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Various types of external beam (teletherapy) radiotherapy units are used in veterinary medicine. These units are usually categorized according to the radiation energy level produced:

Superficial units energy range from 50 to 150 kV
Orthovoltage units energy range from 200 to 300 kV
Supervoltage energy range from 500 to 1000 kV
Megavoltage units energy range > 1MV (includes Cobalt)

In human medicine, orthovoltage units have almost completely been replaced by electron radiotherapy, as have most superficial units. Ortho and megavoltage units are most commonly used in veterinary medicine. The use of megavoltage (which has many advantages over orthovoltage) units in veterinary medicine has made radiotherapy a more successful cancer treatment modality. The main advantages of the megavoltage energy radiation are summarized below:

Table I – Comparison of megavoltage and orthovoltage radiation treatment: Characteristic Megavoltage Orthovoltage Tissue interaction Primarily Comptom with minimal photoelectric absorption and minimal tissue density dependence. Preferential bone absorption due to increased photoelectric absorption. Very tissue density dependent. Dose distribution Results in homogeneous and relatively predictable dose distribution. Predictability allows for accurate computer treatment planning (dosimetry). Results in uneven and unpredictable dose distribution and higher dose to bone. Does not permit accurate dosimetry. Maximum dose Dose builds up to reach a maximum dose (Dmax) at a tissue depth that is specific for the photon energy. The higher the energy the deeper the Dmax and the more skin sparing. Maximum dose is at the surface of the skin. Results in more severe skin side effects due to absence of skin sparing. Treatment depth The more penetrating the photon, the more sparing there is of the superficial tissues (skin). This allows treatment of deep seeded tumors without overdosing the skin surface. Depth is greatly limited by surrounding bone (preferential absorption) and by the lack of skin sparing (the deeper the lesion, the higher the total dose to the skin surface) and the worse the skin lesions. Source montage Most are isocentrically mounted to treat at multiple angles without repositioning. Treating through more than one angle results in a more homogeneous dose pattern over the total field and spreads out the dose distribution of the superficial tissue which spares more skin. X-ray tube is usually fixed to a vertical orientation, requiring patient repositioning if a treatment via multiple angles is desired. Repositioning of patients creates more room for error.

Electron radiotherapy has become the treatment of choice for superficial lesions, because of its deeper-tissue sparing characteristics (a result of rapid dose fall-off). As with megavoltage photon radiation, electron radiotherapy also offers a relatively homogenous dose distribution with no bone preferential absorption. A large amount of power is necessary for electron radiotherapy and is therefore only available with megavoltage units producing at least 6 MV photon energy. These units are widely used in human medicine but, due to their extravagant cost, are only available at a few veterinary facilities in the United States.

Computer treatment planning has also permitted better tissue sparing by allowing dose calculations for multi-portal plans. By using multiple treatment ports, the superficial dose distribution is spread out over a larger area, thus decreasing the total dose to the skin. The dose to certain critical structures can be accurately calculated by the computer, and then divided among multiple portals while still adhering to the total desired dose. This results to better tumor control with a lower complication rate to the surrounding structures. This is particularly important in treating nasal and brain tumors where critical structures such as eyes and normal surrounding brain tissue are included in the treatment area. Because the dose distribution with orthovoltage and superficial radiotherapy is so unpredictable, accurate data cannot be entered into their computer programs for dose calculations. For this reason, treatment planning and dose calculations for orthovoltage and superficial radiotherapy cannot be done by computer with accuracy.