11 Lip and buccal mucosa

Lip and buccal mucosa

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 10/05/2019

Fig 11: Planning CT with isodose curves with plastic tubes technique.

Fig 12: Measure of the distance from the tip with rigid needles tehnique.

Fig 13: Isodose curves in an equidistant implant of six rigid needles forming triangles.

9. TREATMENT PLANNING

surrounding the lip, for small tumours. It is more comfortable, but not so fixed as interstitial techniques 8.1.4 Protection device A custommade protection device can be used, if clinically indicated. Such a device should be prepared to shield the upper lip, the tongue and the lower gum. The protector consists of a 2mm lead shield placed between both lips and the mandible, contained in an acrylic mouthpiece, bended in an L-shape. This reduces the dose to the upper lip and lower gum by a factor of two. The advantage with HDR is that it can be placed only during the few minutes of each session. To make exact dosimetry possible, an identical dummy protector without lead shielding or removable lead plate can be made and used for dose planning. 8.2 Technique for buccal mucosa carcinoma Two implantation techniques were mainly used with LDR: guide gutters replaced by iridium hairpins for superficial, well limited, anterior lesions, close to the lip commissure, smaller than 20 mm in size. And plastic tubes for all other lesions, taking into account the thickness of the tumour and the anatomical characteristics of the cheek, whose thickness increases from the lip commissure to the intermaxillary commissure. The needles are implanted parallel to each other, parallel to the oral mucosa and parallel to the horizontal branch of the mandible. Under digital control, the lines are placed approximately 3-5 mm deep, under the buccal mucosa. They should cover the CTV, 10 mm of normal mucosa behind and in front of the lesion with a recommended spacing of 10-15 mm. A single plane is required to cover the target volume if the thickness of the tumour does not exceed 5 mm. Plastic spacers at both ends keep the tubes parallel. Retention buttons are fixed at both extremities (Fig 6, 7, 8). If the lesion is thicker than 5 mm, a second plane of tubes is implanted between the first one and the skin. A customized lead gutter inside the oral cavity is highly recommended to reduce the risk of osteoradionecrosis. The shielding system may reduce the thickness of the target volume by stretching the cheek and squeezing the tumour so irradiation is better adapted. In some cases rigid needles can be used directly, with a template. An oblique direction is used in cases of lateral commissure involvement. The loops used with LDR are no longer needed with HDR, due to the optimization (Fig 9, 10).

2D dose planning Most of classical implants with LDRwere calculatedwith orthogonal projection images to register the source positions. The prescribed dose to the Minimal Target Dose usually corresponds to 85% of the MCD (Paris System). It was sometimes complemented by CT scan and/or MRI, to determine as closely as possible both GTV and CTV. 3D dose planning Nowadays, a planning CT scan is mandatory for plastic tubes technique, with radiopaque markers around the tumour or on the scar in postoperative cases (fig 11). Different planes are chosen, perpendicular to the radioactive lines if possible: in the frontal plane when the lines are parallel to the buccal mucosa, in the sagittal plane when the lines are perpendicular to the buccal mucosa. GTV and CTV must be drawn, and the bone as organ at risk. The Stepping Source Dosimetry System (SSDS), a modified Paris system, is useful to optimize the isodose curves, finishing with amanual optimization.The dose to bone should not be greater than the prescription dose. For the rigid needles technique with template, the distance between needles is fixed, and the tumour is clearly visible, then the CTV is well defined. Therefore, a theoretical calculation can be done quickly by taking the distance from the tip of the needle to the beginning and the end of the CTV, drawn on the skin or the lip (fig 12). The calculation can be done without planning CT, and optimization is done to volume to the 90% isodose (Fig 13). The whole tumour must be inside that volume. ACT planning can help to delineate the CTV, but it is not mandatory. 3D dose planning with MRI is lacking.

10. DOSE, DOSE RATE, FRACTIONATION

The prescribed dose for LDR was 60-75 Gy at the 85% reference isodose, at dose rates between 45-80 cGy/h. This used to take 4 to 6 days to deliver a dose of 60-65 Gy for T1 and 65-70 for T2 [21]. Although doses up to 70-75 Gy can be given in some large tumours, without unacceptable sequelaes [13] the increase in