivered, since leakage radiation is the dominant component.

Purpose: To develop a customizable tool for enterprise-wide managing of CT protocols and analyzing radiation dose information of CT exams for a variety of quality control applications Methods: All clinical CT protocols implemented on the 11 CT scanners at our institution were extracted in digital format. The original protocols had been preset by our CT management team. A commercial CT dose tracking software (DoseWatch,GE healthcare,WI) was used to collect exam information (exam date, patient age etc.), scanning parameters, and radiation doses for all CT exams. We developed a Matlab-based program (MathWorks,MA) with graphic user interface which allows to analyze the scanning protocols with the actual dose estimates, and compare the data to national (ACR,AAPM) and internal reference values for CT quality control. Results: The CT protocol review portion of our tool allows the user to look up the scanning and image reconstruction parameters of any protocol on any of the installed CT systems among about 120 protocols per scanner. In the dose analysis tool, dose information of all CT exams (from 05/2013 to 02/2014) was stratified on a protocol level, and within a protocol down to series level, i.e. each individual exposure event. This allows numerical and graphical review ofmore » dose information of any combination of scanner models, protocols and series. The key functions of the tool include: statistics of CTDI, DLP and SSDE, dose monitoring using user-set CTDI/DLP/SSDE thresholds, look-up of any CT exam dose data, and CT protocol review. Conclusion: our inhouse CT management tool provides radiologists, technologists and administration a first-hand near real-time enterprise-wide knowledge on CT dose levels of different exam types. Medical physicists use this tool to manage CT protocols, compare and optimize dose levels across different scanner models. It provides technologists feedback on CT scanning operation, and knowledge on important dose baselines and thresholds.« less

Purpose: Intensity modulated arc beams have been newly reported for treating multiple brain metastases. The purpose of this study was to determine the variations in the normal brain doses with increasing number of arc beams for multiple brain metastases treatments via the TrueBeam Rapidarc system (Varian Oncology, Palo Alto, CA). Methods: A patient case with 12 metastatic brain lesions previously treated on the Leksell Gamma Knife Perfexion (GK) was used for the study. All lesions and organs at risk were contoured by a senior radiation oncologist and treatment plans for a subset of 3, 6, 9 and all 12 targets were developed for the TrueBeam Rapidarc system via 3 to 7 intensity modulated arc-beams with each target covered by at least 99% of the prescribed dose of 20 Gy. The peripheral normal brain isodose volumes as well as the total beam-on time were analyzed with increasing number of arc beams for these targets. Results: All intensisty modulated arc-beam plans produced efficient treatment delivery with the beam-on time averaging 0.6–1.5 min per lesion at an output of 1200 MU/min. With increasing number of arc beams, the peripheral normal brain isodose volumes such as the 12-Gy isodose line enclosed normal brain tissue volumes were on average decreased by 6%, 11%, 18%, and 28% for the 3-, 6-, 9-, 12-target treatment plans respectively. The lowest normal brain isodose volumes were consistently found for the 7-arc treatment plans for all the cases. Conclusion: With nearly identical beam-on times, the peripheral normal brain dose was notably decreased when the total number of intensity modulated arc beams was increased when treating multiple brain metastases. Dr Sahgal and Dr Ma are currently serving on the board of international society of stereotactic radiosurgery.

Purpose: Residual uncertainties on the order of 1-2 mm are frequently observed when delivering stereotactic radiosurgery via on-line imaging guidance with a relocatable frame. In this study, a predictive model was developed to evalute potentiral late radiation effects associated with such uncertainties. Methods: A mathematical model was first developed to correlate the peripherial isodose volume with the internal and/or setup margins for a radiosurgical target. Such a model was then integrated with a previoulsy published logistic regression normal tissue complication model for determining the symptomatic radiation necrosis rate at various target sizes and prescription dose levels. The model was tested on a cohort of 15 brain tumor and tumor resection cavity patient cases and model predicted results were compared with the clinical results reported in the literature. Results: A normalized target diameter (D{sub 0}) in term of D{sub 0} = 6V/S, where V is the volume of a radiosurgical target and S is the surface of the target, was found to correlate excellently with the peripheral isodose volume for a radiosurgical delivery (logarithmic regression R{sup 2} > 0.99). The peripheral isodose volumes were found increase rapidly with increasing uncertainties levels. In general, a 1-mm residual uncertainties as calculated to resultmore » in approximately 0.5%, 1%, and 3% increases in the symptomatic radiation necrosis rate for D{sub 0} = 1 cm, 2 cm, and 3 cm based on the prescription guideline of RTOG 9005, i.e., 21 Gy to a lesion of 1 cm in diameter, 18 Gy to a lesion 2 cm in diameter, and 15 Gy to a lesion 3 cm in diameter respectively. Conclusion: The results of study suggest more stringent criteria on residual uncertainties are needed when treating a large target such as D{sub 0}≤ 3 cm with stereotactic radiosurgery. Dr. Ma and Dr. Sahgal are currently serving on the board of international society of stereotactic radiosurgery (ISRS)« less

Purpose: Volumetric modulated arc therapy (VMAT) has enabled rapid treatments of multiple brain tumors with a single or few isocenters. We investigated inter‐lesion dose interplay effects for such a treatment and compared against standard multi‐isocenteric Gamma Knife (GK) or dynamic‐conformal‐arc (DCA) SRS deliveries. Methods: A patient case with 12 intracranial targets and simulated cases with 2–60 targets in the brain parenchyma were used for the study. For the patient case, all targets and organs‐at‐risk were contoured by a senior clinician. A subset of 3, 6, 9 and 12 targets were then planned at different institutions for GK, DCA and VMAT SRS. Identical dose‐volume constraints to the targets and critical structures were applied. Each target was prescribed with 20 Gy covering at least 99% of the target volume. Relationships between the mean 4‐Gy to 12‐Gy isodose volumes per lesion versus increasing number of lesions were analyzed for each modality. Results: For all the cases, 12‐Gy isodose volumes per lesion exhibited negligible dependence with the increasing number of targets for GK SRS and DCA deliveries. However, for VMAT delivery, a strong statistically significant dependence with increasing number of targets was found at all levels of isodose volumes. For example, the increase in the 12‐Gy volumes of the patient case was 0.068+/−0.016 cc/lesion (p=0.05) for the VMAT delivery in contrast to 0.0+/−0.0 cc/lesion (p < 0.0001) for both GK and DCA deliveries. The increase in the 4‐Gy isodose volumes was 2.79+/−0.40 cc/lesion (p=0.02) for the VMAT delivery, and 2.13+/−0.15 cc/lesion (p=0.005) for the DCA delivery in contrast to 0.08+/−0.29 cc/lesion (p=0.10) for the GK delivery. Conclusion: Significant dose interplay effects were found for single‐or few‐isocenter VMAT SRS of multiple lesions, somewhat for multi‐isocenteric DCA SRS, but nearly negligible for GK SRS treatments.

Purpose: Small‐volume biologically effective (BED) dose limits are critical to safe spinal stereotactic body radiotherapy(SBRT) delivery. However, due to mismatch in spatial location of dose hot spots from non‐uniform dose distributions inherent to SBRT, for repeat treatment courses they cannot be simply added by assuming a uniform dose distribution. This study aims to develop a probability‐based biological equivalent dose formula to solve this problem. Methods: A generalized biological equivalent dose (gBED) was formulated via computing damaging or survival probability of repeat spine SBRT treatments. Parameters from the linear‐quadratic model such as α/β =2 Gy for the spinal cord were applied for the gBED calculations. The derived method was applied to both simulated and clinical treatment cases to demonstrate its applicability and usefulness for assessing spinal cord dose limits for repeated SBRT treatment courses. Results: The gBED formula allows direct superposition of dose within a small volume of spinal cord from a non‐uniform dose distribution of varying dose fractionation schemes of SBRT. From the studied examples, traditional BED calculations even with full voxel‐by‐voxel tracking calculations resulted in inconsistent BED values and can underestimate the biological dose to a small‐volume spinal cord by as much as 20%. Such an error tends to increase rapidly with increasing volume of interests such as from 0.1 mL to 2.0 mL. Conclusions: When assessing spinal cord tolerance for repeat spinal SBRT treatments, consistent surrogates such as gBED are needed to avoid potential underestimation of treatment‐induced complications.

Purpose: The purpose of the study was to develop a procedure using selective region mutual information image registration and anatomic landmarks transformation to improve the treatment accuracy of volume‐staged Gamma Knife (GK) radiosurgery for large arteriovenous malformations (AVMs). Methods: The procedure was implemented via Leksell Gamma Plan (LGP version 9.0) and an in‐house point registration program for multiple session staged GK treatments. A requirement of such treatments was to match the partial‐volume dose distribution of a previously delivered treatment with the current treatment. In our procedure, MR images of the current treatment plan were first imported and co‐registered with the initial reference MR studies via selective region mutual information algorithm in LGP. Then the current MR studies were re‐imported and defined for its stereotactic frame. Based on these two MR studies (one registered and one defined), four or more anatomic landmarks surrounding the target region were selected, and then a rigid‐body transformation matrix was calculated from these points to map the original treatment plan onto the current treatment plan for the existing frame coordinate system. We analyzed the accuracy of such procedures performed for a group of 12 patients with 14 staged treatments at our institution since 2007. Results: The average discrepancy in the landmark vector distance between the original treatment plan and the transformed treatment plan was 0.25±0.09 mm. This value was 3–4 times smaller than previously published results. Additional studies on the accuracy of mutual information registration of MR‐MR images of the same objects/patients on our LGP system yielded a mean standard deviation of 0.16±0.07 mm, where the largest deviation was generally observed along the image scanning direction Conclusions: Volume‐staged GK treatments can be delivered to the same level of accuracy as single fraction GK treatments.

Purpose: Planning roboticradiosurgery for patients with 3 or more metastatic brain lesions is due to the need for a large number of constraints and the requirement of prescribing different doses to separate targets. A two‐step optimization technique was developed to improve the plan quality of such treatments.Method and Materials: First, separate treatment plans were developed for individual targets via setting nominal doses to each target disregarding contributions from other lesions. Then the 3D dose matrix associated with each target was exported and a singular‐value‐decomposition algorithm was applied to balance the background dose changes as well as dose interferences among targets via adjusting relative weightings among the dose matrices. Comparisons were carried out for such approach versus a planning‐for‐all approach implemented on the latest commercial system. Results: Significant improvements were found in the target volume coverage with the new technique as compared with the existing plan‐for‐all techniques due to the capability to select individualized isodose levels for each target. Additionally, improvement was particularly noted for low‐level peripheral normal brain sparing. On average, the volume of the normal brain receiving more than 4‐Gy was reduced by 13% with the new technique for the studied cases. The improvement was especially notable when a high number (n>5) of targets are considered. Conclusion: A two‐step optimization technique has been demonstrated to improve treatment planning quality as well as the flexibility of dose prescription for multi‐target treatment planning for roboticradiosurgery.

Purpose: Intracranial radiosurgery commonly employs many isocenters or beams to deliver dose distributions over an extended treatment time (30–100 minutes or more). In this study, we investigate whether the sequential order of delivering these isocenters or beams would significantly influence the biological effectiveness of the treatment and how to optimize a delivery sequence based on such considerations. Method and Materials: A group of patient cases treated with Gamma Knife Perfexion at our institution were analyzed All permutation possibilities of the delivery sequence were reconstructed for each patient. For each possibility, the time‐sequence of 3D dose accumulation within the target and the surrounding normal structures was computed. Then the equivalent uniform dose (EUD) based on a generalized linear‐quadratic model formula was calculated for each possible delivery sequence. Variations in the EUD values from all combinations were calculated. Dependence of EUD values for the alpha‐beta ratios ranging from 2 to 20 was also investigated for each case in order to determine its effect on the results. Results: The maximum EUD and minimum EUD values varied approximately 10–15%, or 1–2 Gy for different order of delivery sequences depending on the target size and the target peripheral dose. The effect was found to be dependent on the total number of isocenters and dose interference among them. Furthermore, discrepancy in the EUD values was found to increase with decreasing alpha‐beta values, suggesting the effect being most relevant for treating lesions such as large AVM, where alpha‐beta ratio is supposedly low and a high number of isocenters are typically used. Conclusion: Optimizing sequential order of beam delivery based on biological considerations is important in minimizing treatment‐related EUD variations and ensuring consistency in treatmentdosimetry.

Purpose: Real time tumor tracking can be performed using the Synchrony respiratory tracking (CyberKnife, Accuray). This study evaluates the range of target correction margins required for treatment of patients with lungtumors using this system. Method and Materials: In this study, twenty‐eight lung patients treated using Synchrony were selected. The moving average of the correction margin (CM), i.e., the difference between the predicted tumor positions versus on‐line x‐ray measured tumor positions, was extracted for each fraction as well as the entire course of the treatment for each patient. The population‐based CM was then derived using the modified van Herk margin formula taking into account the hypofractionated regimen. This modified margin recipe also incorporated the situations of small number of patients, N

Purpose: Non‐random target motions have been reported for hypofractionated spine stereotactic body radiotherapy(SBRT), largely due to prolonged treatment delivery as compared to conventional radiotherapy. In this study, we aim to develop an adaptive correction strategy to overcome such non‐random target motions. Method and Materials: Intra‐fraction target motions of more than 200 treatment sessions were analyzed. These target motions were detected using an in‐room dual kV x‐ray imagingsystem. Non‐random target motions was characterized in six degree of freedom (DOF) that included translations (x‐, y‐, z‐ shifts) and rotations (roll, yaw, pitch). Based on the observed incidence and motion characteristics, a correction strategy based on periodic interventions (e.g., via realigning the patient or the beams) was developed in order to correct the effects of such motions to an acceptable level. The population averaged time intervals for implementing the strategy were calculated for different treatment sites that included cervical, thoracic, and lumbar‐sacral lesions. Results: Non‐random target motions were found to be present for every case studied regardless of target locations. Cervical spine targets were found to possess the highest incidences of non‐random target motions as compared to other sites (p

Purpose: The peripheral dose volume such as 10‐Gy or 12‐Gy volume has been reported to correlate with incidence of radionecrosis for stereotactic radiosurgery(SRS) or radiotherapy (SRT). In this study, we investigated whether variations in peripheral dose fall‐off of individual treatments would significantly affect normal tissue sparing for SRS or SRT. Method and Materials: A power relationship for measuring general dose fall‐off near the target was developed for both isocentric and non‐isocentric intracranial radiosurgery modalities. For the study, we sampled the ranges of variations in the peripheral dose distributions from patients treated with Gamma Knife Perfexion, Cyberknife, or linac‐based delivery system. Equivalent uniform biological effective dose (EUBED) for the normal braintissue was formulated to study the effect of dose‐fall variations on treatments of different targets of varying a/b values. Functional relationship of normal brain EUBED with increasing number of fractions was derived for treating either fast growing tumors with a/b of 10–20) or abnormal tissues such as AVM with possible a/b of 2–5. Results: The average g‐index from the derived power formula was found to be −1.49±0.25 with a range of −1.19 to −1.79. Based on such range, EUBED of the normal brain was calculated and found to decrease with increasing number of fractions for targets with a/b of 10–20. This decrease was most pronounced for fractions fewer than 10. However, the EUBED was found to slightly increase with increasing number of fractions for targets with a/b of 2–5. Conclusion: In delivering SRS or SRT, normal tissue EUBED was found to favor hypofractionated treatments for fast growing tumors with a/b of 10–20, but single fraction treatments for abnormal tissues with a/b of 2–5. The result was found to be insensitive to variations in dose fall‐off from individual cases regardless being treated with Gamma Knife, Cyberknife or Linac‐based modality.

Purpose: To investigate whether a common relationship exists governing the near‐target dose fall‐off for Gamma Knife, Cyberknife and MLC‐based Novalis linac systems. Method and Materials: Three groups of intracranial patient cases treated with Gamma Knife, Cyberknife, Novalis MLC‐based system were selected for the study. Each group contains 10–20 delivered treatment plans. In each case, the target was covered conformally by a peripheral isodose line ranging from 45% (Gamma Knife) to 95% (Novalis) of the maximum dose inside the target. Based on the divergence theorem, the dose fall‐off near the target was derived as V / V 0 = ( D / D 0 ) −γ , where V0 is volume of the prescribed isodose line covering at least 99% of the target volume, γ is a constant fitting parameter, which measures the steepness of the dose fall‐off near the target. The goodness of the fit and the average γ‐values were obtained for all cases of each modality. Results: The formula fitted excellently for all cases with the linear correlation coefficient R2 exceeded 0.99 in the log‐log plot for each case. No obvious dependence was found for the size of the targets, the number of the beams or the isocenters used for each group. An average γ‐value of 1.49± 0.13 was obtained for Gamma Knife, 1.48± 0.41 for Cyberknife, and 1.46 ± 0.19 for Novalis group of cases. No statistical significance was found in the mean value differences among the groups. However, Gamma Knife exhibited the smallest range of deviations in the mean value while Cyberknife exhibited the largest range of deviations in the mean value. Conclusion: A general formula and a common parameter was demonstrated for relating the average dose fall‐off near the target for Gamma Knife, Cyberknife and Novalis MLC‐based Linac systems. Despite large physical differences, nearly identical dose fall‐off was found for these modalities for treating intracranial lesions.

Purpose: Vastly different dose fractionation and dose prescription schemes exist for radiosurgery and body radiotherapy of intracranial and extracranial tumors. A generalized biological equivalent dose (gBED) model has been developed to account for variations in dose fractionations and non‐uniform dose distributions for both targets and critical structures of such treatments. Method and Materials: Assuming cell survival fraction (S) can be expressed as S = e −α gBED , we derived gBED = ∑ i w i BED i where w i = v i S(d i / ∑ i S(d i ), BED i = nd i [1+d i /(α/β)] of the linear‐quadratic model, and vi is the ith voxel receiving the dose of di as in the calculation of the dose volume histogram. In the above gBED formula, wi is the probability‐weighted volume unit which is analogous to the voxel mass (density×volume). As a result, we derived a new histogram by plotting accumulative BEDi versus accumulative wi and we called it Dose Unit Histogram (DUH). To study DUH model, analyses were applied to a group of >20 radiosurgery and body radiotherapy cases of varying fractionations and dose volume histograms. The dependence of gBED on α/β values (ranging from 2–20) and other physical dose parameters were also investigated. Results: The normalized DUH fell consistently below the normalized DVH curves regardless of α/β values. This indicates a decreased effect to target and increased tolerance of the normal structure. For high α/β values such as 20, gBED approached to the conventional BED calculated from the mean dose of a volume. For low values of α/β such as 2, gBED was significantly different, particularly for non‐uniform target dose distributions. Conclusion: A generalized BED formula was developed and used for radiosurgery and body radiotherapy planning analysis. The formula yielded different histogram plots from conventional DVH by accounting for the variations in dose fractionation and non‐uniformity of the dose distributions.

Purpose: With the Gamma Knife Automatic Positioning System, high dose spikes can be placed inside a target using 4‐mm shots to boost the central target dose without affecting adjacent normal brain sparing. Potential applications of the technique include target boost when nearby critical structures such as optical structure limit the peripheral dose or increasing the dose to centrally hypoxic regions. This study investigates how the spatial distribution and height of the spikes can be optimized for this purpose. Method and Materials: A computer program was developed to optimize the pattern of spike boost placed over conventional Gamma Knife plans. The program first extracts the prescription isodose volume and then iteratively adjusts the positions and the heights of the spikes while accounting for the existing dose distribution. Different boost patterns were generated for both patient and phantom cases. The program calculated the stereotactic coordinates and weights of the spike shots, which were manually entered into the Leksell Gamma Plan for dose calculation and dose‐volume evaluations of the modified treatment plans. Results: The total number of dose spikes and the maximum dose for the spike shots correlates with the target size (R2> 0.9). For the cases studied, an increase in average dose to the target of 15–28% was noted while the conformity index of target volume coverage remained unchanged. The dose to the adjacent normal brain (defined as the volume enclosed by half of the prescription isodose line excluding the target) remained unchanged to better than 2% of the maximum dose. Overall, spike patterns arranged along the radial direction consistently give a higher average dose to the target than for spikes arranged along the translational directions. Conclusion: Optimizing the shot weights and the spike patterns enhances dose to the target volume while maintaining normal brain sparing for Gamma Knife radiosurgery.

Purpose: To measure doses delivered outside of the irradiated volume for typical CyberKnife treatments and to compare the results to peripheral doses arising from similar Gamma Knife (GK) and IMRTtreatments.Method and Materials: CyberKnife treatment plans were developed for two hypothetical lesions in an anthropomorphic phantom, one in the thorax and another in the brain. In both cases, 500 cGy was prescribed to the 70% isodose line. Li‐F TLD‐100 capsules were placed within the phantom at various depths and distances from the irradiated volume. For the brain lesion, GK and 6‐MV IMRTtreatment plans were also developed, and peripheral doses were measured. Results: Peripheral doses for the CyberKnife thorax treatment ranged from 3.3±0.13% to 1.2±0.06% of the prescribed dose (Dp) at distances between 15 and 43 cm from the edge of the target. For the target in the brain, CyberKnife peripheral doses ranged from 1.2±0.02% to 0.32±0.02% of Dp at distances between 18 and 71 cm from the target edge. In comparison, the GK peripheral dose ranged from 0.63±0.03% to 0.053±0.002% of Dp, and the IMRT plan resulted in doses between 0.19±0.004% and 0.043±0.002% of Dp over the same range of distances. Conclusion: Doses outside the irradiated volume for Cyberknife treatments are significantly higher than those encountered in standard radiation therapy. Peripheral doses given in AAPM Task Group Report No. 36 (Stovall, et al. Med. Phys. 22:63–82, 1995) for a 6‐MV beam are between 0.2% and 0.02% of the dose at dmax at a distances ranging from 20 to 70 cm from the edge of a 5×5cm2 field. Furthermore, for the same target in the brain, CyberKnife peripheral doses were a factor of 2 to 6 times larger than those for GK, and a at least a factor of 6 times larger than those for IMRT.

Purpose: The CyberKnife Synchrony system allows respiratory motion compensated treatment of lung lesions implanted with fiducials, using a model of the relative motion of the fiducials and infrared diodes on the chest. Often implantation of fiducials in the lesion is not feasible because of the potential for pneumothorax. We compare the following two alternative strategies 1. Synchrony treatment with fiducials in the anterior chest wall 2. Non‐Synchrony treatment with fiducials in the posterior chest wall Method and Materials: The lung lesion patients were scanned using a GE Lightspeed Multi‐Slice scanner to obtain 300 1.25mm thick slices through the area of interest during one breath‐hold. This was performed three times covering full inspiration, full expiration, and mid‐inspiration. Fiducials were implanted percutaneously in the anterior and posterior chest wall. The lesion and position of the fiducials were delineated on all three sets of CT scans. If the motion of the lesion tracked the motion of the anterior chest wall fiducials, then a Synchrony plan was developed to treat the lesion as drawn on the mid‐inspiration CT scan. If the lesion motion did not track with the anterior chest wall fiducials then a non‐Synchrony plan was developed that treated the lesion as seen on all three sets of CT scans using the fiducials in the posterior chest wall for tracking. Results: Using anterior chest wall fiducials, the lesion motion tracked to within 1–2mm in X and Y, and 5–6mm in Z. The target was extended in Z to accommodate this. For non‐Synchrony treatments the target volumes increased by 60 to 75% and the prescription isodose volumes increased by 65 to 90%. Conclusion: The treatment of lung lesions presents a difficult challenge especially if fiducials cannot be implanted in the lesion to allow full Synchrony treatment, but the techniques described will still allow treatment.