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External beam (low energy)


Radiotherapy with low energy beams is performed with superficial x-ray machines (50 - 80 kVp) or orthovoltage x-ray machines (80 - 350 kVp). Clinically, the dose for superficial x-ray beams is most often prescribed at the skin surface, while for treatments with orthovoltage x-ray beams a point at the centre of the target volume is chosen, generally located at depths ranging from a few mm to a few cm. Absolute dose determinations are therefore performed either in air, in combination with backscatter factors, or in a phantom using appropriate depth dose data. Relative dose distributions are generally based on tables published in the literature or on isodose curves provided by the manufacturer of x-ray machines.

Important Principles

Depending on the beam quality, specified in terms of half-value layer (HVL) expressed in thickness (in mm) of aluminum or copper for superficial or orthovoltage x-ray beams, respectively, different types of ionization chambers should be used for absolute dose determination, as recommended in the IAEA TRS 398 Report. Ionization chambers of the plane-parallel type must have a thin entrance window, and may need to have an additional plastic foil when used in beams above 50 kV. The user determines the HVL of the beam and then chooses the calibration coefficient for that particular chamber applying the calibration curve supplied by the standards laboratory. Output factors have to be determined for all x-ray tubes and applicators using either the in-air method or an in-phantom measurement. Central axis depth dose curves are difficult to measure in low energy beams and depth-dependent correction factors may be required. If a suitable detector for relative dosimetry cannot be identified in the clinic, published data can be used.

Introduction to References

A general discussion of the various types of dosimeters is presented in Chapter 3 of the IAEA Handbook. Specific beam characteristics of superficial and orthovoltage x-ray machines can be found in Chapter 5 while kilovoltage dosimetry is discussed in Chapter 9 of the IAEA Handbook. Details of the absorbed dose calibration procedure in low energy external beams are given in the IAEA TRS 398 Report, and in Chapters 18 and 19 in the Handbook of Radiotherapy Physics by Mayles et al.. The AAPM TG61 protocol is an example of an air-kerma based protocol for x-ray beam dosimetry.