(87 days)
Polymer-gel dosimeters may be used for quality assurance procedures for radiotherapy treatments whenever three dimensional dose distributions are required. They can be used for routine measurements of radiation dose distributions produced by irradiation devices delivering doses in the range 0-30 Gy.
Polymer-Gel Dosimeters consist of various monomers dispersed in a gelatin matrix. These monomers are polymerized by the free radicals produced by ionizing radiations, and the extent of polymerization is proportional to the absorbed dose. Both the NMR and optical properties of the irradiated gel are thus changed, and dose distributions may be imaged and quantitated using magneticresonance imaging (MRI) or computerized optical tomographic scanning (currently under development by MGS). The polymerization converts the original solution of monomers into a suspension of microparticles that are each much smaller than one micron, and which are fixed in space by the gel, and so the resolution of polymer-gel dosimeters is limited mainly by the imaging device employed. Using a standard head coil and a clinical MRI, pixel sizes on the order of 1 mm are obtained when imaging 2-4 liter gels. For the higher resolution that may be required for brachytherapy sources, gels of about 0.5 liters can be imaged in small-bore, higher-frequency MRI's and pixel sizes of fractions of a millimeter obtained.
Polymer-Gel Dosimeters contain only organic molecules and water, and so their average and effective atomic numbers, and mass densities are, depending upon the specific formulation, very nearly the same as that for muscle tissue. Also, their dose response curves exhibit little or no radiation-quality dependence over the range of xand gamma-ray energies employed in radiation therapy, nor do they show any doserate dependence for dose rates in the range 0.06-16 Gy/min. Of equally great importance is that polymer gels take the shape of their containers which could simulate various parts of the anatomy, and even contain bone and air cavities. This latter feature will provide data not obtainable by any other practical means, and which can provide a benchmarks for treatment-planning-computer algorithms.
As polymerization of the monomers in polymer-gel dosimeters is inhibited by oxygen, it is essential that the vessel that contains the gel be oxygen-free when it is filled and impermeable to oxygen during the period of irradiation and for one hour post-irradiation during which time polymerization goes to completion. This requirement places severe constraints upon the techniques employed for gel preparation, and upon the materials and methods of fabrication of the gel vessels. At the present time, MGS provides polymer gels in 2-liter, spherical glass vessels for confirmation of stereotactic radiosurgery, and 1.0-liter glass bottles for brachytherapy-source dosimetry. These vessels are shipped in nitrogen-filled pouches made from a unique aluminum/Saran/polyethylene foil. The fabrication of vessels made from Barex plastic, a product of the BP Chemicals Corp., and which is impermeable to oxygen, is also possible.
The images recorded by polymer-gel dosimeters are permanent thus permitting comparisons between, for example, the dose distributions for a particular x-ray beam which were made even years apart.
Here's the information about the acceptance criteria and the study that proves the device meets them, based on the provided text:
Device: BANG Polymer-Gel Dosimeter
1. Table of Acceptance Criteria and Reported Device Performance
| Acceptance Criteria (Implied) | Reported Device Performance |
|---|---|
| Ability to determine dose distributions produced by x-ray, gamma-ray, and electron beams. | The study, "Radiation therapy dosimetry using MRI of polymer gels" published in Medical Physics, 23, 699-705 (figures 3-5), compared the BANG Polymer-Gel Dosimeter with the Wellhofer automated water-tank scanner. |
The reported performance indicates that dose response curves and central-axis depth-dose curves determined for high-energy x-ray and electron beams using the Polymer-Gel Dosimeter were in "close proximity" to those obtained using the Wellhofer automated water-tank scanner. This "close proximity" was deemed "well within the experimental uncertainty limits" of both devices. |
| Substantial Equivalence: Equivalence to the automated water-tank scanner produced by the Wellhofer Corp. (K945321) in terms of dose distribution determination. | Conclusion: "it is the conclusion of MGS Research that the Polymer-Gel Dosimeter is substantially equivalent to the Wellhofer automated water-tank scanner for the determination of dose distributions produced by x-ray, gamma-ray and electron beams."
Further details on performance:
- Tissue Equivalence: Contains only organic molecules and water, with average/effective atomic numbers and mass densities "very nearly the same as that for muscle tissue."
- Radiation Quality Dependence: Exhibits "little or no" dependence over the range of x- and gamma-ray energies used in radiation therapy.
- Dose Rate Dependence: Shows "no dose-rate dependence for dose rates in the range 0.06-16 Gy/min."
- Resolution: Due to small voxel size (1x1x3 mm) obtained by MRI, resolution is generally higher than with water-tank scanners.
- 3D Capabilities: Polymer-Gel Dosimeter records doses to all points simultaneously, allowing for true 3D dose distributions, which are not directly obtainable by the point-by-point method of the water-tank scanner in a practical manner for full 3D. |
2. Sample Size Used for the Test Set and Data Provenance
The document does not explicitly state the specific sample size (e.g., number of measurements, number of gel samples) used in the comparative study in "Radiation therapy dosimetry using MRI of polymer gels."
- The study involved comparing dose response curves and central-axis depth-dose curves for high-energy x-ray and electron beams. This implies multiple measurements were taken to generate these curves for both the Polymer-Gel Dosimeter and the Wellhofer scanner.
- Data Provenance: The study was a prospective comparison, conducted by MGS Research, Inc., likely in a controlled laboratory or clinical phantom setting, given the nature of dosimetry. The country of origin for the data is not explicitly stated in the provided text, but the applicant (MGS Research, Inc.) is based in Madison, CT, USA.
3. Number of Experts Used to Establish the Ground Truth for the Test Set and Their Qualifications
The concept of "experts establishing ground truth" in the typical sense (e.g., radiologists reviewing images) does not directly apply here. For dosimetry devices, the ground truth is established by a reference standard method and physics principles.
- Ground Truth Method: The "ground truth" for the dose measurements in this context was established by the Wellhofer automated water-tank scanner, which is described as the predicate device. This device itself is considered a standard for dosimetry.
- Qualifications: While not explicitly stated, the expertise lies within the field of medical physics and radiation oncology/dosimetry, where the Wellhofer system is a recognized measurement tool. The authors of the referenced "Medical Physics" paper would be the experts conducting and analyzing the study.
4. Adjudication Method for the Test Set
Adjudication methods (like 2+1, 3+1 for expert review) are not applicable here. The comparison is based on quantitative physical measurements. The "adjudication" is the direct comparison of the quantitative dose measurements (dose response curves and depth-dose curves) between the two devices. The determination of "close proximity" was likely based on statistical or experimental uncertainty analysis inherent to physics measurements.
5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study Was Done
No, a Multi-Reader Multi-Case (MRMC) comparative effectiveness study was not done. MRMC studies are typically used for diagnostic imaging devices where human readers interpret images. This device is a dosimeter, designed for physical measurement of radiation dose, not for human interpretation of images in a diagnostic context that would require multiple readers.
6. If a Standalone (i.e., algorithm only without human-in-the-loop performance) Was Done
Yes, a standalone performance study was done for the Polymer-Gel Dosimeter. The study compared the measurements of the Polymer-Gel Dosimeter against an established reference standard (the Wellhofer automated water-tank scanner). The Polymer-Gel Dosimeter, as described, directly measures and records dose distributions, and its output (including MRI image processing to convert relaxation rates to dose maps) is the "algorithm only" performance, given it's a physical measurement device. There is no human interaction in the measurement process itself that alters the fundamental dose output from the gel.
7. The Type of Ground Truth Used
The type of ground truth used was a reference standard measurement by a predicate device. Specifically, the dose measurements obtained from the Wellhofer automated water-tank scanner served as the comparative ground truth.
8. The Sample Size for the Training Set
The concept of a "training set" in the context of machine learning algorithms is not directly applicable to this device as described. The BANG Polymer-Gel Dosimeter is a physical measurement device that relies on the chemical properties of the gel and imaging physics (MRI).
- However, the device does require a calibration kit of identical gel "irradiated to known doses" to establish its dose-response curve (R2 vs Gy). This calibration process is analogous to "training" a measurement instrument, but it's not a machine learning training set with a large sample size of diverse cases.
- The document does not specify the sample size for this calibration kit.
9. How the Ground Truth for the Training Set Was Established
The "ground truth" for the calibration kit (which establishes the R2 vs Gy dose-response curve for the gel) is established by irradiating samples of the gel to "known doses". These "known doses" would be delivered by a precision radiation source at a calibrated facility, where the dose delivered is independently verified using primary or secondary dosimetry standards.
§ 892.5050 Medical charged-particle radiation therapy system.
(a)
Identification. A medical charged-particle radiation therapy system is a device that produces by acceleration high energy charged particles (e.g., electrons and protons) intended for use in radiation therapy. This generic type of device may include signal analysis and display equipment, patient and equipment supports, treatment planning computer programs, component parts, and accessories.(b)
Classification. Class II. When intended for use as a quality control system, the film dosimetry system (film scanning system) included as an accessory to the device described in paragraph (a) of this section, is exempt from the premarket notification procedures in subpart E of part 807 of this chapter subject to the limitations in § 892.9.