Wong Tp

A new radiotherapy surface dose detector: The MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOS-FET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-microns in diameter and consists of a 0.5-microns Al electrode on top of a 1-microns SiO2 and 300-microns Si substrate. Results for % surface dose were within +/- 2% compared to the Attix chamber and within +/- 3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10 x 10-cm and 40 x 40-cm field size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

X-ray surface dose measurements using TLD extrapolation.

Surface dose measurements in therapeutic x-ray beams are of importance in determining the dose to the skin of patients undergoing radiotherapy. Measurements were performed in the 6-MV beam of a medical linear accelerator with LiF thermoluminescence dosimeters (TLD) using a solid water phantom. TLD chips (surface area 3.17 x 3.17 cm2) of three different thicknesses (0.230, 0.099, and 0.038 g/cm2) were used to extrapolate dose readings to an infinitesimally thin layer of LiF. This surface dose was measured for field sizes ranging from 1 x 1 cm2 to 40 x 40 cm2. The surface dose relative to maximum dose was found to be 10.0% for a field size of 5 x 5 cm2, 16.3% for 10 x 10 cm2, and 26.9% for 20 x 20 cm2. Using a 6-mm Perspex block tray in the beam increased the surface dose in these fields to 10.7%, 17.7%, and 34.2% respectively. Due to the small size of the TLD chips, TLD extrapolation is applicable also for intracavity and exit dose determinations. The technique used for in vivo dosimetry could provide clinicians information about the build up of dose up to 1-mm depth in addition to an extrapolated surface dose measurement.

Dosimetry of 6-MV x-ray beam penumbra

The measurement of x-ray beam dose profiles in the penumbral region, using silicon diode, ionization chamber, TLD, and film dosimetry, has been investigated for a 6-MV beam defined by independent collimators. Penumbral width (80%-20%) at dmax, as measured by diode, film, and TLD was found to be 3.6, 3.6, and 3.4 mm, respectively. These results reflect the relative sensitive widths of each of the measurement systems (2.5, 2.0, and 1.0 mm, respectively). An empirical forming function was used to relate the penumbral shape measured with a finite-sized detector to that which would be measured with a point detector, the width of the point detector penumbra calculated from the diode penumbra is 3.4 mm, indicating that the TLD rods are a good approximation to a point detector. An alternative method of determining the width of a point detector penumbra is to extrapolate the penumbral widths obtained using two or more detectors of sensitive width. With this method, using Farmer and RK ionization chambers, a point detector penumbra width of 3.1 mm is obtained. An EGS4 Monte Carlo simulation, where a point source was assumed, gave a penumbral width of 2.8 mm. Negligible differences between the penumbra of beams defined by symmetric and asymmetric collimators was observed.

Medical physics aspects of cancer care in the Asia Pacific region.

Medical physics plays an essential role in modern medicine. This is particularly evident in cancer care where medical physicists are involved in radiotherapy treatment planning and quality assurance as well as in imaging and radiation protection. Due to the large variety of tasks and interests, medical physics is often subdivided into specialties such as radiology, nuclear medicine and radiation oncology medical physics. However, even within their specialty, the role of radiation oncology medical physicists (ROMPs) is diverse and varies between different societies. Therefore, a questionnaire was sent to leading medical physicists in most countries/areas in the Asia/Pacific region to determine the education, role and status of medical physicists. Answers were received from 17 countries/areas representing nearly 2800 radiation oncology medical physicists. There was general agreement that medical physicists should have both academic (typically at MSc level) and clinical (typically at least 2 years) training. ROMPs spent most of their time working in radiotherapy treatment planning (average 17 hours per week); however radiation protection and engineering tasks were also common. Typically, only physicists in large centres are involved in research and teaching. Most respondents thought that the workload of physicists was high, with more than 500 patients per year per physicist, less than one ROMP per two oncologists being the norm, and on average, one megavoltage treatment unit per medical physicist. There was also a clear indication of increased complexity of technology in the region with many countries/areas reporting to have installed helical tomotherapy, IMRT (Intensity Modulated Radiation Therapy), IGRT (Image Guided Radiation Therapy), Gamma-knife and Cyber-knife units. This and the continued workload from brachytherapy will require growing expertise and numbers in the medical physics workforce. Addressing these needs will be an important challenge for the future.

VERIFICATION OF BRACHYTHERAPY DOSIMETRY WITH RADIOCHROMIC FILM

The aim of this work is to empirically validate the optimized dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3) at a distance of 1.0 cm from a stepping source of high-dose-rate-iridium 192 (192Ir). The longitudinal dose distribution at 1.0 cm from a straight pathway of multiple-source positions is measured using radiochromic film and compared with the planning systems calculated results. The optical density of the exposed films was determined with a modified Scanditronix film scanner, and the film was calibrated with 192Ir using manually calculated exposure times. A calibration equation was used to convert scanner output to dose. Our results illustrate the significance of exacting geometry in the experimental setup due to the inverse square law and the small distances involved. The dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3), at a distance of 1.0 cm, is validated to within +/-4% of the measured dose distribution. The advantages and limitations of radiochromic film as a dosimetry tool are also addressed in this work.

Magnetic repulsion of linear accelerator contaminates.

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20 x 30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20 x 20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.

Thermoluminescence dosimetry of therapeutic x-rays with LiF ribbons and rods

Extruded LiF ribbons (3.1*3.1*0.9 mm3) and rods (6*1*1 mm3) are commonly used thermoluminescence dosimeters (TL dosimeters) for clinical dosimetry in radiotherapy. The dose distribution in these crystals was investigated in a 6 MV X-ray beam using smaller LiF TL dosimeter types (thin ribbons with a thickness of 0.14 mm and small cubes with 1 mm side length). Using the thin ribbons, the effective depth of measurement in the normal ribbons was found to be shifted towards the surface. Measuring on the surface of a solid water phantom in a 10*10 cm2 field it was found to be at a depth of 0.4 mm as compared to the physical centre of the ribbons of 0.44 mm. In the investigations with small cubes assembled in form of ribbons and rods it was found that a higher dose was deposited in the centre of the ribbons and rods. Accordingly it was found that TL dosimeters in close contact with each other increase their respective reading. Using two ribbons in contact with each other on the surface of a phantom leads to an overestimation of dose of about 1%. The specific dose response of LiF increases with dosimeter size, this is most likely due to increased electron scatter from the additional LiF material with density higher than unity.

Dosimetry errors in endovascular high-dose-rate brachytherapy.

Monte Carlo data were used to demonstrate the dosimetry of the microSelectron high-dose-rate (HDR) iridium 192 (192Ir) stepping source. These data were used to assess the accuracy of the Nucletron brachytherapy planning system (BPS version 13) for peripheral vessel endovascular brachytherapy. Dose rates from the high-dose-rate (HDR) source are calculated using the Monte Carlo code MCNP4A. Calculations are made at 0.25-cm intervals in the longitudinal direction on sleeves of radii of 1 and 0.25 cm. The Monte Carlo data are summed and weighted to simulate the longitudinal dose distribution at a distance of 1 and 0.25 cm from an 192Ir source stepping through a straight pathway. A comparison is made between the simulated Monte Carlo dosimetry and the Nucletron brachytherapy planning systems dosimetry. This study illustrates and quantifies the dosimetric errors at small distances associated with a point source dose calculation algorithm. The effects of step size, dwell time optimization, and active length on the accuracy of BPS v.13 for HDR endovascular brachytherapy are demonstrated.

Standard linear plans in single channel high dose rate brachytherapy: a dosimetric analysis

The use of standard linear plans is proposed for single channel intraluminal High Dose Rate brachytherapy. This technique employs the optimized dwell times derived from a straight line within a curved geometry. Such standardization of the planning procedure ensures expedient delivery of treatment. The 3-D dose distribution resulting from the use of standard linear plans within various curved geometries is investigated. In this study a comparison is made between the dose delivered to the perimeter of the target volume from both standard linear plans and individually optimized plans. Our results demonstrate that the use of a standard linear plan is acceptable in curved geometries, given the current practice of dose and volume specification for high dose rate intraluminal brachytherapy.