29 Skin Cancer

Skin Cancer

9

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/04/2017

Fig. 31.10: Interstitial implant with three 5 French plastic tubes of an infiltrating >8mm thick squamous cell carcinoma.

Fig. 31.10: Interstitial implant with three 5 French plastic tubes of an infiltrating >8mm thick squamous cell carcinoma.

Fig. 31.12: Squamous carcinoma in the columella. Two plastic tubes HDR 4Gy x 12 twice a day. Results at six months

8.2 Interstitial techniques Local or regional anaesthesia is required. If possible, nerve block anaesthesia is performed to avoid swelling in the target area. This is possible with a mentalis nerve block for the medial two thirds of the lower lip, an infraorbital nerve block for the upper lip and cheek, or partial or total ring anaesthesia around the ear. If local infiltration anaesthesia of the skin is used all visible and palpable tumour should be marked with a pen before the skin is infiltrated with lidocaine 1% or other local anaesthetic. The positioning of the lines is drawn on the skin, taking into account the Paris System and Stepping Source Dosimetry System (SSDS) [24] rules to cover the target volume (for details see 9.A). Most skin cancers can be treated by a single plane implant, using parallel lines, spaced 10 to 16 mm apart (Fig. 31.10, 11, 12). Implants which are too superficial may result in late visible telangiectasia along the source positions. For curved planes, frequently occurring in skin cancers of the scalp, face and limbs the prescription isodose projects deeper at the concave than at the convex side. The arrangement of implanted or applied sources in those cases must take account of this shift in the isodose lines. Lesions thicker than the thickness of the reference prescription isodose of a single plane configuration, have to be treated by double plane implants (the second plane may be constructed "in air" above the tumour adding some bolus or tissue equivalent to cover the catheters). However, sometimes it is easier to shave the exophytic part of the tumour with electrocoagulation. With HDR, the optimization allows to correct the small differences in distance between the plastic tubes. Some catheter spacing and stabilization technique is useful to maintain geometry for multicatheter interstitial implants [25]. If rigid needles are used,

a pair of templates with perforated holes at exact distances, usually triangles or squares 1cm side, are useful to achieve very homogeneous implants. Most of the commercially available Treatment Planning Systems (TPS) are based on the TG-43 assumptions [26] and then the scatter default on this implants type is not considered. In some hospitals, this deviation is theoretically compensated with the use of bolus over the skin surface. In a recent work using Monte Carlo the conclusion is that no bolus is required for Ir-192 but few mm of bolus are required for HDR Co-60 [27].

9. TREATMENT PLANNING

9.1 2D planning In contact brachytherapy, treatment planning is often 2D and the prescription is to distance in depth depending on the contact applicator type. It is always required to report the depth of prescription dose, as well as the surface dose delivered at the epidermal surface. For surface contact applicators the prescription depth is 3-4 mm under the skin surface, no more than 5 mm. Dosimetry can use library pre-calculated curves (atlas) for surface applicators. We must keep inmind that the surface is receiving a higher dose than prescribed, with surface contact applicators (Leipzig™, Valencia™