A Comparison of the Characteristics of Megavoltage Electron Beams and Superficial X-Ray Beams Used in Keloid Treatment
Henry Weatherburn PhD and John Glees MD, FRCR, DMRT
BACKGROUND
At the 5th International Keloid Symposium, a cost-effectiveness comparison of post- operative superficial kilovoltage (kV) X-ray beam treatment and linear-accelerator megavoltage (MeV) electron beam treatment was presented. Both modalities are employed for treatment of keloids and this comparison identified kV X-ray units as being more cost effective and simpler to quickly “set up” for patient treatment
Cancer treatment capacity pressures, limiting linear accelerator access, may also be present as an issue in some centres.
Subsequent post-presentation discussion identified the need for an explicit physical comparison of the beam characteristics of these two treatment modalities to provide a more comprehensive perspective of their relative utility. The present study therefore compares those MeV electron beam and superficial X-ray beam characteristics most relevant to clinical decision-making in post-excisional keloid radiotherapy, namely: beam energy, depth-dose, field flatness/uniformity, penumbra, sensitivity to curvature/stand-off, surface coverage and management of long treatment fields.
CONCLUSION
For post-excisional keloid radiotherapy, 100kV superficial X-ray beams are attractive for lesions where: the basal layer is < 0.5cm in depth; and the field is < 10cm in length. Only narrow (0.5-1.0 cm) treatment margins are required because of their sharp penumbra and simple surface coverage. However, their greater sensitivity to short FSD geometry and a large field “heel effect” limits their usefulness for long or broad targets3,5.
Bolused 6–9 MeV electron beams provide versatile coverage for typical postoperative scar-bed depths, with less persistent dose at greater depth than harder (150kV) superficial X-ray beams and better scalability to long fields. On the basis of beam physics alone, 6 MeV with 0.5 cm bolus appears well suited to many routine postoperative keloid beds, while 9 MeV with 1 cm bolus is preferable when a deeper target is suspected, though wider (1.0-1.5 cm) treatment margins are required for electron beams, due to their wider penumbra2–4,6..
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