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

HDR treatments, especially for sensitive sites such as urogenital, perineal, anal canal or head and neck tumours. However, emerging data suggest that HDR treatments are feasible, though the number of series is still limited. In most cases treated with PDR BT, BT is performed in one session; for other patients - for bulky tumour or where it is difficult to adapt the treated volume to the CTV - two, or sometimes even three sessions may be necessary. In the French experience of PDR BT, the physical prescription ranges from 50 to 60 Gy using continuous hourly pulses of 0.42 Gy per pulse to keep the daily dose at 10 Gy. For patients with macroscopic residuum, the objective is to deliver at least 50 GyEQD2 to 90% of the CTV (CTV D 90 ) and a minimal dose > 55-60 GyEQD2 to 95% of the GTV (α/β = 10 Gy for tumour). For microscopic only disease, the objective is to deliver at least 45-50 GyEQD2 to 90% of the CTV. In the Dutch experience of PDR BT, irradiation typically delivers 32-36 pulses of 1.25 Gy every 2.1 hours up to 42.2-47.5 GyEQD2 (α/β value = 10 Gy for tumour). For HDR treatments, physical doses of 27.5 Gy to 36 Gy are delivered through fractions of 3 to 5.5 Gy (two fractions per day), resulting in EQD2 doses of 35-40 GyEQD2 (α/β-ratio = 10 Gy for tumour). Hyperdose sleeves should be as thin as possible, with minimal confluence of the 150% isodose. The V150/V100 should be ideally less than 25-30%. For interstitial procedures, the hyperdose sleeve (200% idosose) should be less than 8-9 mm width. Few data are available in terms of dose/constraints for paediatric BT and adult data cannot be applied to the paediatric population. Because of smaller organs at risk volumes, dose/volume parameters should be considered very carefully. In particular, dose volume parameters must be adjusted for young children for whom a D 2 cm 3 may not be appropriate due to smaller size. Indeed, D 2 cm 3 refers to excessively large volumes, compared to the patient anatomy in particular for young children and therefore organs at risk D 2 cm 3 doses will likely not adequately predict the risk of complications with regard to the radiation dose exposure. In children, smaller volumes should be considered, such as the D 1cm 3 , the D 0.5 cm 3 or even the D 0.1 cm 3 . Only one series so far has studied dose/volume effects among 78 patients treated with PDR BT for urogenital tumours, median age 2.9 years. It was shown that the rectal D 0.5cm 3 and D 1cm 3 were significant for probability of grade 1 to 3 gastrointestinal complication (p = .009 and p = .017, respectively) and grade 2 to 3 anorectal morbidity (p = .007 and p = .049, respectively). The D 2cm 3 was not significant for the risk of rectal complications. A 10% probability (95% confidence interval, 4%-20%) for anorectal toxicity of grade 2 or greater was reached for a D 0.5cm 3 = 52 Gy. These data suggest higher radiation sensitivity in children, as compared to adults and the need to carefully look at smaller volumes (D 0.5cm 3 ) than in adults [62]. The use of vaginal spacers in female paediatric patients with bladder neck RMS resulted in significantly decreased doses to the rectum and the posterior vaginal wall [38]. More cooperative international research is required to provide evidence-based dose constraints.

as well as data from pre-implant imaging (usually MRI) and data from imaging with the applicator in place (CT scan +/- MRI fused with CT scan). For intracavitary BT, the dose is prescribed to the most external part of the CTV, at mid-level of the implant (usually at 2-3 mm from the applicator, depending on the depth of vaginal wall). Optimization is possible to avoid hyperdose sleeves extent into the vaginal wall. For interstitial procedures, the dose is usually prescribed to the 85% isodose, according to the Paris system rules, as a starting point for treatment planning. The homogeneity criteria are carefully to be considered and no large high dose volumes should be allowed. Dose-volume relations for organs at risk (including soft tissue and bone) are accurately assessed. Manual optimization can be done to improve dose distribution, for example in BP RMS where catheters are usually more distant each at the level of bladder neck than at the level of prostate. In the AMORE procedure CT-based treatment planning with the mould in situ is performed. Preoperative MR-images can be registered with the CT scan, but geometric deviations because of surgery and mould positioning should be considered. The reference isodose (40-45 Gy) typically surrounds the mould surface by 5 mm with limited optimization beyond 5 mm if necessary. Also, a high-risk CTV volume can be identified (tumour site and probable microscopic irradicality) which receives the above mentioned 40-45 Gy and an intermediate risk volume (no visible tumour on MRI, but microscopic tumour extension not excluded) for a lower dose (e.g., 30-36 Gy) (Figure 9). Dose-volume relations for organs at risk (including soft tissue and bone) are accurately assessed. Total dose varies according to the aim of radiotherapy within the specific treatment protocol applied. For interstitial or intracavitary BT alone, the total dose expressed as EQD2 (α/β-ratio = 10 Gy for tumour) usually varies from 45 to 60 GyEQD2: 45 GyEQD2 for standard prognosis (e.g., residual microscopic disease), > 50 up to 60 GyEQD2 for poor prognosis (e.g., gross residual disease) or for recurrent disease. Decisions about dose depend on the individual prognosis of the child according to the specific risk group. The following factors also influence the decision about total dose: tumour site, tumour stage at diagnosis and after surgery (amount of residual disease), histological subtype (good, intermediate, poor prognosis), response to chemotherapy (complete/partial remission, no response), dose volume relations in organs at risk, age of the child, place of BT within the treatment programme. When given as a boost in combination with external beam radiotherapy, one possible scheme is to deliver through PDR (hourly pulses of 0.5 Gy) a dose of 15-20 G EQD2, depending on the dose of external beam radiotherapy. Most of the available literature in paediatric BT comes from LDR, or more recently, from PDR. Because of the theoretical radiobiological superiority, those treatments may be preferred over 10. DOSE, DOSE RATE AND FRACTIONATION