A Rare Case of Extralaryngeal Schwannoma Arising from the Superior Laryngeal Nerve.

Schwannomas are the most common neurogenic tumours arising in the parapharyngeal space. Usually they arise from the vagus nerve but rarely have an extralaryngeal presentation arising from the superior laryngeal nerve. In the case reported here, cytology could not confirm the diagnosis and imaging could not identify the nerve of origin.

Thermoluminescence dosimetry for skin dose assessment during intraoperative radiotherapy for early breast cancer.

Dosimetry for intraoperative radiotherapy (IORT) after wide local excision for breast cancer using a 50 kV X-ray needle (Intrabeam) was performed in vivo using thermoluminescence dosimetry. Eight LiF:Mg,Ti chips were placed on the skin around the incision site after wide local excision while the tumour bed was irradiated to a prescribed dose of 5 Gy 10 mm from the applicator surface. The maximum and mean measured skin dose for 57 patients ranged from 0.64 to 7.1 Gy and 0.56 to 4.78 Gy, respectively, reflecting different tissue thicknesses overlying the applicator. The average maximum dose of 2.93+/-1.46 Gy was below the threshold for severe radiation skin toxicity.

An experimental MOSFET approach to characterize (192)Ir HDR source anisotropy.

The dose anisotropy around a (192)Ir HDR source in a water phantom has been measured using MOSFETs as relative dosimeters. In addition, modeling using the EGSnrc code has been performed to provide a complete dose distribution consistent with the MOSFET measurements. Doses around the Nucletron 'classic' (192)Ir HDR source were measured for a range of radial distances from 5 to 30 mm within a 40 x 30 x 30 cm(3) water phantom, using a TN-RD-50 MOSFET dosimetry system with an active area of 0.2 mm by 0.2 mm. For each successive measurement a linear stepper capable of movement in intervals of 0.0125 mm re-positioned the MOSFET at the required radial distance, while a rotational stepper enabled angular displacement of the source at intervals of 0.9 degrees . The source-dosimeter arrangement within the water phantom was modeled using the standardized cylindrical geometry of the DOSRZnrc user code. In general, the measured relative anisotropy at each radial distance from 5 mm to 30 mm is in good agreement with the EGSnrc simulations, benchmark Monte Carlo simulation and TLD measurements where they exist. The experimental approach employing a MOSFET detection system of small size, high spatial resolution and fast read out capability allowed a practical approach to the determination of dose anisotropy around a HDR source.

A parallel plate chamber for calibration of 192Ir LDR and HDR sources.

A good program of quality assurance requires the user to check the calibration of brachytherapy sources. Conventional well-type ionization chambers have restrictions on the length of the source which can be used due to the variation in the relative ionization with distance from the position of maximum sensitivity. We have fabricated a parallel plate chamber, which can be used for both LDR and HDR sources of 192Ir placed centrally inside. The electrodes are made from printed circuit boards. Sources of lengths up to 15 cm can be calibrated with this chamber. It has a good linear response and stability. It costs little and can be used with any electrometer. The chamber, which has a volume of about 40 cm3, has a response of 820 nCmin(-1) per cGy m2 h(-1).

Levels of leakage radiation from electron collimators of a linear accelerator.

The leakage radiation from electron applicators used with our linear accelerator has been measured. For the applicators 6 X 6 to 25 X 25 cm size, the leakage was measured in the plane of the patient and on the sides of the applicators with the available electron energies of 6, 9, 12, 15 and 18 MeV. The levels were significant. The highest leakage on the side was for the combination of 6 X 6-cm applicator and 9-MeV electrons (32%) and in the plane of the patient for 25 X 25-cm applicator with 18 MeV (10%) relative to the peak dose. Adding lead 1-2 mm, at appropriate locations inside the applicators has reduced the leakages to acceptable levels without affecting the beam parameters.

Dosimetry of cathetron applicators in intracavitary therapy.

Theoretical computations have been done to obtain dose distributions in the paracervical and sagittal planes for various loadings of Cathetron applicators. Uterine catheters of various sizes and shapes and various loadings of ovoid catheters have been considered for the dosimetry. Experimental measurements have been made to verify the computed dose distributions. The computed dose distributions have also been compared with those of a conventional Manchester loading. Although the ratio of dose contribution to point A from vaginal sources to that from the uterine sources is found to be significantly different for Cathetron therapy from the conventional Manchester system, the isodose distributions in the paracervical plane are in good agreement. However, the differences in the isodose distributions in the sagittal plane indicates a higher dose to the rectal region for Cathetron therapy, which can be reduced by the use of rectal retractors.

60Co activity in 192Ir seeds.

The decayed sources of steel-sheathed 192Ir seeds have shown residual 60Co activity. The gamma spectra from such sources show the two distinctive peaks for 60Co, whereas the platinum-sheathed 192Ir sources do not show such peaks. This may be due to the low percentage of cobalt present in the magnetic alloy sheath. Though this would not affect the clinical dosimetry, it may be of concern when handling large numbers of such sources for disposal.

Measurement of dose to the contralateral breast during radiation therapy for breast cancer: reduction by the use of superflab.

The effect of radiation in regard to breast carcinogenesis is well studied and analysed from data from atomic bomb survivors, patients treated for acute mastitis and tuberculosis patients monitored by fluoroscopy. Therefore the radiation received by the untreated breast during breast irradiation of the other breast, is of concern to clinicians. Using thermoluminescence dosimeters, we have measured the dose received by contraleteral breast of six patients during radiation therapy. Most of the dose received will be from the collimator scatter and leakage. We investigated the effect of an absorber, superflab, in reducing the skin dose to contralateral breast. With the overlaying of a cm superflab on the breast, the skin (surface) dose could be reduced by 40-75% of its original value. This is an effective and practical method of reducing significantly the dose to contralateral breast during breast conservation therapy. Superflab can be made conveniently at low cost.

Thickness of shielding required for dose reduction with 4-22 MeV electrons.

In electron therapy, it is common practice to shape the treatment area by the use of lead or cerroband alloy on the skin, or as insert at the electron collimator. We have studied the effect of the position of these shields on the skin dose and the thickness of the shield to reduce the dose to less than 5%. With energies above 12 MeV, it is not practicable to reduce the dose to 5% or less with reasonable thickness of the shield at either position. For lower energies the shield is more effective at the collimator rather than on the skin.

The use of round ended needles for high dose rate brachytherapy implants.

The use of Speiser needles for interstitial implants has resulted in malfunctioning of HDR afterloading equipment due to seepage of blood. We have successfully used rounded ended needles in the patient after removal of the trocar ended needles, preventing the seepage of blood. This technique also helps to prevent any possible cross contamination between patients.

Local shielding of high dose rate brachytherapy in an operating theatre.

A high-dose rate brachytherapy facility was installed into an established operating theatre by using local shielding in the form of mobile lead screens and by taking advantage of the ease with which staff movements can be controlled in an operating suite. This facility was inexpensive to develop, and has proved clinically efficient and entirely adequate from a radiation safety standpoint.

Skin dose with cones and diaphragms in 137Cs teletherapy - a study using lithium borate thermoluminescent dosimeters.

Beam definition in 137Cs teletherapy is done either by use of cones for specified SSDs or by means of adjustable diaphragms. Using lithium borate thermoluminescent dosimeters, the skin dose was studied for conditions of the bare cone without any electron filter, the cone with an affixed perspex cup for cutting off the electrons, the cone with an affixed copper plate filter at its face. It was observed that of the above three arrangements, the case employing the copper filter gave the minimum skin dose. Similar skin dose measurements made for a diaphragm type collimator are discussed in comparison with the values for cones.

Determination of total activity and its distribution in cathetron source pencils.

High intensity remote-controlled afterloading devices such as Cathetron, employ tiny source elements which facilitate free movement through the curvature of the guide tubes. As the activities involved are in the curie level and a number of sources are assembled together in the form of a pencil, conventional methods cannot be employed for determining the activity of individual sources. This paper deals with the determination of the total activity of a source pencil from the measured exposure rate and an autoradiographic method used to determine the activity of individual sources.

Clinical dosimetry of the rectum in cathetron applications for treatment of carcinoma of the uterine cavity.

Rectal dose measurements have been carried out for 60 patients undergoing Cathetron therapy for the treatment of carcinoma of the uterine cervix at the Cancer Hospital, Indore. The measurements show that the maximum rectal dose occurred around 8 cm from the anal verge. In most of the cases (approximately 75%) the maximum rectal dose lies between 50 and 85% of the dose at point A. The combined biologically effective doses from intracavitary and external beam therapy regimes in this centre and the clinical complications have been compared with those of other centres. Although no immediate serious rectal complications have been observed, it is desirable to check the rectal dose using monitoring source pencils, so that the applicators and rectal retractor can be readjusted, if necessary, to minimise the dose.