Biomedical subjects

Tsang Cheung

Publications and source records attributed to Tsang Cheung.

28 records · Page 2Linked to original sources

PubMed2003

Source and detector polarization effects on radiographic film dosimetry.

Kodak X-Omat V radiographic film has been tested for its polarization properties when using polarized and non-polarized light sources and detectors. The radiographic film has been shown to produce a relatively small but not negligible (less than 5%) variation in 90 degrees cycles in measured optical density within the visible spectrum when the light source is fully linear polarized and the film is rotated through 360 degrees angle. Negligible variations are seen when the detector is linearly polarized. If both light source and detector are linearly polarized, variations in measured optical density can reach 35% when the film is rotated through 360 degrees angle. This seems to be due to variations in the degree and axis of rotation in polarized light caused by the radiographic film and is independent of exposure level as the intensity of variation in absolute optical density is relatively the same for all film exposures. We recommend that a polarization test be performed on a densitometry system to establish the extent of its polarization properties before accurate dosimetry is performed with radiographic film.

Artifacts↗

PubMed2003

Rounded end multi-leaf penumbral measurements with radiochromic film.

Multi-leaf penumbral doses have been investigated for 6 MV x-rays and a Varian millennium multi-leaf collimator (MLC) using Gafchromic MD-55-2, radiochromic film and X-omat V radiographic film. An advantage of Gafchromic film for multi-leaf penumbral dose measurement is the relatively low energy dependence of the film. A comparison of penumbral dose measurements has also ascertained the effects of energy response on radiographic film in this region. Similar 80%/20% penumbral doses have been measured with both types of films. Thus there is a relatively low energy effect on penumbral dose measurements in film dosimetry. The 80%/20% dose penumbral distances for rounded leaf end multi-leafs for a 10 cm x 10 cm field at Dmax were found to be 4.6 mm and 4.3 mm for radiochromic and radiographic film respectively. This is compared to 2.6 mm and 2.6 mm for the leaf edge penumbra. Radiochromic film also measured leaf end/interleaf leakage doses in the penumbral region, which was shown to produce approximately 4% of maximum dose wave across the penumbral region with maximum doses delivered at the MLC leaf interfaces.

Equipment Failure Analysis↗

PubMed2003

Polarization effects on a high-sensitivity radiochromic film.

A new high-sensitivity radiochromic film has been tested for its polarization properties. Gafchromic HS film has been shown to produce a relatively small (less than 3%) variation in the optical density measured at 660 nm wavelength when the light source is fully linear polarized and the film is rotated through a 360 angle. Similar variations are seen when the detector is linearly polarized. If both the light source and the detector are linearly polarized, variations in the measured optical density can reach 15% when the film is rotated through a 360 angle. This seems to be due to a phase shift in polarized light caused by the radiochromic film resulting in the polarized light source becoming out of phase with the polarized detector. Gafchromic HS radiochromic film produces a minimal polarization response with varying angle of rotation; however, we recommend that a polarization test be performed on a densitometry system to establish the extent of its polarization properties before accuracy dosimetry is performed with radiochromic HS film.

Absorptiometry, Photon↗

PubMed2002

Corresponding dose response of radiographic film with layered gafchromic film.

This note investigates the dose response of layered HS Gafchromic film compared to Kodak EDR-2 radiographic film. Using five layers of HS type Gafchromic film a dose response greater than EDR-2 film is achieved at the peak wavelength (0.55 OD/Gy versus 0.3 OD/Gy for EDR-2 film). Even over a broader waveband of 30 nm, which is similar to that found in ultra bright LED scanners, the response was found to be 0.38 OD/Gy as opposed to 0.29 OD/Gy. Measurements averaged over the entire visible spectrum produce a relative dose response of 0.165 OD/Gy for five layer HS and 0.29 OD/Gy for EDR-2 film. Due to this high dose response that is achievable, the five layer HS could be used in applications where small doses are delivered to certain areas and a low dependence of energy response is required for measurement.

Dose-Response Relationship, Radiation↗

PubMed2002

High sensitivity radiochromic film dose comparisons.

This short note investigates the dose characteristics of a relatively new high sensitivity radiochromic film (Gafchromic HS) and compares dose and energy response to various Gafchromic film types and radiographic (EDR-2) film. The original MD-55-1 and two improved sensitivity films, MD-55-2 and HS film, were investigated for energy and dose response. Results show that the energy response of the new HS film is relatively the same as the original MD-55-1 and MD-55-2 films with a decrease in sensitivity at lower x-ray energies, with response decreasing down to approximately 0.64 (normalized to 1 for a 6 MV beam) for a 28 keV effective energy beam. This is compared to an over response of 9.2 at the same energy for EDR-2 film. The dose response at the maximum absorption peak was found to be approximately 3.8 and 1.9 times more sensitive than MD-55-1 and MD-55-2 films, respectively. At the absorption peak yielding the maximum optical density change, HS was found to be approximately 0.2 to 0.25 times the sensitivity of EDR-2.

Dose-Response Relationship, Radiation↗

PubMed2002

Effects of water light absorption properties of a radiographic film.

Photon beam dosimetry using Kodak extended dose range (EDR2) radiographic film can provide accurate and high spatial resolution information especially for areas such as IMRT dosimetry where a higher dose level (100-400 cGy) is often required to be delivered for verification. For such dosimetry checks, it may sometimes be useful to place the film in a tank filled with water during irradiation. The effects of water on the film when packaged and when removed from the packaging have been examined. Results have shown that the EDR2 film when supplied in the ready pack form is provided in water proof packages and no significant absorption effects are observed or measured on the film even after 48 h of soaking in a water bath. When the film is removed from the ready packs and exposed to water directly, various effects are seen. In the visible spectrum region, small variations (up to 3%) in recorded optical density (OD) are recorded using a fluorescent light densitometer. These effects become much larger in the infrared region (e.g. 7.5% at 900 nm and 12.5% at 1000 nm) and are wavelength dependent. The changes produced by the water are relatively independent of the exposure time to water from 5 s up to 1 h or whether the water exposure occurred.

Absorptiometry, Photon↗

PubMed2002

Use of a blood glucose meter for radiochromic film analysis in blood irradiation.

The use of a diabetic blood glucose meter for radiochromic film dosimetry in blood irradiation using x-ray beams on a medical linear accelerator has been investigated. The glucose meter provides optical density analysis in the visible and infrared region using a reflectance measurement technique. By comparing the 'blood sugar' level output with standard calibration gafchromic films a calibration curve is produced for quantitative analysis. Results show that a reproducible dose to meter output curve can be fitted using a second order polynomial function and that blood irradiation doses in vitro were measured to within 7.9% mean error (as compared to ionization chamber results) using the blood glucose meter. This level of accuracy falls below that measured with a standard densitometer (4.3%); however, results show that the blood glucose meter, which would be available in any haematology department, produces an adequate measure of gafchromic film optical density for blood irradiation dosimetry.

Blood↗

PubMed2002

Measurement of skin dose variations produced by a silicon-based protective dressing in radiotherapy.

Variations in skin dose caused by a silicon-based burn dressing used in radiotherapy during treatment have been investigated. Measurement of these variations in skin dose has been achieved using thermoluminescent dosimeters (TLDs) and Gafchromic film. For a 6 MV x-ray beam results have shown that an approximately 0.4 mm thick silicon mesh dressing increases the average surface dose by approximately 12.5% to 14% of the maximum and average dose at 1 mm depth and by 4% to 6% of the maximum for field sizes ranging from 5 cm x 5 cm up to 40 cm x 40 cm at 100 cm source to surface distance (SSD). The radiation effective thickness of the silicon dressing was calculated to be 0.5 mm +/- 0.05 mm water equivalent. TLDs of various thicknesses provide point-dose assessment and Gafchromic film can provide a detailed two-dimensional dose map with a high spatial resolution. Results have shown that a large variation in skin dose is delivered under the dressing depending on the amount of material directly above it as defined by the silicon mesh outline.

Humans↗

PubMed2002

Multilayer Gafchromic film detectors for breast skin dose determination in vivo.

Assessment of skin dose delivered to patients from radiotherapy x-ray beams should be performed both inside and outside the prescribed treatment fields. A multilayer Gafchromic film detector which has high sensitivity for detection of radiation can be used to measure skin dose in a two-dimensional map over the skin surface if required. This is an advantage over other detectors, which only provide point dose estimates. A study of 25 patients undergoing breast irradiation was performed to analyse the ability of the multilayer detector to analyse skin dose and to assess both in-field and out-of-field radiation doses delivered during tangent field breast irradiation. Results show that the main contributor to total skin dose within the treatment field was delivered by exit dose. However, outside the field, most dose was delivered by entry beams. Patients with smaller breast separations where found, in general, to receive a higher total skin dose from entry and exiting beams at the central axis. Results also showed that a significant skin dose was delivered outside the treatment field and the main cause of this dose was from electron contamination from entry beams. The multilayer Gafchromic film detector provided adequate skin dose assessment within one fraction of treatment for in vivo results.

Breast↗

PubMed2002

Variations in skin dose associated with linac bed material at 6 MV x-ray energy.

Treatment with radiotherapy x-rays at 6 MV energy produces a build-up effect whereby a smaller dose is delivered to the patient's skin compared to the tumour dose. With anterior fields, no material is normally placed over the patient's skin, thus providing the maximum skin sparing possible with the beam configuration used. A posterior beam normally passes through the treatment couch top and increases the dose delivered to the patient's skin. Both the Mylar sheeting and the support ribbing material produce a significant increase in skin dose. Measurements at 6 MV have shown that the basal cell layer dose can be increased by up to 51% of maximum dose with a carbon fibre/Mylar couch and by 28% for a tennis string/Mylar couch when compared to anterior beams. These values are associated with the position of the carbon fibre or tennis string ribbing. Dermal layer doses are increased by up to 30 and 24% of maximum dose for carbon fibre and tennis string, respectively. These values include a combination of dose due to the support ribbing and the Mylar sheeting. Due to the variability in patient positioning on the couch top, these increases would be spread out over the skin surface producing an average increase per unit area at the basal layer of up to 32 and 20% of the maximum, respectively, for carbon fibre and tennis string couch tops and 21 and 12% at the dermal layer compared to dose at Dmax.

Humans↗