What Do We Mean When We Talk about the Linac Isocenter?

Purpose: The isocenter of a medical linac system is a frequently used concept in clinical practice. However, so far not all the isocenters are rigorously defined. This work is intended as an attempt of deriving consistent and operable isocenter definitions. Methods: The isocenter definition is based on a fundamental concept, the axis of rotation of a rigid body. The axis of rotation is determined using the trajectory of any point on a plane that intersects the rigid body. A point on the axis of rotation is found through the minimal bounding sphere of the trajectory when the rigid body makes a full rotation. The essential mathematical tool of the isocenter definition system is three-dimensional coordinate transformation. Results: The axes of rotation of the linac collimator, gantry, and couch are established first. The linac mechanical isocenter (linac isocenter) is defined as the center of a circle that best fits the trajectory of a select linac X-ray source position. The axis of rotation and the minimal bounding sphere are cornerstones for the rotation isocenters of the collimator, gantry and couch. The definition of radiation isocenter incorporates a surrogate of the useful beam axis. Conclusions: A framework of isocenter definitions for medical linacs is presented in this manuscript. Consistent meanings of the mechanical and radiation isocenters can be achieved using this approach.

A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

Purpose: With usually a millimeter-level PTV margin, stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) pose a stringent requirement on the isocentricity of the Linac. This requirement is partly fulfilled by routine isocenter quality assurance (QA) test to verify the size and location of the isocenter. The current common QA methods such as spoke shot were developed before SBRT/SRS became popular and when IGRT was largely absent and hence have their limitations. In this work, we describe an isocenter QA approach based on portal imaging to provide the community with a superior alternative. Methods: The proposed approach utilizes a BrainLab ball bearing (BB) phantom in conjunction with an electronic portal imaging devices (EPID) imager. The BB phantom was first aligned with a calibrated room laser system. Portal images were then acquired using 6 MV beam with a 2 × 2 cm2 open field and a 15 mm cone on a Varian TrueBeam STx machine. The gantry, collimator, and table were rotated separately at selected angles to acquire a series of portal images in order to determine the isocenter of each rotating system. The location and diameter of these isocenters were determined by calculating the relative displacement of either BB or open field edge between the acquired EPID images. The demonstration of the reproducibility and robustness of this EPID-based approach was carried out by repeating measurements 10 times independently for each rotating system and simulating clinical scenarios of asymmetric jaws and misalignment of BB phantom, respectively. Results: For our TrueBeam STx machine, the isocenter diameter derived from open-field EPID images was roughly 0.15 mm, 0.18 mm, 0.49 mm for the collimator, table, and gantry, respectively. For the collimator and gantry, images taken with the cone gave considerably smaller isocenter diameter. Results remained almost unchanged despite the presence of simulated BB misalignment and asymmetric jaws error, and between independent measurements. Isocenter location and diameter derived from images obtained at a limited number of angles (≤11) were adequately accurate to represent those derived from images of densely sampled angles. Conclusions: An EPID-based isocenter QA approach is described and demonstrated to be accurate, robust, and reproducible. This approach provides a superior alternative to conventional isocenter QA methods with no additional cost. It can be implemented with convenience for any linear accelerator with an EPID imager.

Planning issues on linac-based stereotactic radiotherapy

This work aims to summarize and evaluate the current planning progress based on the linear accelerator in stereotactic radiotherapy(SRT).The specific techniques include 3-dimensional conformal radiotherapy,dynamic conformal arc therapy,intensity-modulated radiotherapy,and volumetric-modulated arc therapy(VMAT).They are all designed to deliver higher doses to the target volume while reducing damage to normal tissues;among them,VMAT shows better prospects for application.This paper reviews and summarizes several issues on the planning of SRT to provide a reference for clinical application.