Radiotherapy

Basal Cell Carcinoma

Radiotherapy as the Primary Treatment

Radiotherapy has slightly lower cure rates than surgical excision, at about 95-97% compared with 97-99%. It also results in late complications such as fibrosis, telangiectasia or second malignancies which may be avoided with primary surgical excision. Radiotherapy has the benefits of minimally invasive treatment, no requirement for anaesthesia and tissue preservation. Radiotherapy is also useful when a basal cell carcinoma is so locally advanced that surgical excision is impossible - for instance, when perineural invasion to the base of skull has occurred. Radiotherapy is therefore recommended in the following circumstances:

  • When the patient is elderly
  • When the patient has medical comorbidities that prevent anaesthesia
  • When the tumour is located on a difficult cosmetic site, such as the eyelid, earlobe or lip
  • When the tumour is not resectable due to significant perineural or other local invasion
  • Where the size of the tumour means surgical excision would lead to a poor cosmetic outcome

Choice of Radiation Modality

Most radiation modalities can be used for treatment of BCC.

Kilovoltage X-Rays

Kilovoltage x-rays have numerous benefits for treatment of BCC. A kilovoltage machine capable of producing energies from 100 keV to 300 keV is desirable for the treatment of tumours of varying thickness. Kilovoltage x-rays have a sharp penumbra at surface and depth, rapid dose fall-off with depth, and maximum dose at the skin surface. Kilovoltage x-rays are also rapidly attenuated by shielding due to increased photoelectric interactions with the high atomic number atoms of the shielding material.

Megavoltage X-Rays

Megavoltage x-rays are not well suited for treatment of BCC but have a role to play in certain circumstances. They have a large surface sparing effect, slow fall-off of dose with depth, and a broader penumbra than kilovoltage beams.

  • Bolus applied to the tumour site can negate the surface sparing effect. This is particularly useful for tumours of the external ear.
  • Tumours with significant perineural invasion can not be adequately treated with kV photons. MV photons can penetrate to much greater depths to treat this spread, if it is present.

Electrons

Electrons are commonly used in departments with modern linear accelerators. This has seen a decline in the use of kilovoltage x-ray units. Electrons have a spectrum of surface doses - surface dose increases with increasing energy (as opposed to kV and MV photons). The depth dose curve for electrons falls rapidly after a particular depth (estimated by dividing the energy of the beam by 3) and minimal dose reaches deeper structures. Electron fields have a large penumbra and higher energy beams show increased lateral scatter at depth Algorithms for electron dose distribution are poorer than photon beam calculations - this makes plans using electron fields less reliable.

Brachytherapy

Brachytherapy is a potential treatment for BCC but is more commonly used in extensive SCC of the skin. A surface mould can be created that fits the treatment area. Brachytherapy applicators are run through the mould. Brachytherapy requires fewer fractions but often costs significantly more due to the time required for mould creation.

Choice of Field Size

Field size depends on:

  • The size of the lesion
  • The margin required for coverage of microscopic disease
  • The margin to account for the penumbra at the edge of the beam
  • The margin to account for setup errors to avoid geographical miss

Coverage of microscopic disease

For small BCCs with no aggressive features, a margin of 5 mm is likely to be sufficient. Larger BCCs, or those with infiltrative or morpheic histopathology should receive a margin of 1 cm.

Accounting for penumbra

Kilovoltage beams have virtually no penumbra if collimation is placed on the patient surface, minimising field expansion. Megavoltage photon beams require a further expansion of about 5 mm, whereas electrons fields require an expansion of about 1 cm due to penumbra.

Accounting for set up errors

It is important to add a margin to account for set up errors. This is typically 5 mm in most departments.

Summary

Kilovoltage photons require an expansion of about 1 cm (0.5 to account for microscopic spread, 0.5 for setup errors)
Electrons require an expansion of 2 cm (as above, but 1 cm to account for broad penumbra)
Megavoltage photons require an expansion of 1.5 cm.
Lesions at high risk of microscopic extension (sclerosing/morphaeic types) should have an additional 0.5 added to the microscopic margin.


Choice of Dose and Fractionation

There are a large number of fractionation schedules for basal cell carcinoma.
For electron based therapy and megavoltage photon therapy, addition of 4-5 Gy in total dose is required to account for the decreased relative biological effectiveness of electrons/high energy photons with kilovoltage photons.

Standard choices

The best cosmesis is typically achieved through the use of protracted fractionation regimes. Common schedules include:

  • 60 Gy in 30 fractions provides the best tumour control, but this dose level is only required for larger lesions in most cases
  • 50 Gy in 20 fractions (2.5 Gy/#) is a commonly used fractionation schedule, particularly in younger patients and with smaller lesions
  • 45 Gy in 15 fractions (3 Gy/#) may be used for smaller lesions where cartilage will not be included in the radiotherapy field (eg. nose)
  • 30 Gy in 5 fractions (6 Gy/#) may be used for small lesions

Where the BCC is > 2 cm and cosmesis is important

Higher total doses are required for tumour control when a large number of clonagens are present. Higher doses are required in most cases.

  • 60 - 66 Gy in 30 - 33 fractions (2 Gy/#)
  • 50 - 55 Gy in 20-22 fractions (2.5 Gy/#)

Where the patient has a limited life expectancy

If a patient has a limited life expectancy and difficulty in attending the department, hypofractionation is commonly used:

  • 40 Gy in 8 - 10 fractions (4 - 5 Gy/#) for elderly patients
  • 30 Gy in 5 fractions
  • 20 Gy in 1 fraction (for patients with numerous comorbidities and limited life expectancy)

Radiotherapy as Adjuvant Treatment

Radiotherapy can be used adjuvantly with surgery in selected situations:

  • When the lesion has been incompletely resected and
    • Where the lesion can not be removed without a poor cosmetic outcome or
    • Where the residual tissue is inoperable
  • Where there is a combination of adverse risk factors such as perineural invasion, size, or recurrent disease

Younger patients who are likely to survive for an extended period may benefit from surgical excision and reconstruction with a free flap due to poor cosmesis over time with radiotherapy.


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