Kilovoltage X-Ray equipment has been used therapeutically for the treatment of skin cancers for more than 100 years. More recently, in some centres, its use has diminished as mega-voltage X-Rays; usually not appropriate for most skin cancers and electrons have replaced it for many clinical applications .
Kilovoltage X-Ray units have the advantage of being low in cost, compared with mega-voltage units because of the relative simplicity of design and operation and the use of simple collimation and beam shaping, additionally, the housing of such equipment requires less in the way of radiation protection compared to the linear accelerators. Consequently the use of superficial or orthovoltage radiotherapy equipment allows departments to optimise the use of their mega-voltage machines for treating other non-cutaneous malignancies .
Confirmation of the diagnosis, histologically or cytologically; skin scrape, punch biopsy or excision biopsy, is usually recommended so that a rational treatment approach can be made, but sometimes a clinical diagnosis alone is sufficient. Initially an assessment of the depth of the tumour is made; often there can be deep penetration of the tumour at sites of embryonic fusion planes such as the nasolabial fold, ala nasi, tragus and post-auricular area and care should be taken to ensure adequate margins at depth at these sites.
The gross tumour volume (GTV) is drawn. This is the extent of macroscopic disease visible. The definition of tumour margin is best done with a magnifying glass in a well–illuminated room. A margin around the GTV is then drawn to encompass sub clinical extension of disease and to take account of any potential change in position of the patient during fractionated radiotherapy. This additional margin is known as the planning target volume (PTV). The size of this is dependent on the tumour size, histology, aggressiveness and evidence of perineural invasion .
- Small lesions; less than 0.5 mm with well-defined margins, require between 5 mm and 8 mm.
- Larger lesions; greater than 0.5 mm and particularly squamous cell carcinomas, require 1 cm margins.
The appropriate beam energy can then be chosen depending on the clinical assessment of depth of the lesion.
Most departments have standard lead ‘cut-outs’ for regular-shaped treatment volumes. Alternatively, in preparation for treatment, patients may attend the ‘mould room’ for a customised lead cut-out to be fashioned to fit the treatment volume of their tumour. The thickness of lead is dependent on the energy beam used. For 90 to 150 kV 1.5 mm is sufficient .
When treating tumours of the eyelid near the inner or outer canthus, an internal lead eye shield is used to protect the conjunctiva, cornea and lens. These are either internal or spade shaped eye shields .
The patient is positioned appropriately on the treatment couch with the lead cut-out in place over the lesion. The machine applicator of suitable diameter is then gently positioned over the area to be treated. The daily dose and fractionation of radiotherapy is largely dependent on the site, size and histology of the lesion, in addition to the age of the patient and their ability to attend for treatment. Many different regimes have been shown to be effective and the chosen regime should have optimum cure rate with least normal tissue damage .
Common prescribed doses are:
- Basal cell carcinoma; 40·5 Gy in 9 daily fractions.
- Squamous cell carcinomas; 45 Gy in 10 daily fractions.
- For elderly patients the ‘Sambrook Split’ technique is recommended which requires two visits, five weeks apart, giving 12 Gy at each visit. (Provided the area to be treated is not more than 4.5 cm) .
Other prescription regimes used include 32·5 Gy in five daily fractions or one single 18 Gy fraction . Both of these regimes are only suitable for basal cell carcinomas where the lesion is less than 3 cm .
However, please note that the author does not use either of these regimes as the acute radiation reaction is quite severe.