Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters.

Thermoluminescence dosimeters (TLD) are widely used to verify absorbed doses delivered from radiation therapy beams. Specifically, they are used by the Radiological Physics Center for mailed dosimetry for verification of therapy machine output. The effects of the random experimental uncertainties of various factors on dose calculations from TLD signals are examined, including: fading, dose response nonlinearity, and energy response corrections; reproducibility of TL signal measurements and TLD reader calibration. Individual uncertainties are combined to estimate the total uncertainty due to random fluctuations. The Radiological Physics Center's (RPC) mail out TLD system, utilizing throwaway LiF powder to monitor high-energy photon and electron beam outputs, is analyzed in detail. The technique may also be applicable to other TLD systems. It is shown that statements of +/- 2% dose uncertainty and +/- 5% action criterion for TLD dosimetry are reasonable when related to uncertainties in the dose calculations, provided the standard deviation (s.d.) of TL readings is 1.5% or better.

Results of photon absorbed-dose measurements using the AAPM TG-21 protocol for accelerating potentials up to 26 MV.

The AAPM Task Group 21 protocol for the calibration of high-energy photon and electron beams was produced to accomplish essentially two goals: (1) incorporate the latest physical data available for calculating absorbed dose from ionization measurements and (2) to eliminate inconsistencies in absorbed dose measurements made with various ion chamber and phantom combinations. The ability of the protocol was assessed to consistently determine x-ray absorbed dose from measurements made with four Farmer-type chambers and one parallel-plate chamber in water, polystyrene, and acrylic phantoms. The measurements were performed using seven high-energy x-ray beams from 60Co to 26-MV nominal accelerating potential. The absorbed dose to water calculated from measurements made with the various chamber and phantom combinations were found to be consistent. The doses calculated for the two most common phantom materials, water and polystyrene, were found to be in excellent agreement. This resolved a 1.6% discrepancy in the absorbed dose determined from the two phantoms using the SCRAD protocol. The doses for acrylic phantoms were found to be approximately 1.2%, low for nominal accelerating potentials less than 8.8 MV. For accelerating potentials of 8.8 MV or greater the agreement was considerably better. The mean dose determined for the parallel-plate chamber from measurements in polystyrene was found to be within 0.7% of the mean dose determined using Farmer-type ion chambers in all phantom materials.

RPC technical report #18: information that should be included in every patient's radiotherapy treatment record (external beam).

The Radiological Physics Center (RPC) has frequently been asked to develop a model daily treatment record. It became obvious that there was some minimum information that must be in patient records. However, equally obvious was the fact that practices at institutions could be very different, so organization of information in records was different. We have compiled a list of data that must be in a patient's treatment record to assure that the radiation treatment can be reconstructed at a later date. The ordering of this information in forms and records is left to the discretion of the institution.

Verification of total body photon irradiation dosimetry techniques.

A method of verifying the dosimetry of patients undergoing total body irradiation (TBI) with photon beams having energies from cobalt-60 to 25 MV is presented. A simple set of spot checks at the TBI axis has been used to verify data used for TBI dosimetry. Calculations to verify dose delivered to TBI patients are done in the same manner as those irradiated at standard treatment distances. A simple method of effective field size determination for various anatomical locations in a typical adult is presented. Measurements in an Alderson phantom with thermoluminescent dosimeters and an ion chamber at several anatomical locations indicate that this calculational method can predict the dose along the patient axis to within 4% for 60Co and 18-MV photon beams, provided the dosimetry data are appropriate (as determined by the spot checks). Results of intercomparisons of TBI beam calibration, off-axis and depth-dose data at various institutions visited by the Radiological Physics Center are also presented.

Mailable TLD system for photon and electron therapy beams.

The mailable TLD system developed by the Radiological Physics Center for monitoring calibration of photon beam energies from cobalt 60 to 25 MV and electron beam energies from 6 to 20 MeV has been in use since 1977 for photons and since 1982 for electron beams. Design considerations, proper use of the system and calibration techniques are detailed. The accuracy of the system is comparable to that of ion chamber measurements made in a water phantom, although it shows less precision.

Electron beam central axis depth dose measurements.

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron-producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.

A comparison of high-energy accelerator depth dose data.

Accurate depth dose information is necessary for the use of high-energy radiotherapy photon beam units. It would be useful, therefore, to have one set of published data available for each different type unit manufactured to which physicists can compare their measured data. Pertinent questions are raised regarding the similarity between accelerators and their central axis depth dose characteristics, the availability of adequate published central axis depth dose data, and the minimum amount of data needed to determine the applicability of published data to a particular machine. Data taken by the Radiological Physics Center (RPC) for 4-10 MV units are analyzed and compared with published data in an attempt to answer these questions.