Radiant energy – Supported for nonsignalling objects of irradiation – With source support
Reexamination Certificate
2001-06-05
2004-07-13
Pham, Hai (Department: 2861)
Radiant energy
Supported for nonsignalling objects of irradiation
With source support
C250S370070
Reexamination Certificate
active
06762418
ABSTRACT:
FIELD OF INVENTION
The invention relates to the fields of dosimetry and calorimetry, in particular routine dosimetry using calorimetry for the routine monitoring and control of ionizing radiation processing procedures.
BACKGROUND OF THE INVENTION
Radiation processing, the treatment of items with radiation, plays an important role in the production of many products. A radiation process is a method or procedure which uses radiation processing, such as radiation sterilization. Radiation may be used in a radiation process for the sterilization of materials, particularly for medical instruments and accessories, for the pasteurization of food products, and for material processing, such as for inducing or enhancing the polymerization of materials or the introduction of dopants or impurities into substantially pure materials. Electron-beam radiation is a common form of radiation used in radiation processes for sterilization, pasteurization, and for alteration of the properties of materials.
A radiation dosage is an amount of radiation absorbed by an irradiated item. Dosimetry is the measurement of an amount of radiation.
Measurement, by means of a dosimetry system, and reporting of the amount of radiation absorbed by items to be sterilized or otherwise processed by radiation is an important quality control measure. Reporting requirements in highly regulated industries, such as those typically using radiation processing, are stringent and often require much effort. Calibration of the dosimetry system, comprising comparison of the dosimetry system measured values with those of national standard radiation sources with different control settings, is typically performed before use of a radiation source, and optionally at other times thereafter, to determine whether the dosimetry system is providing accurate radiation dosage measurements. The calibrated dosimetry system is used, if necessary, to adjust the radiation source so that it performs as desired. Quality control measurements may be routine measurements taken infrequently or frequently, and often at regular intervals, to monitor the performance of the radiation processing on an on-going basis. Such quality control measurements of radiation delivery during radiation processing are termed routine dosimetry measurements, and differ from calibration measurements in that calibration measurements are used to assure the accuracy of the dosimetry system, and to verify that dose measurements obtained are valid, while routine dosimetry measurements track the performance of a radiation process during use.
Two general categories of dosimeters exist, reference standard dosimeters and routine dosimeters. The objective of the reference standard dosimeters is to provide a link between national standard dosimetry calibration laboratories and production radiation processing facilities. The key criteria is control and accuracy of measured dose. Practicality is thus not an issue for reference standard dosimeters since their use during calibration of routine dosimeters is infrequent. Routine dosimeters, on the other hand, are used for regular quality control within a radiation processing plant. Practicality is of utmost importance, combined with reasonable accuracy.
The amount of radiation delivered by an electron beam may be measured in a number of ways. For example, the current induced by the passage of radiation past capacitor plates was reported as a measure of the radiation beam dosage by Taumann, U.S. Pat. No. 4,427,890, while the current produced in a coaxial sensor placed in an electron beam was reported to be proportional to an electron beam current intercepted by the sensor, and so was said to be useful as a measure of the beam dosage (Fiorito et al., U.S. Pat. No. 4,629,975). The current collected by a beam stop was said by Lawrence et al., U.S. Pat. No. 5,661,305, to be a useful measure of the absorbed dose in a product irradiated by an electron beam. All patents cited herein, both supra and infra, are hereby incorporated by reference in their entirety.
A listing of and a discussion of the advantages and disadvantages of various dosimetry methods may be found in Annex C of “Dosimeters, dosimetry and associated equipment” ANSI/AAMI/ISO 11137-1995 (1995) (referred to herein as “ISO 11137”). For example, calorimetry is listed therein for use as a reference standard dosimeter, while several spectrophotometric methods are listed as examples of routine dosimeters.
One method of detecting and measuring radiation is to measure the optical density of radiation-sensitive films (also known as radiochromic films) exposed to radiation. Commonly, calibration is performed by irradiation of unexposed radiation-sensitive film routine dosimeters together with controlled reference standard dosimeters from national standards laboratories. The results of such film dosimetry are determined by the amount of exposure of the film following irradiation, such as by measuring the optical density of the developed film. Results of the reference standard dosimeters are then correlated with the results of the routine film dosimeters to form calibration curve for the film dosimeters. Film dosimetry is performed for routine dosimetry to confirm that the appropriate amount of radiation is being delivered to the items to be irradiated, and to correct the exposure if it is found to be inaccurate. However, there is a delay in obtaining film dosimetry readings while the film is developed, and film dosimetry is subject to problems of reliability and consistency due to variation between films, variability of film placement on or within the items being irradiated, variability in the time between irradiation and film development, and the effect of temperature, humidity and ultraviolet light levels on the exposure characteristics of the film (see, for example, Table C5, ISO 11137).
The absorption of radiation heats an object that has been irradiated. Calorimetry, which measures heat, may be used as a method of dosimetry by measuring a temperature change in an irradiated item and correlating the radiation dosage absorbed with the temperature change. However, in order to be accurate, these measurements must be made without allowing any significant loss of heat. Radiation processing with gamma radiation, for example, may take several hours. Electron beam sources typically require a shielding maze and multiple passes within the maze, requiring an hour or more for radiation processing. Significant amounts of heat transfer might occur during this time, making calorimetry measurements difficult and inaccurate under these circumstances. Attempts to calibrate an electron beam source radiation by placing a calorimeter on an arm that swings into the path of the electron beam require that radiation processing of other items be stopped, and the system reconfigured for normal operation before radiation processing is able to begin again. However, such a system cannot be used for routine dosimetry measurements because the calorimeter interrupts the normal functioning of the system.
The variability in the measuring tools presently used in routine dosimetry hinders routine measurement of the performance and reliability of radiation sources. However, radiation processing, such as for sterilization, is a heavily audited and critical process in the production of medical implements and medical instrumentation, in food processing, and in the processing of many materials. The ability to minimize quality control issues related to routine dosimetry would improve present methods and significantly reduce the risk of non-compliance with the strict regulations that are typically applied to radiation processing.
Accordingly, what is required are systems and methods for radiation processing and for routine dosimetry that are not affected by sensor location, variability in sensing elements, and other such problems, and are capable of being used without interference with the normal operation of radiation processing methods.
SUMMARY OF THE INVENTION
The present invention is directed to a method and system for determini
Coomber Allen
Lambert Byron
Riggs Brian
Tang Fuh-Wei
Volpone Michael
Advanced Cardiovascular Systems Inc.
Fulwider Patton Lee & Utecht LLP
Pham Hai
LandOfFree
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