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.

Electrons↗

PubMedDate not supplied

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.

Biophysical Phenomena↗

PubMedDate not supplied

The penumbra of a 6-MV x-ray beam as measured by thermoluminescent dosimetry and evaluated using an inverse square root function.

Data on the dose distribution in the penumbral region of megavoltage x rays are of importance for most radiotherapy planning systems. For medical linear accelerators the distance between the points representing 20% and 80% of the central axis dose (20/80) is typically only a few millimeters. To achieve good spatial resolution a radiation detector with small physical size has to be used for penumbra measurements. The penumbra in a 6-MV therapeutic x-ray beam was investigated for field sizes of 10 x 10 cm2 and 20 x 20 cm2 at depth of maximum dose (dmax = 1.5 cm) and at 10-cm depth in a solid water phantom. In addition, the field edge of an independent jaw driven to the center of the axis of the primary collimator was investigated. LiF thermoluminescent (TL) ribbons and rods were used embedded in solid water in different geometries resulting in relative detector sizes of 1, 3.1, and 6 mm. A forming function based on an inverse square root function was used to fit the experimental penumbra measurements. For the asymmetric field an amendment to the function is proposed to give a better fit for the experimental data. From the penumbra measured with the three different detector sizes, a penumbra can be extrapolated for an infinitesimal small detector. The extrapolated penumbral width (20/80) was found to be 2.3, 3.2, and 2.7 mm at dmax for the 10 x 10-cm2 symmetric, 10 x 10-cm2 asymmetric, and 20 x 20-cm2 field sizes, respectively. The 20/80 values at 10-cm depth in the solid water phantom for the same radiation fields were 4.2, 4.3, and 4.1 mm, respectively.

Biophysical Phenomena↗

PubMedDate not supplied

Fine needle aspiration cytologic diagnosis of metastatic Merkel cell carcinoma in the parotid gland.

A case of Merkel cell (trabecular) carcinoma metastatic to the parotid gland and diagnosed clinically by fine needle aspiration cytology is described. The primary tumor was on the left upper eyelid and had been resected four months earlier. This unusual presentation and the incidence of secondary tumors involving the parotid gland are discussed.

Adenocarcinoma↗