13 Head and Neck - Oropharynx

Head and Neck - Oropharynx

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 15/02/2021

9. TREATMENT PLANNING

into account all the extensions of the disease and CTV. Hence in a way the planning process is initiated in the operation theatre itself. The details of contouring are given in the target volumes section. It is advisable to also contour the critical structures such as mandible, bilateral parotids, spinal cord, and submandibular glands similar to EBRT. Planning Loading of the sources is based on the contouring of the target. In situations where the target is not drawn the radiation oncologist decides the loading based on the clinical and radiological findings. Basal dose points can be generated as per the Paris system rules and normalization is done. The dose is prescribed on an isodose representing 85% of the mean basal dose rate (mean central dose rate,), calculated in the central plane. However sometimes while prescribing at 85%, the volume of the 150% and 200% isodoses may increase. Hence the prescription isodose is typically decided based on the target coverage, reducing the V150% and V200% to minimum. In the 3D era the prescription at 85% is replaced by a minimal target dose (MTD). Figure 26 shows 3D planning of brachytherapy to the soft palate in the coronal, sagittal and axial planes along with the 3D dose distribution.

CT scan After recovery from general anaesthesia the next step is treatment planning. Three-dimensional computed tomography (3DCT) based planning is recommended. Tubes are numbered and dummy catheters are inserted in each tube. A radiotherapy planning CT scan with 1-5 mm cuts is obtained in the supine position. Overlapping images of 2mm may result in better quality for 3D catheter reconstruction. In the case of smaller tumours thin cuts of 1mm may be taken in the area of interest. After the CT scan each tube in the implant is measured and recorded methodically. Catheter reconstruction Multiplanar reconstruction software is useful for accurate localization of the tubes. Reconstruction can be started from the tip end or connector end. The same should be used for all the tubes. The tip of the tube can be identified by the radiopaque buttons which are placed at the entry and exit of the catheters. Contouring It is advisable that contouring be done by the same oncologist who has done the BT implant. Typically the needle placement takes

Figure 26: Coronal, sagittal, axial and 3D dose distribution for a soft palate implant. Please note the superior-most tube is inserted via lateral-lateral technique just anterior to the mandible, while the other two tubes are inserted as loops.