34. Uncommon indications for brachytherapy - The GEC-ESTRO Handbook of Brachytherapy

Uncommon indications for brachytherapy

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/04/2025

3. TUMOUR AND TARGET VOLUME

External Beam Radiation Therapy (EBRT) The target definition rules of EBRT component in combination with brachytherapy follow the same rules as for patients treated with EBRT alone. When using preoperative EBRT, the CTV volumes encompassed with EBRT include the GTV (gadolinium-enhanced T1-weighted MRI) with an expansion (2-4 cm longitudinally and 1-1.5 cm radially) that is manually edited to encompass peritumoural edema (defined on T2-weighted MRI) [14,15]. With postoperative EBRT, the CTV should also account for areas of potential tumour seeding during surgery (i.e., the scar, drain port, and surgical tumour bed)[16]. The CTV needs to be defined by anatomic boundaries (i.e., fascia, bone, joint, or compartment) and potential areas of seeding from previous procedures (i.e., unresected biopsy track).

The target definition rules for the brachytherapy and the EBRT components are the same irrespective of their use as a single modality or a combined treatment.

Brachytherapy Different Clinical Target Volume (CTV) definitions have been used. Traditionally, the CTV was defined as the entire surgical bed because local recurrences were presumed to occur due to tumour cell shedding during surgical manipulation with subsequent propagation through the post-surgical seroma. Nowadays, CTV definition has evolved to more conservative proposals that may be equally useful and less toxic. The American Brachytherapy Society consensus statement for soft tissue sarcoma brachytherapy [13] advises construction of the CTV with a tumour bed expansion of 2 cm craniocaudally and 1 cm lateral to the tumour bed. Although traditional brachytherapy practice has emphasized the futility of defining a PTV that compensates for inaccuracies incurred during the treatment process due to the presumed stability of the implant with regards to the CTV, this may not be the case in all scenarios. PTVs may not need to be created in small CTVs fully covered with firmly anchored double-button catheters. However, catheter displacement may occur in large CTVs with single-button catheters. In this situation, a CTV expansion in the direction of the tubes to create a PTV may be required if the position of the catheters cannot be verified before each treatment or due to postoperative conditions (i.e, swelling, etc.) .

4. TECHNIQUE

Perioperative implants are performed at the time of surgery. After determining the CTV according to the surgical and pathological findings, as well as the preoperative imaging (MRI), the plastic tubes are implanted. The majority of CTVs can be encompassed with a single-plane implant. Larger CTVs may require two or three planes (Figure 34.1). The number of planes depends on the size of the primary tumour and the tumour bed closure technique. A large tissue defect can be converted into a smaller CTV if the different components of the tumour bed are sutured onto each other.

Figure 34.1: Multiple plane implant in a large myxoid liposarcoma of the thigh. Axial MRI at the central plane of the implant (left panel). Planning CT showing the deep and superficial catheter planes. Note outstanding tumour bed displacement after resection. The CTV (red) is encompassed by the 100% 4Gy isodose line (blue) with 150% isodose 6Gy lines (white) accounting for a small part of the irradiated volume due to the large number of catheters and dwell positions (right panel). A 5-mm tissue thickness (yellow lines) is usually enough to separate the organs at risk away from the high-dose regions.