Purpose Patient-specific IMRT QA measurements are important components of processes designed to identify discrepancies between calculated and delivered radiation doses. Discrepancy tolerance limits are neither well defined nor consistently applied across centers. The AAPM TG-218 report provides a comprehensive review aimed at improving the understanding and consistency of these processes as well as recommendations for methodologies and tolerance limits in patient-specific IMRT QA. Methods The performance of the dose difference/distance-to-agreement (DTA) and γ dose distribution comparison metrics are investigated. Measurement methods are reviewed and followed by a discussion of the pros and cons of each. Methodologies for absolute dose verification are discussed and new IMRT QA verification tools are presented. Literature on the expected or achievable agreement between measurements and calculations for different types of planning and delivery systems are reviewed and analyzed. Tests of vendor implementations of the γ verification algorithm employing benchmark cases are presented. Results Operational shortcomings that can reduce the γ tool accuracy and subsequent effectiveness for IMRT QA are described. Practical considerations including spatial resolution, normalization, dose threshold, and data interpretation are discussed. Published data on IMRT QA and the clinical experience of the group members are used to develop guidelines and recommendations on tolerance and action limits for IMRT QA. Steps to check failed IMRT QA plans are outlined. Conclusion Recommendations on delivery methods, data interpretation, dose normalization, the use of γ analysis routines and choice of tolerance limits for IMRT QA are made with focus on detecting differences between calculated and measured doses via the use of robust analysis methods and an in-depth understanding of IMRT verification metrics. The recommendations are intended to improve the IMRT QA process and establish consistent, and comparable IMRT QA criteria among institutions.

Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218.

Intensity modulated radiation therapy (IMRT) poses a number of challenges for properly measuring commissioning data and quality assurance (QA) radiation dose distributions This report provides a comprehensive overview of how dosimeters, phantoms, and dose distribution analysis techniques should be used to support the commissioning and quality assurance requirements of an IMRT program The proper applications of each dosimeter are described along with the limitations of each system Point detectors, arrays, film, and electronic portal imagers are discussed with respect to their proper use, along with potential applications of 3D dosimetry Regardless of the IMRT technique utilized, some situations require the use of multiple detectors for the acquisition of accurate commissioning data The overall goal of this task group report is to provide a document that aids the physicist in the proper selection and use of the dosimetry tools available for IMRT QA and to provide a resource for physicists that describes dosimetry measurement techniques for purposes of IMRT commissioning and measurement-based characterization or verification of IMRT treatment plans This report is not intended to provide a comprehensive review of commissioning and QA procedures for IMRT Instead, this report focuses on the aspects of metrology, particularly the practical aspects of measurements that are unique to IMRT The metrology of IMRT concerns the application of measurement instruments and their suitability, calibration, and quality control of measurements Each of the dosimetry measurement tools has limitations that need to be considered when incorporating them into a commissioning process or a comprehensive QA program For example, routine quality assurance procedures require the use of robust field dosimetry systems These often exhibit limitations with respect to spatial resolution or energy response and need to themselves be commissioned against more established dosimeters A chain of dosimeters, from secondary standards to field instruments, is established to assure the quantitative nature of the tests This report is intended to describe the characteristics of the components of these systems; dosimeters, phantoms, and dose evaluation algorithms This work is the report of AAPM Task Group 120

/pdf/dosimetry-tools-and-techniques-for-imrt-48s74o7shq.pdf

Dosimetry tools and techniques for IMRT

Distributed applications are difficult to program reliably and securely. Dependently typed functional languages promise to prevent broad classes of errors and vulnerabilities, and to enable program ver- ification to proceed side-by-side with development. However, as recursion, effects, and rich libraries are added, using types to reason about programs, specifications, and proofs becomes challenging.
We present F⋆, a full-fledged design and implementation of a new dependently typed language for secure distributed programming. Our language provides arbitrary recursion while maintaining a logically consistent core; it enables modular reasoning about state and other effects using affine types; and it supports proofs of refinement properties using a mixture of cryptographic evidence and logical proof terms. The key mechanism is a new kind system that tracks several sub-languages within F⋆ and controls their interaction. F⋆ subsumes two previous languages, F7 and Fine. We prove type soundness (with proofs mechanized in Coq) and logical consistency for F⋆.
We have implemented a compiler that translates F⋆ to .NET bytecode, based on a prototype for Fine. F⋆ provides access to libraries for concurrency, networking, cryptography, and interoperability with C#, F#, and the other .NET languages. The compiler produces verifiable binaries with 60% code size overhead for proofs and types, as much as a 45x improvement over the Fine compiler, while still enabling efficient bytecode verification.
We have programmed and verified nearly 50,000 lines of F⋆ including new schemes for multi- party sessions; a zero-knowledge privacy-preserving payment protocol; a provenance-aware curated database; a suite of web-browser extensions verified for authorization properties; a cloud-hosted multi-tier web application with a verified reference monitor; the core F⋆ typechecker itself; and programs translated to F⋆ from other languages such as F7 and JavaScript.

/pdf/secure-distributed-programming-with-value-dependent-types-q2vncf6bg5.pdf

Secure distributed programming with value-dependent types

Challenges in calculation of the gamma index in radiotherapy - Towards good practice.

Quantitative comparisons of dose distributions are an integral component of a medical physicist's responsibility of assuring high quality radiation therapy dose delivery. While overlays and other displays of multiple dose distributions are useful for such evaluations, quantitative evaluations require a mathematical comparison. The dose-difference is the most straightforward method for comparing two dose distributions, but it can show large differences in steep dose gradient regions, even for relatively small misalignments. A tool, termed y,was developed to take both dose and spatial difference into account. It does this automatically, evaluating distributions for dose difference and spatial discrepancies in regions of shallow and steep dose gradients, respectively. The tool has been used extensively in commercial dose measurement and evaluation software. This chapter describes the tool, some alternative techniques, and limitations of the tool.

https://iopscience.iop.org/article/10.1088/1742-6596/250/1/012071/pdf

Gamma Dose Distribution Evaluation Tool

The gamma dose comparison tool has been used by numerous investigators to quantitatively compare multidimensional dose distributions. The gamma tool requires the specification of dose and distance-to-agreement (DTA) criteria for acceptable variations between the dose distributions. The tool then provides a comparison that simultaneously evaluates the dose difference and distance to agreement of the two dose distributions. One of the weaknesses of the tool is that the comparison requires one of the dose distributions to have a relatively high spatial resolution, with points spaced significantly closer than the DTA criterion. The determination of gamma involves an exhaustive search process, so the computation time is significant if an accurate gamma is desired. The reason for the need for high spatial resolution lies with the fact that the gamma tool measures the closest point in one of the dose distributions (the evaluated distribution) with individual points of the other distribution (the reference distribution) when the two distributions are normalized by the dose difference and DTA criteria for the dose and spatial coordinates, respectively. The closest point in the evaluated distribution to a selected reference distribution point is the value of gamma at that reference point. If individual evaluated dose distribution points are compared, the closest point may not accurately reflect the closest value of the evaluated distribution as if it were interpolated on an infinite resolution grid. Therefore, a reinterpretation of the gamma distribution as the closest geometric distance between the two distributions is proposed. This is conducted by subdividing the evaluated distribution into simplexes; line segments, triangles, and tetrahedra for one, two, and three-dimensional (3D) dose distributions. The closest distance between any point and these simplexes can be straightforwardly computed using matrix multiplication and inversion without the need of interpolating the original evaluated distribution. While an exhaustive search is still required, not having to interpolate the evaluated distribution avoids the drastic growth of calculation time incurred by interpolation and makes the gamma tool more practical and more accurate. In our experiment, the geometric method accurately computes gamma distributions between 3D dose distributions on a 200 x 200 x 50 grid within two minutes.

Geometric interpretation of the gamma dose distribution comparison technique: interpolation-free calculation.

Operational Type Theory (OpTT) is a type theory allowing possibly diverging programs while retaining decidability of type checking and a consistent logic. This is done by distinguishing proofs and (program) terms, as well as formulas and types. The theory features propositional equality on type-free terms, which facilitates reasoning about dependently typed programs. OpTT has been implemented in the Guru verified programming language, which includes a type- and proof-checker, and a compiler to efficient C code. In addition to the core OpTT, Guru implements a number of extensions, including ones for verification of programs using mutable state and input/output. This paper gives an introduction to verified programming in Guru.

/pdf/verified-programming-in-guru-4y8d38jre7.pdf

Verified programming in Guru

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Geometric interpretation of the gamma dose distribution comparison technique: interpolation-free calculation.

Verified programming in Guru