10 General Aspects of Head and Neck Brachytherapy

General Aspects of Head and Neck Brachytherapy

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/01/2019

8.2. Target Definition The Gross Tumor Volume (GTV) is the visible or palpable primary tumor volume (GTV-T) ormetastatic lymph nodes (GTV-N) defined at diagnosis by clinical examination and imaging techniques. The GTV should be precisely recorded, especially if external beam irradiation or chemoradiation is delivered before brachytherapy. Placement of radioopaque markers (e.g. clips, seeds, etc.) or tattoos can be very helpful in delineating the tumor volume before any shrinkage occurs. Gold seeds also aid in subsequent external irradiation if image-guided radiation therapy (IGRT) is used. The Clinical Target Volume (CTV) is the tissue volume presumed to containmicroscopic disease at a certain probability level.There is always a CTV around each of the GTVs (generated by asymmetrical expansion of 5-10 mm) and as many CTVs as areas felt to contain microscopic disease. In the postop setting, tumor bed clips are very useful in defining the post-resection CTV corresponding to the tumor bed. The ABS [1,14] and the GEC-ESTRO [2,15] guidelines do not detail specific CTV definition rules applicable to different disease scenarios such as the CTV definition for tumors treated with Brachytherapy alone (Section 6.1.), CTV definition for tumors treated with HN BT after External irradiation or chemoradiation (Section 6.2.) or CTV definition for tumors treated with surgery and adjuvant HN BT (Sections 6.3. and 6.4.). The Planning Treatment Volume (PTV) is the margin provided around the CTV to account for organ motion and setup errors that guarantees that the CTV actually receives the prescribed dose. It has traditionally assumed that in HN BT the effect of organ motion and setup errors is negligible and, therefore, that a PTV is unnecessary [2]. Due to the lack of recommendations for target definition, the leading Brachytherapy Societies should create a task force aimed to produce target definition guidelines for tumors treated with HN BT alone (Section 6.1.), tumors treated treated with HN BT after External irradiation or chemoradiation (Section 6.2.) or tumors treated with surgery and adjuvant HN BT (Sections 6.3. and 6.4.). It is important to note that the adaptative target definition concept should be considered in the HN BT boosting after external irradiation/chemoradiation in a similar way to that used in gynaecological brachytherapy. 8.3. OAR definition and Tissue Sparing Devices Standardized Organ at Risk (OAR) dose–volume constraints in HN brachytherapy are lacking. It is wise, however, to keep the dose in bone, nerves, vessels and other dose-limiting organs as low as possible provided that the CTV coverage is adequate. Adoption of OAR constraints used for other highly conformal radiation techniques such as IMRT or V-MAT is a prudent alternative in the absence of HN BT-specific OAR constraints. The spinal cord should be contoured in all the cases. Although the spinal cord will receive very low doses from the HN BT implant, this may be sometimes used in the management of locally recurrent tumors after prior irradiation in whom the spinal cord tolerance has already been reached during the first course of irradiation. The mandible should be contoured to evaluate and minimize the risk of osteonecrosis, especially in implants adjacent to the jaw or in patients with recurrent disease after prior irradiation. A prosthesis including lead shielding should be made when brachytherapy of

Fig 10. A custom-designed dental shield lined with lead. A dental cast is used to make the shield. Courtesy Prof.D.Peiffert.

Fig11.3DPlanningofaperioperativeHDRbrachytherapyafterresectionofapreviously irradiated, recurrent squamous cell cancer of the left border of the tongue with level Ib positive nodes. Structures delineated include mandible (orange), carotid artery (green) and spinal cord (yellow). CTV-T (magenta) is delineated by the 100% isodose line of 4Gy (traslucid green) produced by 9 afterloading catheters.

Fig 12. DVH Planning of a perioperative HDR brachytherapy after resection of a previously irradiated, recurrent squamous cell cancer of the left border of the tongue with level Ib positive nodes. Structures delineated include mandible (orange), carotid artery (green) and spinal cord (yellow). CTV-T (magenta) and CTV-N (red).