Jao-Perng Lin

The measurement of photoneutrons in the vicinity of a Siemens Primus linear accelerator

This study involves the measurement of photoneutron contamination emitted from a Siemens Primus medical linear accelerator by using BD-PND bubble detectors. Various bubble detectors were arranged around the linac head with the interval of I m and at the same height as the isocenter to measure the dose equivalent distribution in the treatment room. The measurements were performed for 15 MV X-rays with 40 x 40cm2 and 0 x 0cm2 fields and for 15,18, and 21 MeV electrons with 25 x 25 cm2 electron cone. Neutron dose equivalent rate at the points of measurement in the treatment room decreased with increasing distance to the isocenter. The maximum neutron dose equivalents were at the isocenter, and the values for 15MV 40 x 40 and 0 x 0 cm2 were 1843+/-90 and 169.9+/-59.9 microSv per Gy X-ray, respectively. The values for 15, 18 and 21 MeV electrons with 25 x 25 cm2 cones were 100.0+/-20.4, 262.7+/-61.2 and 349.0+/-29.6 microSv per Gy electron, respectively. The neutron contamination of electrons less than 12 MeV was below the detection limit.

Monte Carlo simulation of a clinical linear accelerator

The effects of the physical parameters of an electron beam from a Siemens PRIMUS clinical linear accelerator (linac) on the dose distribution in water were investigated by Monte Carlo simulation. The EGS4 user code, OMEGA/BEAM, was used in this study. Various incident electron beams, for example, with different energies, spot sizes and distances from the point source, were simulated using the detailed linac head structure in the 6 MV photon mode. Approximately 10 million particles were collected in the scored plane, which was set under the reticle to form the so-called phase space file. The phase space file served as a source for simulating the dose distribution in water using DOSXYZ. Dose profiles at Dmax (1.5 cm) and PDD curves were calculated following simulating about 1 billion histories for dose profiles and 500 million histories for percent depth dose (PDD) curves in a 30 x 30 x 30 cm3 water phantom. The simulation results were compared with the data measured by a CEA film and an ion chamber. The results show that the dose profiles are influenced by the energy and the spot size, while PDD curves are primarily influenced by the energy of the incident beam. The effect of the distance from the point source on the dose profile is not significant and is recommended to be set at infinity. We also recommend adjusting the beam energy by using PDD curves and, then, adjusting the spot size by using the dose profile to maintain the consistency of the Monte Carlo results and measured data.

Skin dose measurement by using ultra-thin TLDs.

The treatment schedule for radiation therapy is often interrupted because of complicated skin reactions. Quantitative information relating beam parameters and skin reactions will be helpful. Measurements were performed for 6-15 MV photons and 6-21 MeV electrons with ultra thin TLD films (GR-200F, surface area 0.5 x 0.5cm2, nominal thickness 5 mg cm(-2)). The skin doses for various field sizes, ranging from 10 x 10 to 40 x 40 cm2, and various incident angles of beam from 0 degrees to 80 degrees were measured. The ratios of skin dose to maximum dose in phantom for 10 x 10 cm2 are 16.10+/-0.68%, 14.03+/-1.04% and 10.59+/-0.64% for 6, 10 and 15 MV, respectively. Such ratios increase with a larger field size. For electrons the ratios are 72.59+/-1.72%, 78.52+/-2.99%, 78.89+/-2.86%, 86.08+/-2.62%. 87.75+/-1.94% and 86.33+/-3.09% for 6, 9, 12, 15, 18 and 21 MeV, respectively. They also increase with a larger size. The oblique factors also increase with larger incident angle.

The effect of a metal hip prosthesis on the radiation dose in therapeutic photon beam irradiations.

Prostate and cervical cancer patients are often treated with external X-ray beams of bi-lateral incidence. Such treatment may incur some dose effect that cannot be predicted precisely in commercial treatment planning systems (TPS) for patients having undergone total hip replacement. This study performs a Monte Carlo (MC) simulation and an analytical calculation (convolution superposition algorithm which is implemented in ADAC TPS) of a 6 MV, 5 x 5 cm2 X-ray beam incident into water with the existence of hip prosthesis, e.g. Ti6A14V and CoCrMo alloy. The results indicate that ADAC TPS cannot precisely account for the scatter and backscatter radiation that a metal hip prosthesis causes. For percent depth dose (PDD) curves, the maximum underdosage of ADAC TPS up to 5mm above the interface between dense material and water is 5%, 20% and 27% for PDD(Bone), PDD(Ti) and PDD(Co), respectively. The dose re-buildup, which occurs behind the hip region, becomes more and more obvious for denser medium existed in water. Increasing inhomogeneity also enhances the underdosage of ADAC for greater depth (> 10cm), as the figures of nearly 2% in PDD(Bone), PDD(Ti) and 4-5% in PDD(Co) reveal. Overestimating the attenuated power of high-density non-water material in ADAC TPS causes this underdosage. For dose profiles, no significant differences were found in Profile(Bone) at any depth. Profile(Ti) reveals that MC slightly exceeds ADAC at off-axis position 1.0-2.0 cm. Profile(Co) reveals this more obviously. This finding means that scatter radiation from these denser materials is significant and cannot be predicted precisely in ADAC.

Accuracy of the convolution/superposition dose calculation algorithm at the condition of electron disequilibrium

Using Monte Carlo simulation and the convolution/superposition algorithm, this work examines percent depth dose curves of the central axis in an acrylic phantom (20 x 20 x 20 cm(3)) with variously sized air cavities (20 x 20 x 1.0, 20 x 20 x 2.0, 20 x 20 x 3.0, 20 x 20 x 4.0 and 20 x 20 x 4.95 cm(3) for study of longitudinal electron disequilibrium (ED) and 3.6 x3.6 x 4.95, 4.5 x 4.5 x 4.95, 5.4 x 5.4 x 4.95 and 20 x 20 x 4.95 cm(3) for study of lateral ED). Radiochromic film samples are also measured to verify the Monte Carlo results. The Monte Carlo simulation is performed using OMEGA/BEAM and DOSXYZ codes, and the convolution/superposition calculation relies on an ADAC commercial treatment planning system. Underestimating the dose kernel expansion leads to overestimating the dose of what was found in the air cavity of ED using the convolution/superposition algorithm. Consequently, the dose in the rebuild-up region is influenced. The influenced region is on the acrylic phantom surface to a depth of about 0.5 cm. The density scaling method of the convolution/superposition algorithm, applied to heterogeneous media, should be enhanced to account for the over-expansion of the dose kernel in the cavity of ED.

The response of a thermoluminescent dosimeter to low energy protons in the range 30-100 keV

This study demonstrates the thermoluminescence (TL) response of CaF2:Tm (commercial name TLD-300) to 30-100 keV protons which were generated by means of a Cockcroft-Walton accelerator. The phenomenon in which the total thermoluminescent output from CaF2:Tm (TLD-300) decreases with proton energy from 30 to 100 keV (with increase of LET) can be interpreted by the track structure theory (TST). The analysis of the glow peaks: P2 (131 degrees C), P3 (153.5 degrees C) and P6 (259 degrees C), of TLD-300 show the oscillatory decreasing phenomenon as a function of incident proton energy, which can be interpreted with the TST and the oscillatory emission of electrons in a thermoluminescent dosimeter (TLD) that is caused by resonant or quasi-resonant charge transfer in ion-atom interactions in this TLD-300.

Dose compensation of the total body irradiation therapy.

The aim of the study is to improve dose uniformity in the body by the compensator-rice and to decrease the dose to the lung by the partial lung block. Rando phantom supine was set up to treat bilateral fields with a 15 MV linear accelerator at 415cm treatment distance. The experimental procedure included three parts. The first part was the bilateral irradiation without rice compensator, and the second part was with rice compensator. In the third part, rice compensator and partial lung block were both used. The results of thermoluminescent dosimeters measurements indicated that without rice compensator the dose was non-uniform. Contrarily, the average dose homogeneity with rice compensator was measured within +/- 5%, except for the thorax region. Partial lung block can reduce the dose which the lung received. This is a simple method to improve the dose homogeneity and to reduce the lung dose received. The compensator-rice is cheap, and acrylic boxes are easy to obtain. Therefore, this technique is suitable for more studies.

Indoor light on thermoluminescence of CVD diamond film used as a high-energy photon dosimeter.

The effect of light on polycrystalline diamond film that was produced by chemical vapor deposition and is used as a thermoluminescent dosimeter should be considered, although some researchers have indicated that such an effect was theoretically unlikely to happen. A 15 min exposure to a normal desk light bulb induces significant thermoluminescence (TL) comparable to a 0.5 Gy exposure to high-energy photons. This light-induced TL will be saturated within 2 h. The saturated TL intensity depends on the frequency of the light and the blue light dominates. The TL peak area at a temperature of 605 K is insensitive to light but is sensitive to high-energy photons. Another peak at about 410 K is caused by light only, because the TL from the ionization radiation at the same location is bleached. The effect of light could be easily distinguished by a numerical or an experimental method. Lamps with a green lampshade or pure red lights are suggested for use as indoor light sources. To reduce the effect of light, pre-heating treatment before readout is also suggested.

Monte Carlo simulation of surface percent depth dose.

This work studied the surface percent depth dose of 6 and 15 MV X-rays, 10 x 10 cm2 and 20 x 20 cm2 fields by Monte Carlo simulation. The OMEGA/BEAM code, an EGS4 user code developed by the NRCC, was used. The linac, Siemens PRIMUS, was accurately modeled according to the ion chamber and CEA film measurement, and the phase space data generated from this linac were collected to simulate dose distribution in water. The water phantom had radius 30 cm and thickness 10 cm. The percent depth doses at zero depth, PDDsurface, for 6 MV X-rays were 13.85 +/- 0.11% and 23.21 +/- 0.20% for the 10 x 10 cm2 and 20 x 20 cm2 fields, respectively. For 15 MV X-rays, PDDsurface values were 8.83 +/- 0.07% and 18.60 +/- 0.12% for the 10 x 10 cm2 and 20 x 20 cm2 fields, respectively.

Surface dose with grids in electron beam radiation therapy

This investigation attempts to solve the problem of the lack of skin-sparing effect in electron radiation therapy and to increase the tolerance of skin to radiation using the grid technique. Electron grid therapy involves the mounting of a Cerrobend grid in the electron cone. Film dosimetry was employed to measure the relative surface dose and the percentage depth dose profile of electron grid portals. Various grid hole diameters (d = 0.45, 1.0, 1.5 cm) and grid hole spacings (s = 0.4, 0.2 cm) were considered for electron beams from 6 to 14 MeV. Experimental results indicate that the electron grid technique can reduce the relative surface dose in electron radiation therapy. Degradations of the relative surface dose depend on the percentage of open area in the grid portal. A proper grid design allows the surface dose to be reduced and the range of nonhomogeneous doses to be limited to a depth at which the target volume can receive a homogeneous dose. The grid technique can lower the surface dose in electron radiation therapy.