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AAPM Task Group Report 135.B covers new technology components that have been added to an established radiosurgery platform and updates the components that were not well covered in the previous report. Considering the current state of the platform, this task group (TG) is a combination of a foundational task group to establish the basis for new processes/technology and an educational task group updating guidelines on the established components of the platform. Because the technology discussed in this document has a relatively small user base compared to C-arm isocentric linacs, the authors chose to emphasize the educational components to assist medical physicists who are new to the technology and have not had the opportunity to receive in-depth vendor training at the time of reading this report. The TG has developed codes of practice, introduced QA, and developed guidelines which are generally expected to become enduring practice. This report makes prescriptive recommendations as there has not been enough longitudinal experience with some of the new technical components to develop a data-based risk analysis., (© 2024 The Author(s). Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Background and Purpose: Effective respiratory motion management reduces healthy tissue toxicity and ensures sufficient dose delivery to lung cancer cells in pulmonary stereotactic body radiation therapy (SBRT) with high fractional doses. An articulated robotic arm paired with an X-ray imaging system is designed for real-time motion-tracking (RTMT) dose delivery. However, small tumors (<15 mm) or tumors at challenging locations may not be visible in the X-ray images, disqualifying patients with such tumors from RTMT dose delivery unless fiducials are implanted via an invasive procedure. To track these practically invisible lung tumors in SBRT, we hereby develop a deep learning-enabled template-free tracking framework, SAFE Track., Methods: SAFE Track is a fully supervised framework that trains a generalizable prior for template-free target localization. Two sub-stages are incorporated in SAFE Track, including the initial pretraining on two large-scale medical image datasets (DeepLesion and Node21) followed by fine-tuning on our in-house dataset. A two-stage detector, Faster R-CNN, with a backbone of ResNet50, was selected as our detection network. 94 patients (415 fractions; 40,348 total frames) with low tumor visibility who thus had implanted fiducials were included. The cohort is categorized by the longest dimension of the tumor (<10 mm, 10-15 mm and > 15 mm). The patients were split into training (n = 66) and testing (n = 28) sets. We simulated fiducial-free tumors by removing the fiducials from the X-ray images. We classified the patients into two groups - fiducial implanted inside tumors and implanted outside tumors. To ensure the rigor of our experiment design, we only conducted fiducial removal simulation in training patients and utilized patients with fiducial implanted outside of the tumors for testing. Commercial Xsight Lung Tracking (XLT) and a Deep Match were included for comparison., Results: SAFE Track achieves promising outcomes to as accurate as 1.23±1.32 mm 3D distance in testing patients with tumor size > 15 mm where Deep Match is at 4.75±1.67 mm and XLT is at 12.23±4.58 mm 3D distance. Even for the most challenging tumor size (<10 mm), SAFE Track maintains its robustness at 1.82 plus or minus 1.67 mm 3D distance, where Deep Match is at 5.32 plus or minus 2.32 mm, and XLT is at 24.83±12.95 mm 3D distance. Moreover, SAFE Track can detect some considerably challenging cases where the tumor is almost invisible or overlapped with dense anatomies., Conclusion: SAFE Track is a robust, clinically compatible, fiducial-free, and template-free tracking framework that is applicable to patients with small tumors or tumors obscured by overlapped anatomies in SBRT., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Purpose: CT Hounsfield Units (HUs) are converted to electron density using a calibration curve obtained from physical measurements of an electron density phantom. HU values assigned to an MRI-derived synthetic computed tomography (sCT) may present a different relationship with electron density compared to CT HU. Correct assignment of sCT HU values is critical for accurate dose calculation and delivery. The goals of this work were to develop a sCT calibration curve using patient data acquired on a clinically commissioned CT scanner and assess for CyberKnife- and volumetric modulated arc therapy (VMAT)-based MR-only treatment planning of prostate SBRT., Methods: Same-day CT and MRI simulation in the treatment position were performed on 10 patients treated with SBRT to the prostate. Dixon in-phase and out-of-phase MRIs were acquired on a 3T scanner using a 3D T1-weighted gradient-echo sequence to generate sCTs using a commercial sCT algorithm. CT and sCT datasets were co-registered and HU values compared using mean absolute error (MAE). An optimized HU-to-density calibration curve was created based on average HU values across an institutional patient database for each of the four sCT tissue types. Clinical CyberKnife and VMAT treatment plans were generated on each patient CT and recomputed onto corresponding sCTs. Dose distributions computed using CT and sCT were compared using gamma criteria and dose-volume-histograms., Results: For the optimized calibration curve, HU values were -96, 37, 204, and 1170 and relative electron densities were 0.95, 1.04, 1.1, and 1.7 for adipose, soft tissue, inner bone, and outer bone, respectively. The proposed sCT protocol produced total MAE of 94 ± 20HU. Gamma values mean ± std (min-max) were 98.9% ± 0.9% (97.1%-100%) and 97.7% ± 1.3% (95.3%-99.3%) for VMAT and CyberKnife plans, respectively., Conclusion: MRI-derived sCT using the proposed approach shows excellent dosimetric agreement with conventional CT simulation, demonstrating the feasibility of MRI-derived sCT for prostate SBRT treatment planning., (© 2024 The Author(s). Journal of Applied Clinical Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Background and Purpose: Motion management is essential to reduce normal tissue exposure and maintain adequate tumor dose in lung stereotactic body radiation therapy (SBRT). Lung SBRT using an articulated robotic arm allows dynamic tracking during radiation dose delivery. Two stereoscopic X-ray tracking modes are available - fiducial-based and fiducial-free tracking. Although X-ray detection of implanted fiducials is robust, the implantation procedure is invasive and inapplicable to some patients and tumor locations. Fiducial-free tracking relies on tumor contrast, which challenges the existing tracking algorithms for small (e.g., <15 mm) and/or tumors obscured by overlapping anatomies. To markedly improve the performance of fiducial-free tracking, we proposed a deep learning-based template matching algorithm - Deep Match., Method: Deep Match consists of four self-definable stages - training-free feature extractor, similarity measurements for location proposal, local refinements, and uncertainty level prediction for constructing a more trustworthy and versatile pipeline. Deep Match was validated on a 10 (38 fractions; 2661 images) patient cohort whose lung tumor was trackable on one X-ray view, while the second view did not offer sufficient conspicuity for tumor tracking using existing methods. The patient cohort was stratified into subgroups based on tumor sizes (<10 mm, 10-15 mm, and >15 mm) and tumor locations (with/without thoracic anatomy overlapping)., Results: On X-ray views that conventional methods failed to track the lung tumor, Deep Match achieved robust performance as evidenced by >80 % 3 mm-Hit (detection within 3 mm superior/inferior margin from ground truth) for 70 % of patients and <3 mm superior/inferior distance (SID) ∼1 mm standard deviation for all the patients., Conclusion: Deep Match is a zero-shot learning network that explores the intrinsic neural network benefits without training on patient data. With Deep Match, fiducial-free tracking can be extended to more patients with small tumors and with tumors obscured by overlapping anatomy., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Background: Image-guided radiation-therapy (IGRT)-based robotic radiosurgery using magnetic resonance imaging (MRI)-only simulation could allow for improved target definition with highly conformal radiotherapy treatments. Fiducial marker (FM)-based alignment is used with robotic radiosurgery treatments of sites such as the prostate because it aids in accurate target localization. Synthetic CT (sCT) images are generated in the MRI-only workflow but FMs used for IGRT appear as signal voids in MRIs and do not appear in MR-generated sCTs, hindering the ability to use sCTs for fiducial-based IGRT., Purpose: In this study we evaluate the fiducial tracking accuracy for a novel artificial fiducial insertion method in sCT images that allows for fiducial marker tracking in robotic radiosurgery, using MRI-only simulation imaging (MRI-only workflow)., Methods: Artificial fiducial markers were inserted into sCT images at the site of the real marker implantation as visible in MRI. Two phantoms were used in this study. A custom anthropomorphic pelvis phantom was designed to validate the tracking accuracy for a variety of artificial fiducials in an MRI-only workflow. A head phantom containing a hidden target and orthogonal film pair inserts was used to perform end-to-end tests of artificial fiducial configurations inserted in sCT images. The setup and end-to-end targeting accuracy of the MRI-only workflow were compared to the computed tomography (CT)-based standard. Each phantom had six FMs implanted with a minimum spacing of 2 cm. For each phantom a bulk-density sCT was generated, and artificial FMs were inserted at the implantation location. Several methods of FM insertion were tested including: (1) replacing HU with a fixed value (10000HU) (voxel-burned); (2) using a representative fiducial image derived from a linear combination of fiducial templates (composite-fiducial); (3) computationally simulating FM signal voids using a digital phantom containing FMs and inserting the corresponding signal void into sCT images (simulated-fiducial). All tests were performed on a CyberKnife system (Accuray, Sunnyvale, CA). Treatment plans and digital-reconstructed-radiographs were generated from the original CT and sCTs with embedded fiducials and used to align the phantom on the treatment couch. Differences in the initial phantom alignment (3D translations/rotations) and tracking parameters between CT-based plans and sCT-based plans were analyzed. End-to-end plans for both scenarios were generated and analyzed following our clinical protocol., Results: For all plans, the fiducial tracking algorithm was able to identify the fiducial locations. The mean FM-extraction uncertainty for the composite and simulated FMs was below 48% for fiducials in both the anthropomorphic pelvis and end-to-end phantoms, which is below the 70% treatment uncertainty threshold. The total targeting error was within tolerance (<0.95 mm) for end-to-end tests of sCT images with the composite and head-on simulated FMs (0.26, 0.44, and 0.35 mm for the composite fiducial in sCT, head-on simulated fiducial in sCT, and fiducials in original CT, respectively., Conclusions: MRI-only simulation for robotic radiosurgery could potentially improve treatment accuracy and reduce planning margins. Our study has shown that using a composite-derived or simulated FM in conjunction with sCT images, MRI-only workflow can provide clinically acceptable setup accuracy in line with CT-based standards for FM-based robotic radiosurgery., (© 2023 American Association of Physicists in Medicine.)

Purpose: To perform a propensity-score matched analysis comparing stereotactic body radiation therapy (SBRT) boost and high-dose-rate (HDR) boost for localized prostate cancer., Methods and Materials: A single-institution retrospective chart review was conducted of men treated with pelvic external beam radiation therapy (EBRT) and SBRT boost (21 Gy and 19 Gy in 2 fractions) to the prostate for prostate cancer. A cohort treated at the same institution with HDR brachytherapy boost (19 Gy in 2 fractions) was compared. Propensity-score (PS) matching and multivariable Cox regression were used for analysis. Outcomes were biochemical recurrence freedom (BCRF) and metastasis freedom (MF)., Results: One hundred thirty-one men were treated with SBRT boost and 101 with HDR boost with median follow-up of 73.4 and 186.0 months, respectively. In addition, 68.8% of men had high-risk and 26.0% had unfavorable-intermediate disease, and 94.3% received androgen deprivation therapy. Five- and 10-year unadjusted BCRF was 88.8% and 85.3% for SBRT and 91.8% and 74.6% for HDR boost (log-rank P = .3), and 5- and 10-year unadjusted MF was 91.7% and 84.3% for SBRT and 95.8% and 82.0% for HDR (log-rank P = .8). After adjusting for covariates, there was no statistically significant difference in BCRF (hazard ratio [HR] 0.81; 95% confidence interval [CI], 0.37-1.79; P = .6) or MF (HR 1.07; 95% CI, 0.44-2.57; P = .9) between SBRT and HDR boost. Similarly, after PS matching, there was no statistically significant difference between SBRT and HDR (BCRF: HR 0.66, 0.27-1.62, P = .4; MF: HR 0.84, 0.31-2.26, P = .7). Grade 3+ genitourinary and gastrointestinal toxicity in the SBRT cohort were 4.6% and 1.5%, and 3.0% and 0.0% in the HDR cohorts (P = .4, Fisher exact test)., Conclusions: SBRT boost plus pelvic EBRT for prostate cancer resulted in similar BCRF and MF to HDR boost in this single institution, PS matched retrospective analysis. Toxicity was modest. Prospective evaluation of SBRT boost for the treatment of unfavorable-intermediate and high-risk prostate cancer is warranted., (Copyright © 2020 Elsevier Inc. All rights reserved.)

The goal of this study is to demonstrate the feasibility of a novel fully-convolutional volumetric dose prediction neural network (DoseNet) and test its performance on a cohort of prostate stereotactic body radiotherapy (SBRT) patients. DoseNet is suggested as a superior alternative to U-Net and fully connected distance map-based neural networks for non-coplanar SBRT prostate dose prediction. DoseNet utilizes 3D convolutional downsampling with corresponding 3D deconvolutional upsampling to preserve memory while simultaneously increasing the receptive field of the network. DoseNet was implemented on 2 Nvidia 1080 Ti graphics processing units and utilizes a 3 phase learning protocol to help achieve convergence and improve generalization. DoseNet was trained, validated, and tested with 151 patients following Kaggle completion rules. The dosimetric quality of DoseNet was evaluated by comparing the predicted dose distribution with the clinically approved delivered dose distribution in terms of conformity index, heterogeneity index, and various clinically relevant dosimetric parameters. The results indicate that the DoseNet algorithm is a superior alternative to U-Net and fully connected methods for prostate SBRT patients. DoseNet required ~50.1 h to train, and ~0.83 s to make a prediction on a 128  ×  128  ×  64 voxel image. In conclusion, DoseNet is capable of making accurate volumetric dose predictions for non-coplanar SBRT prostate patients, while simultaneously preserving computational efficiency.

Stereotactic body radiotherapy for prostate cancer is rapidly growing in popularity. Stereotactic body radiotherapy plans mimic those of high-dose rate brachytherapy, with tight margins and inhomogeneous dose distributions. The impact of interfraction anatomical changes on the dose received by organs at risk under these conditions has not been well documented. To estimate anatomical variation during stereotactic body radiotherapy, 10 patients were identified who received a prostate boost using robotic stereotactic body radiotherapy after completing 25 fractions of pelvic radiotherapy with daily megavoltage computed tomography. Rectal and bladder volumes were delineated on each megavoltage computed tomography, and the stereotactic body radiotherapy boost plan was registered to each megavoltage computed tomography image using a point-based rigid registration with 3 fiducial markers placed in the prostate. The volume of rectum and bladder receiving 75% of the prescription dose (V75%) was measured for each megavoltage computed tomography. The rectal V75% from the daily megavoltage computed tomographies was significantly greater than the planned V75% (median increase of 0.93 cm 3 , P < .001), whereas the bladder V75% on megavoltage computed tomography was not significantly changed (median decrease of -0.12 cm 3 , P = .57). Although daily prostate rotation was significantly correlated with bladder V75% (Spearman ρ = .21, P = .023), there was no association between rotation and rectal V75% or between prostate deformation and either rectal or bladder V75%. Planning organ-at-risk volume-based replanning techniques using either a 6-mm isotropic expansion of the plan rectal contour or a 1-cm expansion from the planning target volume in the superior and posterior directions demonstrated significantly improved rectal V75% on daily megavoltage computed tomographies compared to the original stereotactic body radiotherapy plan, without compromising plan quality. Thus, despite tight margins and full translational and rotational corrections provided by robotic stereotactic body radiotherapy, we find that interfraction anatomical variations can lead to a substantial increase in delivered rectal doses during prostate stereotactic body radiotherapy. A planning organ-at-risk volume-based approach to treatment planning may help mitigate the impact of daily organ motion and reduce the risk of rectal toxicity.

The purpose of this study was to evaluate the performance of a commercially avail-able CyberKnife system with a multileaf collimator (CK-MLC) for stereotactic body radiotherapy (SBRT) and standard fractionated intensity-modulated radiotherapy (IMRT) applications. Ten prostate and ten intracranial cases were planned for the CK-MLC. Half of these cases were compared with clinically approved SBRT plans generated for the CyberKnife with circular collimators, and the other half were compared with clinically approved standard fractionated IMRT plans generated for conventional linacs. The plans were compared on target coverage, conformity, homogeneity, dose to organs at risk (OAR), low dose to the surrounding tissue, total monitor units (MU), and treatment time. CK-MLC plans generated for the SBRT cases achieved more homogeneous dose to the target than the CK plans with the circular collimators, for equivalent coverage, conformity, and dose to OARs. Total monitor units were reduced by 40% to 70% and treatment time was reduced by half. The CK-MLC plans generated for the standard fractionated cases achieved prescription isodose lines between 86% and 93%, which was 2%-3% below the plans generated for conventional linacs. Compared to standard IMRT plans, the total MU were up to three times greater for the prostate (whole pelvis) plans and up to 1.4 times greater for the intracranial plans. Average treatment time was 25min for the whole pelvis plans and 19 min for the intracranial cases. The CK-MLC system provides significant improvements in treatment time and target homogeneity compared to the CK system with circular collimators, while main-taining high conformity and dose sparing to critical organs. Standard fractionated plans for large target volumes (> 100 cm3) were generated that achieved high prescription isodose levels. The CK-MLC system provides more efficient SRS and SBRT treatments and, in select clinical cases, might be a potential alternative for standard fractionated treatments.

Purpose: The purpose of this study was to evaluate the dose-volume relationships of genitourinary toxicity after stereotactic body radiation therapy (SBRT) monotherapy for prostate cancer., Methods and Materials: Fifty-six patients diagnosed with low- to intermediate-risk prostate cancer treated with SBRT alone were reviewed retrospectively. All patients received a total dose of 38 Gy in 4 fractions with a planning target volume expansion of 2 mm. Overall, acute, and late genitourinary toxicity were documented according to the Common Terminology Criteria for Adverse Events (version 4) and International Prostate Symptom Scores (IPSS)., Results: The median age at treatment was 68 years, and the median prostate volume was 45.5 mL, with a median baseline IPSS of 9.95. The median prescription isodose line was 68%. The median clinical follow-up was 35.49 months. Acute grade 1, 2, and 3 genitourinary toxicities occurred in 41.1%, 35.7%, and 0% of patients. All acute genitourinary toxicities resolved except 1 patient with grade 2 toxicity that progressed to grade 3 late toxicity. No dose-volume relationships were associated with acute genitourinary grade 2+ toxicity. Late grade 1, 2, and 3 genitourinary toxicity occurred in 19.6%, 19.6%, and 3.6% of cases, respectively. Of the cases with late toxicities, 16.7% were persistent. Late grade 2+ genitourinary toxicity was associated with prostate volume ≥50 mL, lower homogeneity index, and urethral maximum point dose ≥47 Gy. The overall risk of any grade 2+ genitourinary toxicity was associated with baseline IPSS >7, prostate volume ≥50 mL, urethral volume receiving 44 Gy, and bladder volume receiving 19 Gy., Conclusions: SBRT for prostate cancer appears well tolerated, with mostly transient low-grade toxicity. Urethral sparing should be used with a maximum point dose <47 Gy, volume receiving 120 Gy <50% of the prostate, and bladder volume receiving 19 Gy <15 mL in 4 fraction treatments. Patients with prostate volumes ≥50 mL should be counseled regarding the increased risk of moderate-grade genitourinary toxicity., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Purpose: A unique capability of the CyberKnife system is dynamic target tracking. However, not all patients are eligible for this approach. Rather, their tumors are tracked statically using the vertebral column for alignment. When using static tracking, the internal target volume (ITV) is delineated on the four-dimensional (4D) CT scan and an additional margin is added to account for setup uncertainty [planning target volume (PTV)]. Treatment margins are difficult to estimate due to unpredictable variations in tumor motion and respiratory pattern during the course of treatment. The inability to track the target and detect changes in respiratory characteristics might result in geographic misses and local tumor recurrences. The purpose of this study is to develop a method to evaluate the adequacy of ITV-to-PTV margins for patients treated in this manner., Methods: Data from 24 patients with lesions in the upper lobe (n = 12), middle lobe (n = 3), and lower lobe (n = 9) were included in this study. Each patient was treated with dynamic tracking and underwent 4DCT scanning at the time of simulation. Data including the 3D coordinates of the target over the course of treatment were extracted from the treatment log files and used to determine actual target motion in the superior-inferior (S-I), anterior-posterior (A-P), and left-right (L-R) directions. Different approaches were used to calculate anisotropic and isotropic margins, assuming that the tumor moves as a rigid body. Anisotropic margins were calculated by separating target motion in the three anatomical directions, and a uniform margin was calculated by shifting the gross tumor volume contours in the 3D space and by computing the percentage of overlap with the PTV. The analysis was validated by means of a theoretical formulation., Results: The three methods provided consistent results. A uniform margin of 4.5 mm around the ITV was necessary to assure 95% target coverage for 95% of the fractions included in the analysis. In the case of anisotropic margins, the expansion required in the S-I direction was larger (8.1 mm) than those in the L-R (4.9 mm) and A-P (4.5 mm) directions. This margin accounts for variations of target position within the same treatment fraction., Conclusions: The use of bony alignment for CyberKnife lung stereotactic body radiation therapy requires careful considerations, in terms of the potential for increased toxicity or local miss. Our method could be used by other centers to determine the adequacy of ITV-to-PTV margins for their patients.

Stereotactic body radiation therapy (SBRT) to central lung tumors is associated with normal -tissue toxicity. Highly conformal technologies may reduce the risk of complications. This study compares physical dose characteristics and anticipated risks of radiation pneumonitis (RP) among three SBRT modalities: robotic radiosurgery (RR), helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT). Nine patients with central lung tumors ≤5 cm were compared. RR, HT and VMAT plans were developed per RTOG 0831. Dosimetric comparisons included target coverage, conformity index, heterogeneity index, gradient index, maximal dose at 2 cm from target (D2 cm), and dose-volume parameters for organs at risk (OARs). Efficiency endpoints included total beam-on time and monitor units. RP risk was derived from Lyman-Kutcher-Burman modeling on in-house software. The average GTV and PTV were 11.6 ± 7.86 cm(3) and 36.8 ± 18.1 cm(3). All techniques resulted in similar target coverage (p = 0.64) and dose conformity (p = 0.88). While RR had sharper fall-off gradient (p = 0.002) and lower D2 cm (p = 0.02), HT and VMAT produced greater homogeneity (p < 0.001) and delivery efficiency (p = 0.001). RP risk predicted from whole or contralateral lung volumes was less than 10%, but was 2-3 times higher using ipsilateral volumes. Using whole (p = 0.04, p = 0.02) or ipsilateral (p = 0.004, p = 0.0008) volumes, RR and VMAT had a lower risk of RP than HT. Using contralateral volumes, RR had the lowest RP risk (p = 0.0002, p = 0.0003 versus HT, VMAT). RR, HT and VMAT were able to provide clinically acceptable plans following the guidelines provided by RTOG 0813. All techniques provided similar coverage and conformity. RR seemed to produce a lower RP risk for a scenario of small PTV-OAR overlap and small PTV. VMAT and HT produced greater homogeneity, potentially desirable for a large PTV-OAR overlap. VMAT probably yields the lowest RP risk for a large PTV. Understanding subtle differences among these technologies may assist in situations where multiple choices of modality are available., (© The Author(s) 2014.)

Treatment plans for prostate cancer patients undergoing stereotactic body radiation therapy (SBRT) are often challenging due to the proximity of organs at risk. Today, there are no objective criteria to determine whether an optimal treatment plan has been achieved, and physicians rely on their personal experience to evaluate the plan's quality. In this study, we propose a method for determining rectal and bladder dose constraints achievable for a given patient's anatomy. We expect that this method will improve the overall plan quality and consistency, and facilitate comparison of clinical outcomes across different institutions. The 3D proximity of the organs at risk to the target is quantified by means of the expansion-intersection volume (EIV), which is defined as the intersection volume between the target and the organ at risk expanded by 5 mm. We determine a relationship between EIV and relevant dosimetric parameters, such as the volume of bladder and rectum receiving 75% of the prescription dose (V75%). This relationship can be used to establish institution-specific criteria to guide the treatment planning and evaluation process. A database of 25 prostate patients treated with CyberKnife SBRT is used to validate this approach. There is a linear correlation between EIV and V75% of bladder and rectum, confirming that the dose delivered to rectum and bladder increases with increasing extension and proximity of these organs to the target. This information can be used during the planning stage to facilitate the plan optimization process, and to standardize plan quality and consistency. We have developed a method for determining customized dose constraints for prostate patients treated with robotic SBRT. Although the results are technology specific and based on the experience of a single institution, we expect that the application of this method by other institutions will result in improved standardization of clinical practice.

This paper investigates the dosimetric characteristics of stereotactic body radiotherapy (SBRT) treatment plans of spine patients in the prone position compared to the supine position. A feasibility study for treating spine patients in the prone position using a fiducial-less tracking method is presented. One patient with a multilevel spinal metastasis was simulated for SBRT treatment in both the supine and prone position. CT scans of the patient were acquired, and treatment plans were created using the CyberKnife® planning platform. The potential advantage of the prone setup as a function of lesion location and number of vertebral bodies involved was studied for targets extending over 1, 2 and 3 consecutive vertebral bodies in the thoracic and lumbar spine. The same process was repeated on an anthropomorphic phantom. A dose of 30 Gy in 5 fractions was prescribed to 95% of the tumor volume and the dose to the cord was limited to 25 Gy. To investigate the feasibility of a fiducial-less tracking method in the prone setup, the patient was positioned prone on the treatment table and the spine motion was monitored as a function of time. Patient movement with the respiratory cycle was reduced by means of a belly-board. Plans in the prone and supine position achieved similar tumor coverage and sparing of the critical structures immediately adjacent to the spine (such as cord and esophagus). However, the prone plans systematically resulted in a lower dose to the normal structures located in the anterior part of the body (such as heart for thoracic cases; stomach, lower gastrointestinal tract and liver for lumbar cases). In addition, prone plans resulted in a lower number of monitor units compared to supine plans.

Purpose: High dose rate (HDR) brachytherapy has been established as an excellent monotherapy or after external-beam radiotherapy (EBRT) boost treatment for prostate cancer (PCa). Recently, dosimetric studies have demonstrated the potential for achieving similar dosimetry with stereotactic body radiotherapy (SBRT) compared with HDR brachytherapy. Here, we report our technique, PSA nadir, and acute and late toxicity with SBRT as monotherapy and post-EBRT boost for PCa using HDR brachytherapy fractionation., Patients and Methods: To date, 38 patients have been treated with SBRT at the University of California-San Francisco with a minimum follow-up of 12 months. Twenty of 38 patients were treated with SBRT monotherapy (9.5 Gy × 4 fractions), and 18 were treated with SBRT boost (9.5 Gy × 2 fractions) post-EBRT and androgen deprivation therapy. PSA nadir to date for 44 HDR brachytherapy boost patients with disease characteristics similar to the SBRT boost cohort was also analyzed as a descriptive comparison., Results: SBRT was well tolerated. With a median follow-up of 18.3 months (range, 12.6-43.5), 42% and 11% of patients had acute Grade 2 gastrourinary and gastrointestinal toxicity, respectively, with no Grade 3 or higher acute toxicity to date. Two patients experienced late Grade 3 GU toxicity. All patients are without evidence of biochemical or clinical progression to date, and favorably low PSA nadirs have been observed with a current median PSA nadir of 0.35 ng/mL (range, <0.01-2.1) for all patients (0.47 ng/mL, range, 0.2-2.1 for the monotherapy cohort; 0.10 ng/mL, range, 0.01-0.5 for the boost cohort). With a median follow-up of 48.6 months (range, 16.4-87.8), the comparable HDR brachytherapy boost cohort has achieved a median PSA nadir of 0.09 ng/mL (range, 0.0-3.3)., Conclusions: Early results with SBRT monotherapy and post-EBRT boost for PCa demonstrate acceptable PSA response and minimal toxicity. PSA nadir with SBRT boost appears comparable to those achieved with HDR brachytherapy boost., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The rate of dose accumulation within a given area of a target volume tends to vary significantly for non-isocentric delivery systems such as Cyberknife stereotactic body radiotherapy. In this study, we investigated whether intra-target temporal dose distributions produce significant variations in the biological equivalent dose. For the study, time courses of ten patients were reconstructed and calculation of a biologically equivalent uniform dose (EUD) was performed using a formula derived from the linear quadratic model (α/β = 3 for prostate cancer cells). The calculated EUD values obtained for the actual patient treatments were then compared with theoretical EUD values for delivering the same physical dose distribution except that the whole target being irradiated continuously (e.g. large-field 'dose-bathing' type of delivery). For all the case, the EUDs for the actual treatment delivery were found to correlate strongly with the EUDs for the large-field delivery: a linear correlation coefficient of R² = 0.98 was obtained and the average EUD for the actual Cyberknife delivery was somewhat higher (5.0 ± 4.7%) than that for the large-field delivery. However, no statistical significance was detected between the two types of delivery (p = 0.21). We concluded that non-isocentric small-field Cyberknife delivery produced consistent biological dosing that tracked well with the constant-dose-rate, large-field-type delivery for prostate stereotactic body radiotherapy.

Object: Technical improvements in commercially available radiosurgery platforms have made it practical to treat a large number of intracranial targets. The goal of this study was to investigate whether the dose to normal brain when planning radiosurgery to multiple targets is apparatus dependent., Methods: The authors selected a single case involving a patient with 12 metastatic lesions widely distributed throughout the brain as visualized on contrast-enhanced CT. Target volumes and critical normal structures were delineated with Leksell Gamma Knife Perfexion software. The imaging studies including the delineated contours were digitally exported into the CyberKnife and Novalis multileaf collimator-based planning systems for treatment planning using identical target dose goals and dose-volume constraints. Subsets of target combinations (3, 6, 9, or 12 targets) were planned separately to investigate the relationship of number of targets and radiosurgery platform to the dose to normal brain., Results: Despite similar target dose coverage and dose to normal structures, the dose to normal brain was strongly apparatus dependent. A nonlinear increase in dose to normal brain volumes with increasing number of targets was also noted., Conclusions: The dose delivered to normal brain is strongly dependent on the radiosurgery platform. How general this conclusion is and whether apparatus-dependent differences are related to differences in hardware design or differences in dose-planning algorithms deserve further investigation.

Object: The Leksell Gamma Knife and the Accuray CyberKnife systems have been used in the radio surgical treatment of trigeminal neuralgia. The 2 techniques use different delivery methods and different treatment parameters. In the past, CyberKnife treatments have been associated with an increased incidence of treatment-related complications, such as facial numbness. The goal of this study was to develop a method for planning a CyberKnife treatment for trigeminal neuralgia that would reproduce the dosimetric characteristics of a Gamma Knife plan. A comparison between Gamma Knife and CyberKnife treatment plans obtained with this method is presented., Methods: Five patients treated using the Gamma Knife Perfexion Unit were selected for this study. All patients underwent CT cisternography to accurately identify the position of the trigeminal nerve. The Gamma Knife plans used either one 4-mm-diameter collimator or two coincident 4-mm collimators (one open and one with sector blocking) placed at identical isocenter coordinates. A maximum local dose of 80 Gy was prescribed. Critical structures and representative isodose lines were outlined in GammaPlan and exported to the CyberKnife treatment planning platform. CyberKnife treatments were developed using the 5-mm-diameter cone and the trigeminal node set, which provides an effective collimation diameter of 4 mm at the isocenter. The 60-Gy isodose volume imported from GammaPlan was used as the target in the CyberKnife plans. The CyberKnife treatments were optimized to achieve target dose and critical structure sparing similar to the Gamma Knife plans. Isocentric and nonisocentric delivery techniques were investigated. Treatment plans were compared in terms of dosimetric characteristics, delivery, and planning efficiency., Results: CyberKnife treatments using the 5-mm cone and the trigeminal node set can closely reproduce the dose distribution of Gamma Knife plans. CyberKnife isocentric and nonisocentric plans provide comparable results. The average length of the trigeminal nerve receiving a dose of 60 Gy was 4.5, 4.5, and 4.4 mm for Gamma Knife, nonisocentric CyberKnife, and isocentric CyberKnife, respectively. However, minimizing the dose to the critical structures was more difficult with the CyberKnife and required the use of tuning structures. In addition, the dose fall off away from the target was steeper in Gamma Knife plans, probably due to the larger number of beams (192 beams for perfexion vs ~ 100 beams for cyberknife). While the treatment time with the cyberknife is generally shorter, the planning time is significantly longer., Conclusions: CyberKnife radiosurgical parameters can be optimized to mimic the dose distribution of Gamma Knife plans. However, Gamma Knife plans result in superior sparing of critical structures (brainstem, temporal lobe,and cranial nerves VII and VIII) and in steeper dose fall off away from the target. The clinical significance of these effects is unknown. (DOI: 10.3171/2010.8.GKS101002)

Purpose: The treatment planning quality between nonisocentric CyberKnife (CK) and isocentric intensity modulation treatment was studied for hypofractionated prostate body radiotherapy. In particular, the dose gradient across the target and the critical structures such as the rectum and bladder was characterized., Methods and Materials: In the present study, patients treated with CK underwent repeat planning for nine fixed-field intensity-modulated radiotherapy (IMRT) using identical contour sets and dose-volume constraints. To calculate the dose falloff, the clinical target volume contours were expanded 30 mm anteriorly and posteriorly and 50 mm uniformly in other directions for all patients in the CK and IMRT plans., Results: We found that all the plans satisfied the dose-volume constraints, with the CK plans showing significantly better conformity than the IMRT plans at a relative greater dose inhomogeneity. The rectal and bladder volumes receiving a low dose were also lower for CK than for IMRT. The average conformity index, the ratio of the prescription isodose volume and clinical target volume, was 1.18 +/- 0.08 for the CK plans vs. 1.44 +/- 0.11 for the IMRT plans. The average homogeneity index, the ratio of the maximal dose and the prescribed dose to the clinical target volume, was 1.45 +/- 0.12 for the CK plans vs. 1.28 +/- 0.06 for the IMRT plans. The average percentage of dose falloff was 2.9% +/- 0.8%/mm for CK and 3.1% +/- 1.0%/mm for IMRT in the anterior direction, 3.8% +/- 1.6%/mm for CK and 3.2% +/- 1.9%/mm for IMRT in the posterior direction, and 3.6% +/- 0.4% for CK and 3.6% +/- 0.4% for IMRT in all directions., Conclusion: Nonisocentric CK was as capable of producing equivalent fast dose falloff as high-number fixed-field IMRT delivery., (Copyright (c) 2010 Elsevier Inc. All rights reserved.)

Purpose: To investigate whether dose fall-off characteristics would be significantly different among intracranial radiosurgery modalities and the influence of these characteristics on fractionation schemes in terms of normal tissue sparing., Methods and Materials: An analytic model was developed to measure dose fall-off characteristics near the target independent of treatment modalities. Variations in the peripheral dose fall-off characteristics were then examined and compared for intracranial tumors treated with Gamma Knife, Cyberknife, or Novalis LINAC-based system. Equivalent uniform biologic effective dose (EUBED) for the normal brain tissue was calculated. Functional dependence of the normal brain EUBED on varying numbers of fractions (1 to 30) was studied for the three modalities., Results: The derived model fitted remarkably well for all the cases (R(2) > 0.99). No statistically significant differences in the dose fall-off relationships were found between the three modalities. Based on the extent of variations in the dose fall-off curves, normal brain EUBED was found to decrease with increasing number of fractions for the targets, with alpha/beta ranging from 10 to 20. This decrease was most pronounced for hypofractionated treatments with fewer than 10 fractions. Additionally, EUBED was found to increase slightly with increasing number of fractions for targets with alpha/beta ranging from 2 to 5., Conclusion: Nearly identical dose fall-off characteristics were found for the Gamma Knife, Cyberknife, and Novalis systems. Based on EUBED calculations, normal brain sparing was found to favor hypofractionated treatments for fast-growing tumors with alpha/beta ranging from 10 to 20 and single fraction treatment for abnormal tissues with low alpha/beta values such as alpha/beta = 2., (Copyright (c) 2010 Elsevier Inc. All rights reserved.)

Purpose: To characterize nonrandom intrafraction target motions for spine stereotactic body radiotherapy and to develop a method of correction via image guidance. The dependence of target motions, as well as the effectiveness of the correction strategy for lesions of different locations within the spine, was analyzed., Methods and Materials: Intrafraction target motions for 64 targets in 64 patients treated with a total of 233 fractions were analyzed. Based on the target location, the cases were divided into three groups, i.e., cervical (n = 20 patients), thoracic (n = 20 patients), or lumbar-sacrum (n = 24 patients) lesions. For each case, time-lag autocorrelation analysis was performed for each degree of freedom of motion that included both translations (x, y, and z shifts) and rotations (roll, yaw, and pitch). A general correction strategy based on periodic interventions was derived to determine the time interval required between two adjacent interventions, to overcome the patient-specific target motions., Results: Nonrandom target motions were detected for 100% of cases regardless of target locations. Cervical spine targets were found to possess the highest incidence of nonrandom target motion compared with thoracic and lumbar-sacral lesions (p < 0.001). The average time needed to maintain the target motion to within 1 mm of translation or 1 degrees of rotational deviation was 5.5 min, 5.9 min, and 7.1 min for cervical, thoracic, and lumbar-sacrum locations, respectively (at 95% confidence level)., Conclusions: A high incidence of nonrandom intrafraction target motions was found for spine stereotactic body radiotherapy treatments. Periodic interventions at approximately every 5 minutes or less were needed to overcome such motions.

Object: The new capability of composite sector collimation in Gamma Knife Perfexion produces complex, nonspherical, and nonelliptical dose distributions. In this study, the authors investigated the effect of composite sector collimation on average dose fall-off compared with the previous Gamma Knife model., Methods: A general formalism was derived to describe the peripheral dose distribution of all Gamma Knife models in the form of (V/V(0)) = (D/D(0))(gamma), where V is the volume of the peripheral isodose line with the value of D, V(0) is the reference prescription isodose volume, D(0) is the prescription dose, and gamma is the fitting parameter that determines how fast the dose falls off near the target. Based on this formula, the authors compared 40 cases involving patients treated with Gamma Knife Perfexion with 40 similar cases involving patients treated with Gamma Knife model 4C. The cases were grouped based on the use of the sector collimators in the treatment planning process. For each group as well as all cases combined, the mean gamma values were compared by means of the Student t-test for varying ranges of the peripheral dose distribution-from 100% of the prescription dose to 75, 50, and 25% of the prescription dose., Results: The fit of general formula to the data was excellent for both Gamma Knife Perfexion and Gamma Knife 4C with R(2)> 0.99 for all the cases. The overall gamma values (mean +/- 2 standard deviations) were as follows: gamma = -1.74 +/- 0.47 (Model 4C) versus -1.77 +/- 0.40 (Perfexion) within 100-75% of the prescription dose; gamma = -1.57 +/- 0.26 (Model 4C) versus -1.58 +/- 0.25 (Perfexion) within 100-50% of the prescription dose; gamma = -1.47 +/- 0.18 (Model 4C) versus -1.50 +/- 0.16 (Perfexion) within 100-25% of the prescription dose. No statistical significance between the mean differences for Gamma Knife Perfexion and Model 4C was found within these ranges. The probability values were 0.65, 0.84, and 0.22, respectively., Conclusions: The use of composite sector collimators in Gamma Knife Perfexion demonstrated no statistically significant effects on the volume-averaged dose fall-off near a target periphery for typical treatment cases.

The mechanical accuracy of Gamma Knife radiosurgery based on single-isocenter measurement has been established to within 0.3 mm. However, the full delivery accuracy for Gamma Knife treatments of large lesions has only been estimated via the quadrature-sum analysis. In this study, the authors directly measured the whole-procedure accuracy for Gamma Knife treatments of large lesions to examine the validity of such estimation. The measurements were conducted on a head-phantom simulating the whole treatment procedure that included frame placement, computed tomography imaging, treatment planning, and treatment delivery. The results of the measurements were compared with the dose calculations from the treatment planning system. Average agreements of 0.1-1.6 mm for the isodose lines ranging from 25% to 90% of the maximum dose were found despite potentially large contributing uncertainties such as 3-mm imaging resolution, 2-mm dose grid size, 1-mm frame registration, multi-isocenter deliveries, etc. The results of our measurements were found to be significantly smaller (>50%) than the calculated value based on the quadrature-sum analysis. In conclusion, Gamma Knife treatments of large lesions can be delivered much more accurately than predicted from the quadrature-sum analysis of major sources of uncertainties from each step of the delivery chain.

Purpose/objectivesFor lung stereotactic body radiation therapy (SBRT), real-time tumor tracking (RTT) allows for less radiation to normal lung compared to the internal target volume (ITV) method of respiratory motion management. To quantify the advantage of RTT, we examined the difference in radiation pneumonitis risk between these two techniques using a normal tissue complication probability (NTCP) model.Materials/method20 lung SBRT treatment plans using RTT were replanned with the ITV method using respiratory motion information from a 4D-CT image acquired at the original simulation. Risk of symptomatic radiation pneumonitis was calculated for both plans using a previously derived NTCP model. Features available before treatment planning that identified significant increase in NTCP with ITV versus RTT plans were identified.ResultsPrescription dose to the planning target volume (PTV) ranged from 22 to 60 Gy in 1-5 fractions. The median tumor diameter was 3.5 cm (range 2.1-5.5 cm) with a median volume of 14.5 mL (range 3.6-59.9 mL). The median increase in PTV volume from RTT to ITV plans was 17.1 mL (range 3.5-72.4 mL), and the median increase in PTV/lung volume ratio was 0.46% (range 0.13-1.98%). Mean lung dose and percentage dose-volumes were significantly higher in ITV plans at all levels tested. The median NTCP was 5.1% for RTT plans and 8.9% for ITV plans, with a median difference of 1.9% (range 0.4-25.5%, pairwise P < 0.001). Increases in NTCP between plans were best predicted by increases in PTV volume and PTV/lung volume ratio.ConclusionsThe use of RTT decreased the risk of radiation pneumonitis in all plans. However, for most patients the risk reduction was minimal. Differences in plan PTV volume and PTV/lung volume ratio may identify patients who would benefit from RTT technique before completing treatment planning.

Stereotactic body radiation therapy is a well-tolerated modality for the treatment of primary and metastatic liver lesions, and fiducials are often used as surrogates for tumor tracking during treatment. We evaluated respiratory-induced liver deformation by measuring the rigidity of the fiducial configuration during the breathing cycle. Seventeen patients, with 18 distinct treatment courses, were treated with stereotactic body radiosurgery using multiple fiducials. Liver deformation was empirically quantified by measuring the intrafiducial distances at different phases of respiration. Data points were collected at the 0%, 50%, and 100% inspiration points, and the distance between each pair of fiducials was measured at the 3 phases. The rigid body error was calculated as the maximum difference in the intrafiducial distances. Liver disease was calculated with Child-Pugh score using laboratory values within 3 months of initiation of treatment. A peripheral fiducial was defined as within 1.5 cm of the liver edge, and all other fiducials were classified as central. For 5 patients with only peripheral fiducials, the fiducial configuration had more deformation (average maximum rigid body error 7.11 mm, range: 1.89-11.35 mm) when compared to patients with both central and peripheral and central fiducials only (average maximum rigid body error 3.36 mm, range: 0.5-9.09 mm, P = .037). The largest rigid body errors (11.3 and 10.6 mm) were in 2 patients with Child-Pugh class A liver disease and multiple peripheral fiducials. The liver experiences internal deformation, and the fiducial configuration should not be assumed to act as a static structure. We observed greater deformation at the periphery than at the center of the liver. In our small data set, we were not able to identify cirrhosis, which is associated with greater rigidity of the liver, as predictive for deformation. Treatment planning based only on fiducial localization must take potential intraorgan deformation into account.

The mechanical accuracy of Gamma Knife radiosurgery based on single-isocenter measurement has been established to within 0.3 mm. However, the full delivery accuracy for Gamma Knife treatments of large lesions has only been estimated via the quadrature-sum analysis. In this study, the authors directly measured the whole-procedure accuracy for Gamma Knife treatments of large lesions to examine the validity of such estimation. The measurements were conducted on a head-phantom simulating the whole treatment procedure that included frame placement, computed tomography imaging, treatment planning, and treatment delivery. The results of the measurements were compared with the dose calculations from the treatment planning system. Average agreements of 0.1–1.6 mm for the isodose lines ranging from 25% to 90% of the maximum dose were found despite potentially large contributing uncertainties such as 3-mm imaging resolution, 2-mm dose grid size, 1-mm frame registration, multi-isocenter deliveries, etc. The results of our measurements were found to be significantly smaller (>50%) than the calculated value based on the quadrature-sum analysis. In conclusion, Gamma Knife treatments of large lesions can be delivered much more accurately than predicted from the quadrature-sum analysis of major sources of uncertainties from each step of the delivery chain. [ABSTRACT FROM AUTHOR]