30. Paediatric malignancies - The GEC-ESTRO Handbook of Brachytherapy

Paediatric malignancies

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 01/09/2023

7. TARGET VOLUME

2/ in combination with conservative surgery, when radical surgery would lead to mutilation (e.g., bladder RMS, vaginal RMS, limbs RMS, head and neck RMS). For combined treatments, BT is delivered postoperatively or (preferably) as a perioperative approach to guide catheter placement according to operative findings, including extemporaneous analysis. If the BT decision is taken postoperatively, based on histopathological findings, it may be necessary to re-operate to place BT catheters in the tumour bed. 3/ as adjuvant treatment, in cases with adverse histopathological factors (e.g., involvement of resection margins, alveolar histology). 4/ in combination with external beam radiotherapy, to increase focally the dose to area at high risk for local relapse and minimize dose to organs at risk. For residual tumours > 5 cm in size, achieving a good dose distribution with exclusive BT may be complex, and combination of BT and external irradiation and/ or debulking surgery may be discussed. In case of lymph node extension, BT is usually associated with external radiotherapy to treat locoregional tumour extension. Based on adult data, BT is also usually not delivered as exclusive treatment and rather proposed as a boost following external irradiation for tumour sites at high risk for necrosis (e.g., anal canal) [56]. 5/ for salvage treatments. Local treatment is crucial in patients having a local relapse after primary treatment and radiotherapy and/or BT have a major role in this situation. Salvage BT may be indicated when the tumour recurrence occurs in an area that has not been previously irradiated and that is technically accessible to implantation. When the patient has a relapse in a previously irradiated area, this is a very complex situation, and no definitive recommendation can be made here. Treatment should be discussed on a case per case basis according to previous radiation dose, BT feasibility and the possibility of alternative salvage surgery. Only tumours which relapse with small volume disease can be treated with reirradiation through BT. Reports have shown the feasibility and satisfactory outcome of performing salvage treatment with limited surgery and BT for selected cases [68] Tumour bone extension is a contra-indication for solely BT. Selected patients with bone invasion however can be treated with combination of surgery and brachytherapy. Radiation hypersensitivity syndromes (e.g., Fanconi Anemia, Ataxia Telangiectasia) are also definitive contra-indications for BT. Fertility sparing approaches: Preserving fertility of the child is a major objective of treatment. The indication for BT should be carefully weighed against other possible local treatments. For example, patients with RMS of the uterine cervix may be preferably treated with trachelectomy, rather than with uterovaginal BT, in order to avoid sterility that will be caused by high uterine and ovarian radiation doses. The same approach should be discussed in patients with cervical and/ or upper vaginal tumours, where trachelectomy +/- upper partial colpectomy may lead to less fertility impairment than cervico vaginal BT. Patients requiring pelvic irradiation should undergo a temporary transposition of the gonads to limit radiation exposure, if it is expected that dose exposure will exceed usual dose/volume constraints and if oncologically feasible. This transposition can be performed by paediatric surgeons at the time of BT implantation [16, 17, 29].

The target volume in RMS depends on various factors. These include: tumour topography, tumour characteristics (infiltrative or expansive with well-defined tumour borders), relationships to organs at risk and possible involvement, tumour stage at diagnosis (organ confined tumour, regional or metastatic extent), amount of residual disease after surgery (presence of a microscopic or macroscopic residuum), tumour histology (embryonal or alveolar subtype), response to chemotherapy (complete or partial response, stable disease) and age of the patient. When defining the clinical target volume (CTV) for BT, the tumour volume at diagnosis and after induction chemotherapy must be taken into consideration. A precise clinical examination, if necessary, under general anaesthesia, should be done. The findings from this examination are integrated with information about tumour volume and topography from the different imaging procedures. Post-implant 3D imaging is mandatory to guide target definition and include any residual tumour as part of the CTV. T2 weighted MRI is the best modality to define the residual tumour site, because of higher soft tissue contrast in comparison with CT scans. A discussion with expert radiologists may be necessary to guide target delineation. Residual hyper-T2 regions are parts of the residual gross tumour volume (GTV). In addition to MRI, a CT scan is usually necessary to see with high accuracy the catheters and the seed markers. If a surgical procedure is associated with BT the target volume is defined by the radiation oncologist together with the surgeon and the pathologist. After complete excision, the benefit of MRI is lower than for intact tumours, and a CT scan may be enough to delineate tumour bed. With an intraoperative procedure, the target volume is more easily defined because of the direct view of the structures to be treated [20]. Proper identification of the target volume requires a close collaboration between surgeons and brachytherapists, considering peri-operative findings, including frozen sections, and surgical seed markers. The target volume must be as small as possible because of the high risk of radiation morbidity, in particular the impairment of soft tissue growth and its possible adverse impact on cosmetic and functional outcome. However, for reasons of local tumour control, it is also necessary to consider the initial tumour volume. In fact, the clinical target volume is a compromise between initial and residual tumour volume considering the different variables listed below. For multimodal treatment (induction chemotherapy, surgical resection) there are in principle three situations that are most important for clinical target volume definition: • Residual macroscopic gross tumour: CTV is defined including at least the gross tumour volume (GTV) after induction chemotherapy plus a minimum safety margin of 5 mm. In any case the initial GTV must also be considered. • Residual microscopic disease (confirmed by pathology): For CTV the region of microscopic disease is included with some safety margin (at least 5 mm), also considering the dimensions of GTV at diagnosis, and considering postoperative anatomical changes. • No residual tumour as determined by clinical examination, imaging, or biopsy: if BT is considered, a CTV is defined, which will most likely prevent local recurrence without inducing major morbidity (e.g., prostate, cervix, tongue). This CTV is defined