23. Anal Cancer - The GEC-ESTRO Handbook of Brachytherapy

Anal Cancer

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 01/03/2023

8. TECHNIQUE

For this purpose, many teams perform implantation and dosimetry according to the Paris system: the needles are implanted parallel and equidistant using Papillon’s anal plate as a guide (figure 7). After recovery from general anesthesia, a planning CT scan (or a planningMRI, if needles are compatible) is acquired in the supine position to confirm needle positions and delineate the target volume (defined by the combination of information including imaging (CT, MRI and/or US) and clinical examination, with a safety margin) (figure 8). The advantage of image-based brachytherapy planning is that differential loading of needles can be done with better sparing of uninvolved circumferential anorectal mucosa (figure 9). Finally, dose calculation is performed using geometrical and manual optimization which, in case of imperfect implantation, allows to adapt the coverage to the target volume by adjusting the positions and rest times of the radioactive sources in PDR and HDR techniques.

Preparation Preparation for brachytherapy includes an enema (usually one day before and on the day of brachytherapy). Dietary advice and a residue-free diet are given to the patient to avoid bowel movements during brachytherapy. The procedure is performed under general or epidural anaesthesia. Local anesthaesia alone is possible in case of contraindication to general anaesthesia. The patient is placed in the lithotomy position. Since the patient must remain lying down during the entire irradiation, a urinary catheter is essential. The procedure starts with a new clinical evaluation (rectal examination and anoscopy) to evaluate the tumour response to the chemoradiation to confirm that brachytherapy is appropriate and to define the modalities of implantation. The longitudinal length, circumferential involvement and thickness of the residual tumour should be marked on a new diagram. The upper and lower poles of the tumour (or anal margin) can be marked by submucosal implantation of silver clips. Implantation procedure For anal canal cancers, interstitial brachytherapy is used. The number of needles necessary is determined by the initial and the post-chemoradiation extension of the disease. Implantation follows preferably the rules of the Paris system [34]: needles are implanted equidistant and in parallel using a ring template, perforated at 10-15 mm intervals (figure 6). The implantation of the needles is carried out under control of digital rectal examination. In case of anterior implantation in women, a digital vaginal examination is also essential to check that the needles have not perforated the vaginal wall. A minimal distance of 4-5mm from the needles to the anal mucosa should be respected. The needles are implanted so that they are positioned beyond the cranial pole of the CTV according to the implantation rules of the Paris system and by at least 10 mm to take retraction of the needles into account.Improved catheter placement may be achieved using advanced imaging techniques such as 3D endoluminal ultrasound [35]. Needles are then fixed in the applicator, which is sutured to the perineum. The needle tip positions and the implanted height must be checked radioscopically after the patient's legs have been extended. There is usually 5 to 10 mm of needle retraction when moving from the lithotomy position to the supine position, which is the patient position during irradiation. An anal dilatator is placed at the end of brachytherapy implant to maintain the contralateral side of anal canal and normal rectal mucosa away from the needles, and to allow the evacuation of gases and fecal materials.

10. DOSE, DOSE RATE, FRACTIONATION

The standard treatment scheme for anal canal cancer is concomitant chemoradiation therapy, combining 45 Gy (1.8 Gy × 25) pelvic external beam radiotherapy and two courses of 5-fluorouracil

9. TREATMENT PLANNING

Central to good brachytherapy planning practice is the importance of good implant geometry. The spacing of sources or catheters in regular arrays and patterns mitigates the later need to overmodulate the source dwell times or position.

Figure 7: The Paris Dosimetry System (single curved plane implant) A) Calculation of the basal dose points BDi - B) Different isodoses around the sources