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Oropharynx 283

the adjacent tongue. Parallelism between this loop and the tubes implanted into the faucial arch can be optimised using fluoroscopy.

8 Dosimetry Dosimetry is based on a 3-D reconstruction of the catheters of the applicator (source positions). At least two projection images (radiographs) are needed for such a reconstruction of the implant loaded with dummy sources. CT images and/or MR images may also be used. The rules of the Paris system of dosimetry are often followed and the resulting distribution is calculated. The dose is prescribed on an isodose representing 85% of the mean basal dose rate (mean central dose rate), calculated in the central plane. For the base of tongue, the central plane passes through the middles of the parallel branches of the loops For tumours of the faucial arch, the central plane passes through the midpoints of the two wires and the centre of the quadrilateral basis formed by their extremities. It may be useful to calculate the dose delivered at points of interest marked with metallic seeds during implantation. With the stepping source technology, some optimisation facility can often improve dose distribution, i.e. reduce the inhomogeneity inside the implant and decrease the dose outside the implant to some extend (see physics chapter). Dose, Dose Rate, Fractionation In almost all cases, the implantation is performed following 45 - 50 Gy external beam irradiation of the primary tumour and cervical lymph nodes. The implant delivers then an additional dose of 25 - 30 Gy LDR-PDR to the tumour bed in tonsillar region tumours, and 30 - 35 Gy in base of tongue tumours. For tumours of the base of tongue it may sometimes be useful to fuse the base of the tongue by boosting the whole base to 65 - 70 Gy and the macroscopic tumour site to 75 - 80 Gy. When the implant is carried out in a previously irradiated area for a recurrent tumour or a new tumour, a dose of 60 Gy is required. It is advisable to deliver the dose at 0.3 to 0.6 Gy.h -1 or by hourly 0.5 Gy pulses. For high dose-rate brachytherapy, fraction sizes of less than 6 Gy and an interval between fractions of 6 hours or more are recommended. Typical treatment protocols combine 50 Gy external beam radiation therapy and a high dose rate interstitial brachytherarapy delivered in 4 - 6 fractions of 4 Gy. 10 Monitoring Patients should receive adequate analgesics. Steroids, antibiotics and a feeding tube may be indicated. The implantation site is inspected at least twice daily. Removal of the plastic tubes should always be carried out in an operating room equipped with adequate illumination and suction facilities and means of dealing with extensive bleeding. Arterial 9