Surendra N. Rustgi

Dosimetric characterization of radiosurgical beams with a diamond detector.

Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using a p-type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (S c.p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (+/- 1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. The S c.p factors obtained with the ionometric detectors also agreed well (+/- 1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smaller S c.p factor, as a result of the steep dose gradients across their sensitive volumes. Cross-beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.

Evaluation of the dosimetric characteristics of a diamond detector for photon beam measurements.

The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a silicon p-type photon diode and a 0.14 cm3 ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0 degree (parallel to the detector stem) to a direction transverse to the detector stem (90 degrees), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%-22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3 ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter < or = 4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.

Dose perturbation caused by high-density inhomogeneities in small beams in stereotactic radiosurgery

The influence of high-density tissue heterogeneities in small-diameter beams used in stereotactic radiosurgery has been investigated. Dose perturbation immediately behind aluminium sheets, used to simulate a high-density tissue inhomogeneity such as bone, was studied in a solid water phantom. Dose reduction factors (DRFs), which are the ratios of the dose in the presence of the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs exhibit dependence on both the inhomogeneity thickness and the beam diameter. The DRF decreases with inhomogeneity thickness. The DRF initially decreases with increase in the beam diameter from 12.5 to 25 mm. For fields greater than 25 mm, the DRFs are nearly constant. The commonly used algorithms such as the TAR ratio method underestimate the magnitude of the measured effect. A good agreement between these measurements and Monte Carlo calculations is obtained. The influence of the high-density inhomogeneity on the tissue maximum ratio (TMR) was also measured with the inhomogeneity at a fixed depth dmax from the entrance surface. The TMR is reduced for all detector-inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose points) immediately beyond the inhomogeneity is unaffected by the high-density inhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This reduction in dose outside the beam edge is caused by the reduced range of the secondary radiation (photons and electrons) in the high-density medium.

Influence of air inhomogeneities in radiosurgical beams

The effect of air inhomogeneity on dose distribution in small diameter beams used in stereotactic radiosurgery (SRS) has been investigated. The measurements of the surface dose and central axis dose were made for a 6 MV photon beam with a diamond detector which has suitable radiologic properties for such measurements. Measurements made in a uniform density solid water phantom and in the presence of four air gaps indicate significant dose perturbation immediately beyond the air-solid water interface. The reduction in dose at the surface for a 12.5 mm diameter field is 11%, 17%, 23% and 33% for air gap thicknesses of 3, 4.6, 6 and 9.2 mm, respectively. The corresponding dose reduction for a 25 mm diameter field is 3%, 4%, 7% and 13%, respectively. The ratio of the dose with and without air inhomogeneity is highly dependent on field diameter and approaches 1.0 as the field diameter increases from 12.5 mm to 40 mm. The dose perturbation also increases with increase in air inhomogeneity thickness for all field diameters investigated. A dose buildup phenomenon is observed beyond the air gap with a shallow dmax of approximately 4-6 mm. Beyond the buildup region, a higher dose value compared to a homogeneous phantom is observed at all depths due to reduced photon attenuation in the air gap. The dose profiles beyond the air gap, measured with radiographic films, demonstrate no significant increase in the beam diameter but a pronounced broadening of the beam penumbra (80%-20% and 90%-10%) leading to enhanced dose outside the primary beam geometric edge and reduced dose inside the edge. The dose enhancement outside the beam edge increases with increase in air gap thickness.

Dosimetry of Small Field Electron Beams

Measurements of the central axis depth dose curves, isodose profiles and field size dependence of the output factors for small field electron beams from a Varian Clinac 18 linear accelerator have been performed. Energies of electron beams studied were 6, 9, 12, 15, and 18 MeV. Circular fields of 1, 2, 3, 5, and 8 cm diameter were obtained from cerrobend shields attached to the bottom face of a 15 x 15 cm2 electron cone applicator. Measurements were carried out in a water phantom with an ionization chamber and a silicon diode detector and in a polystyrene phantom with Kodak XV-2 film. For all energies studied, as the field size of the electron beam becomes smaller: (1) the depth of maximum dose shifts toward the surface, (2) the depth of 90% and 80% dose, which are common dose prescription depths, becomes smaller, (3) the surface dose increases, and (4) the dose fall-off region becomes more gradual. The output of the electron beam reduces significantly with reduction in electron beam field size as a consequence of lack of lateral equilibrium. Hence, for accurate treatment planning for small electron fields, it is essential that the beam characteristics be individually measured.

Photon spectral characteristics of a new double-walled Iodine-125 sourcea)

The photon spectral characteristics of a recently designed Iodine-125 source have been measured. The source has a physical length of 5 mm and a diameter of 0.8 mm. A thin tungsten filament coated with radioactive Iodine-125 is used as a radiographic marker and is encapsulated in a double wall titanium shell of uniform thickness all around. The photon spectral characteristics, measured with an intrinsic germanium (Ge) detector coupled to a multichannel analyzer, reveal that the seed emits the 27.4-keV K alpha and 31.4-keV K beta x rays and 35.5-keV gamma photons from the decay of Iodine-125. Because of their low energy, the tungsten x rays are not observed in the spectrum. The anisotropy of the radiation fluence for each of the above-mentioned photon energies was measured in planes containing the seed short and long axes. The 4 pi-averaged anisotropy factor for the total radiation fluence, i.e., sum of the above three photon energies is 0.92. The photon intensity radiated along the seed long axis is approximately equal to the intensity in the seed transverse direction due to the absence of end welds. The new Iodine-125 source is characterized by good radiographic visualization, greater structural strength due to double wall encapsulation design, and emission of more isotropic Iodine-125 photon spectrum.

An afterloading 192Ir surface mold.

Patients with basal cell skin cancer can be treated with afterloading 192Ir surface molds constructed from dental wax. High activity 192Ir seeds in ribbons are distributed in the mold to uniformly irradiate the target volume. The treatment is preplanned with a treatment planning system to achieve a uniform dose distribution in the planned target volume. The implant parameters optimized include the seed strength and number of seeds, inter- and intracatheter spacing between 192Ir seeds, and the distance between the implant and treatment planes. The radioactive 192Ir strands are afterloaded in the catheters embedded in the wax mold and the position secured with buttons. The treatment area drawn on the patient surface is visually overlapped with the uniformly irradiated area sketched on the mold surface. The mold is taped to the head to secure this position. The dose rate on the surface of the mold in contact with the patient skin is measured with calibrated LiF TLD chips and is within +/- 5% of the computer preplanned dose rate value. This technique is a viable alternative to external beam treatments when daily treatments are not feasible and a dose distribution conforming to the treatment area is desirable.

Advantages of using high activity 125I seeds in temporary interstitial breast implants.

There has been considerable interest in the use of high activity 125I sources as a substitute for 192Ir seeds for removable implants of the breast and prostate. 125I seeds with an initial activity of approximately 5 mCi per seed, loaded in special afterloading nylon catheters, are used to improve dose distribution in the tumor volume and minimize dose to the adjacent critical organs and normal tissues. Seed spacing in strands is adjusted to maintain a dose rate of 40-60 cGy per hour at a distance of 5 mm from the plane of the implant. Implants custom loaded with 125I sources achieve superior isodose distribution compared to implants loaded with standard 192Ir seed strands. High activity 125I seeds also offer the advantage of reduced exposure to radiation oncology staff, nurses, and visitors leading to better patient care. Due to reduced exposure to personnel, the accuracy of the actual implant geometry can be verified by taking a localization film with actual 125I sources placed in the tumor bed.