Biomedical subjects

D S Followill

Publications and source records attributed to D S Followill.

7 recordsLinked to original sources

PubMed2003

Predictability of electron cone ratios with respect to linac make and model.

In the past, the Radiological Physics Center (RPC) has developed standard sets of photon depth-dose and wedge-factor data, specific to the make, model, and wedge design of the linear accelerator (linac). In this paper, the RPC extends the same concept to electron-cone ratios. Since 1987, the RPC has measured and documented cone-ratio (CR) values during on-site dosimetry review visits to institutions participating in National Cancer Institute cooperative clinical trials. Data have been collected for approximately 500 electron beams from a wide spectrum of linac models. The analysis presented in this paper indicates that CR values are predictable to 2% to 3% (two standard deviations) for a given make and model of linac with a few exceptions. The analysis also revealed some other interesting systematics. For some models, such as the Varian Clinac 2500 and the Elekta/Philips SL18, SL20, and SL25, CR values were nearly identical for cone sizes 15 cm x 15 cm (or 14 cm x 14 cm) and 20 cm x 20 cm across the range of available energies. Certain models of the same make of linac, such as the Mevatron MD, KD, and 6700 series models or the Clinac 2100 and 2300 models, exhibited indistinguishable CRs. Irrespective of linac model, two consistent general trends were observed: namely, an increase in CR value with incident beam energy for cone sizes smaller than 10 cm x 10 cm and a decrease with energy for cone sizes larger than 10 cm x 10 cm. These data are valuable to the RPC as a quality assurance remote-monitoring tool to identify potential dosimetry errors. The physics community will also find the data useful in several ways: as a redundant check for clinical values in use, to validate the values measured during commissioning of new machines or to ensure consistency of values measured during annual quality assurance procedures.

Electrons↗

PubMed1998

A first order approximation of field-size and depth dependence of wedge transmission.

The Radiological Physics Center, through its dosimetry review visits to participating institutions, is aware that many institutions ignore the field-size and depth dependence of wedge transmission values. Reference wedge transmission values are normally measured by the Radiological Physics Center for a 10 cm x 10 cm field at the calibration depth of 5 or 7 cm. Recently, additional measurements (1) for a 10 cm x 10 cm field at 20-cm depth and (2) for a 20 cm x 20 cm field at the calibration depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transmission values for 138 photon beams from 88 separate linear accelerators (4-25 MV) and 60Co units were measured. Our data suggest that the dependence of the wedge transmission on field-size and depth, in the first approximation, depends on the absolute value of the transmission under reference conditions. For wedges with a transmission value greater than 0.65%, field-size dependence and change in depth dose are typically less than 2%. However, for wedges with transmission values less than 0.65%, field-size dependence increases with decreasing reference wedge transmission. The change in wedge transmission with depth is significant (> 2%) only for photon energies less than or equal to 10 MV and can exceed 5% for thick wedges. Failure to include the depth and field-size dependencies of wedge transmission in patient dosimetry calculations can result in significant tumor-dose discrepancies.

Biophysical Phenomena↗

PubMed1998

Equilibration of air temperature inside the thimble of a Farmer-type ion chamber.

Ionization chambers are frequently moved from one environment to another, sometimes with significant differences in temperature between the chamber and measurement phantom. To obtain reliable ionization data, the temperature of the air in the chamber must be allowed to equilibrate with the measuring phantom. The air temperature inside a thimble of a Farmer-type ion chamber was measured as a function of time for various phantom materials (air, water, and plastic). Equilibration rates for the various conditions are presented. Heat-diffusion theory is presented to explain the characteristics of the measured data. Waiting times for temperature equilibration down to 10% of the initial temperature difference ranges from 1 to 18 min, depending on the phantom material and use of bare or covered thimble. Radiation measurements confirm the temperature data.

Equipment Design↗

PubMed1998

An empirical relationship for determining photon beam quality in TG-21 from a ratio of percent depth doses.

A key component of the Radiological Physics Center's (RPC) on-site dosimetry review visits are photon beam calibrations for which determination of the energy of the x ray is a key element. The ratio of ionizations, TPR20/TPR10, for a 10 cm x 10 cm field at depths of 20 and 10 cm for a constant SCD is used as a quantitative measure of beam quality in the Task Group 21 protocol. The RPC has measured both TPR20/TPR10 and the corresponding ratio of percent depth dose (D20/D10) at a constant SSD for 685 photon beams (4-25 MV) for most makes and models if accelerators. A strong correlation between TPR20/TPR10 and D20/D10 is presented which allows the determination of the TPR ratio from the measurement of the ratio of percent depth doses. An analysis of the uncertainty introduced in the TG-21 factors (L/rho, Pwall, Prepl) caused by the spread in the measured data and translated into the determination of the TPR ratio results in an insignificant error (< 0.3%). This empirical relationship provides an alternate technique for quantifying the beam quality defined in the TG-21 protocol without surrendering any loss of precision in output calibration. This technique may be found by those who calibrate at a fixed SSD to be an easier and quicker method.

Calibration↗

PubMed1997

TG-21 versus TG-25: a comparison for electrons.

Since 1984, the Radiological Physics Center (RPC) has used the American Association of Physicists in Medicine Task Group 21 (TG-21) protocol (absorbed dose determination) as the basis of its On-site Dosimetry Review visits to institutions participating in the National Cancer Institute's cooperative clinical trials. Subsequent to the TG-21 protocol, the Task Group 25 (TG-25) report on electron-beam dosimetry was published. The TG-25 report was not intended to supercede the TG-21 protocol, but to supplement it for depths other than dmax. However, both reports included measurement techniques and data regarding the calibration of electron beams. TG-25 was not intended for absolute calibrations made clear by the fact that it does not present all of the data required for plastic phantom calibrations, i.e., unrestricted stopping power ratios. As a result, some confusion has arisen at various institutions as to which protocol should be used for machine calibration. In this study, possible discrepancies that arise when using TG-21, a version of TG-21 modified by the RPC, and TG-25 are compared. The differences in the results are calculated as a function of energy (6 and 20 MeV), chamber type (cylindrical or parallel plate), and the type of phantom material (water, polystyrene, or acrylic). The largest discrepancies noted were between TG-25 and the two TG-21 methods for low-energy electrons in either water or polystyrene. The mean difference for all conditions was 0.8% with a maximum value of 3.3% in polystyrene. The definition of the effective point of measurement; determination of the mean nominal incident energy (E0), mean energy at depth (EZ) and most probable energy at the surface (Ep,0) for each protocol, and subsequent stopping power ratio, chamber replacement factor, and electron fluence correction factor are the major contributors to the calculated differences.

Clinical Protocols↗

PubMed1995

Differential expression of collagen types I and III in consequential and primary fibrosis in irradiated mouse colon.

These studies were undertaken to understand further the pathogenesis of consequential and primary fibrosis in mouse colon after irradiation. The distal 2.5 cm of colon of C3Hf/Kam mice was irradiated with either a single dose of 27 Gy or a split dose of 2 x 14.75 Gy separated by 10 days to induce a consequential or primary fibrotic lesion, respectively. The amount of total collagen in the two lesions was quantified by hydroxyproline, and tensile strength, an assay of tissue rigidity, was measured as a function of dose and time after irradiation. The relative distribution of collagen types I, III and IV in the colon was visualized by immunohistochemistry. Collagen types I, III and IV were quantified by immunoblot techniques, and in situ hybridization was used to identify and score the cells producing procollagen mRNA types I and III as a function of time after irradiation. The hydroxyproline and tensile strength measurements demonstrated that both lesions contained significantly increased amounts of collagen compared to controls. However, the ulcerated lesion of consequential fibrosis contained three times as much collagen and required a three- to fourfold increase in the peak force to rupture the colon as did the non-ulcerative lesion of primary fibrosis. The fibrosis accompanying the consequential lesion contained elevated levels of both collagen types I and III, but primary fibrosis contained only elevated levels of type I collagen compared to controls. The in situ hybridization studies showed cells producing increased amounts of procollagen mRNA 8 and 25 weeks before the elevated levels of collagen were detected for consequential and primary fibrosis, respectively. The cells producing the excess collagen mRNA were identified as fibroblasts. No distinction between the two lesions could be made based on the cell types producing the collagen. The distribution of labeled cells was localized to tissue areas showing specific immunofluorescence for the collagen types. These data show that the two histologically distinct lesions in irradiated colon, consequential and primary fibrosis, contain different collagen types and quantities of each type, suggesting that the underlying pathogenesis of these two lesions may be different.

Animals↗

PubMed1993

Histological changes in mouse colon after single- and split-dose irradiation.

These studies were undertaken to determine the relationship between acute mucosal damage and late obstructions in the colorectal region in the mouse after exposure to radiation. Radiation doses that either permanently depleted the mucosal epithelial cells or spared the mucosal lining by allowing epithelial regeneration were used. The distal 2.5 cm of colon and rectum of male C3Hf/Kam mice was irradiated with either a range of single doses (15-35 Gy) or two equal doses ranging from 9.75 to 14.75 Gy separated by 10 days. The time of onset and the incidence of obstructions and strictures in the bowel were recorded as a function of dose and time after irradiation. Acute damage in the mucosa and subsequent histological changes in the bowel were documented by sequential histological studies. Doses greater than 20 Gy caused acute crypt depletion followed by nonproductive attempts at regeneration and repopulation that culminated in persistent epithelial denudation. In these mice, obstructions appeared as early as 4 weeks and were characterized histologically by a mucosal ulceration extending deep into the muscularis. Single doses of less than 20 Gy and the split doses produced acute crypt cell depletion followed by successful regeneration, repopulation, and restoration of the colonic mucosa. In these mice, obstructions did not appear until at least 40 weeks after irradiation and were characterized by an intact mucosa with a thickened and fibrotic submucosa. Animals given a single dose of 20 Gy developed obstructions throughout the duration of the experiment. Those obstructions that occurred before 6 months were characterized by ulcerations, whereas those that appeared after this time exhibited only fibrosis in the submucosa with no mucosal ulceration. Based on these data, we suggest that two types of late obstructions occur in the bowel, one that depends on persistent epithelial denudation, i.e., a "consequential" response, and the other in the absence of epithelial denudation, i.e., a true late effect.

Animals↗