For treatment with superficial and orthovoltage therapeutic X–Ray beams, the dose at the skin surface is arguably the most important factor, given the rationale of treatment. X–Rays at the higher end of the energy spectrum (6 to 10 mV) are able to penetrate into the deep tissues of the body and are therefore of use when targeting deep seated tumours. When treating superficial tumours such as skin lesions, this level of penetration is unnecessary and would cause unwarranted dose to ‘normal’ tissues. Furthermore, the maximum dose would be delivered to underlying tissues causing tumour under dosage and the potential for recurrence .
Kilovoltage X-Ray beams can be subdivided into two energy ranges according to the AAPM TG-61 report : Low energy 10kV to 100 kV energy beams; used for superficial therapy, where the intended treatment region is within the first few mm of the dermis.
Medium energy 100 kV to 300 kV energy beams; used where the intended therapeutic depth is up to 20 mm.
In the IPEM code of practice used in the UK , the energy range is subdivided into medium 160 to 300 kV, low 50 to 160 kV and very low 8 to 50 kV.
Several methods of absolute dosimetry are used, depending upon the specific point of clinical interest. For low energy X–Rays, calibration is either performed at the surface of a phantom [4, 5] or in-air at the aperture of the reference applicator [4, 6], whereas for medium energy X-Rays the calibration is either performed at 2 cm depth in water [4, 5, 6], or in-air at the aperture of the reference applicator [4, 6] depending on which code of practice is referenced.