Background: This work explores how the choice of prescription isodose line (IDL) affects the dose gradient, target coverage, and treatment time for Gamma Knife radiosurgery when a smaller shot is encompassed within a larger shot at the same stereotactic coordinates (shot within shot technique). Methods: Beam profiles for the 4, 8, and 16 mm collimator settings were extracted from the treatment planning system and characterized using Gaussian fits. The characterized data were used to create over 10,000 shot within shot configurations by systematically changing collimator weighting and choice of prescription IDL. Each configuration was quantified in terms of the dose gradient, target coverage, and beam-on time. By analyzing these configurations, it was found that there are regions of overlap in target size where a higher prescription IDL provides equivalent dose fall-off to a plan prescribed at the 50% IDL. Furthermore, the data indicate that treatment times within these regions can be reduced by up to 40%. An optimization strategy was devised to realize these gains. The strategy was tested for seven patients treated for 1-4 brain metastases (20 lesions total). Results: For a single collimator setting, the gradient in the axial plane was steepest when prescribed to the 56-63% (4 mm), 62-70% (8 mm), and 77-84% (16 mm) IDL, respectively. Through utilization of the optimization technique, beam-on time was reduced by more than 15% in 16/20 lesions. The volume of normal brain receiving 12 Gy or above also decreased in many cases, and in only one instance increased by more than 0.5 cm
Objective: To determine whether robotic stereotactic radiotherapy of 70-75 Gy delivered in five fractions results in an improved therapeutic ratio, compared with three fractions, in the treatment of peripheral non-small-cell lung cancer (NSCLC), in which case doses of up to 85 Gy in five fractions may be feasible. Materials and Methods: Between December 2006 and May 2010, 20 patients (9 female, 11 male, aged 65 to 88) were treated using the CyberKnife Õ Robotic Radiosurgery System for NSCLC with doses ranging from 67 Gy to 75 Gy based on location, histopathological type, grade of histopathological differentiation, tumor diameter/volume, and normal tissue constraints, with the doses being delivered in five fractions over 5 to 8 days. Tumor diameters ranged from 1.5 cm to 3.4 cm (median: 2.5 cm). Patients with Stage I to IV NSCLC were treated, and the results and observations were analyzed for clinical characteristics and outcomes including toxicity. All patients, except one who had refused surgery, had co-morbid conditions that precluded a lobectomy. Results: Twenty patients were followed every three months by positron emission tomography/computed tomography (PET/ CT). Mean follow-up was 23 months (range: four to 58 months). Local control was achieved in all treated tumors. Three patients expired, and three developed new regional metastases, none of which was within the planning target volume (PTV). The remainder of the patients demonstrated no evidence of recurrence or continued growth detectable by PET/ CT. There was no toxicity above Grade 1. Conclusions: It is feasible to treat peripheral NSCLC with individualized maximal tolerable doses ranging from 67 Gy to 75 Gy in five fractions chosen on the basis of location, histopathological type, grade of histopathological differentiation, tumor diameter/volume, and normal tissue constraints.
Purpose: Stereotactic body radiotherapy (SBRT) is used for spine treatments as it precisely delivers high radiation dose to tumors in close proximity to organs-at-risk (OARs). The goal of this work is to evaluate dosimetric properties of SBRT for spinal treatments with linear accelerators and CyberKnife (CK). Materials and methods: Plans of 27 patients, treated with CK for spine tumors, were also retrospectively optimized for linac-based (LB) intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). One nine-field IMRT plan and five VMAT plans were generated for each patient. The LB target volumes were uniformly expanded by 0.1 cm to accommodate for the uncertainty in patient positioning. All plans were optimized to cover 90% of the target volumes with a prescription dose of 27 Gy in three fractions. If dose constraints to OARs were not met, the prescription dose was decreased to 24 Gy. Target dose conformity and falloff were evaluated with Paddick's conformity (CI) and gradient (GI) indices. Results: PTV expansion resulted in a 31.5% volume increase in the LB plans. The three full-arcs VMAT (VMAT_3full) plans resulted in the best average CI(0.820) compared to CK(0.758) with worst average from one half-arcs VMAT (VMAT_1half) plans (0.747). Dose falloff was also superior with the VMAT_3full plans with an average GI value of 3.596, in comparison to CK(3.786) and IMRT(4.447). In 6 cases CK plans were unable to meet OAR constraints and the prescription dose was decreased to 24 Gy, compared to only 2 for VMAT_3full. Conclusion: Regardless of the larger target volumes, LB plans were comparable to CK plans. Conformity of target doses of the VMAT_3full plans were better than CK in all cases and dose fall-off was better 23 of 27 plans. Dose to OARs were lower for CK, but constraints met for all plans. The use of VMAT would reduce the treatment time.
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