Immobilisation

Due to the nature of treatment techniques, immobilisation for superficial and orthovoltage therapy differs somewhat from that used for Linac (linear accelerator) treatments; the treatment couch is an independent couch, fully moveable with an adjustable height and backrest, thus enabling the patient to sit up comfortably. Further, it has a soft top to aid patient comfort and can offer additional cot sides if required.

Patients are treated in whatever position they find most comfortable; utilising pillows as well as additional supports, and if necessary, can even be treated in their own wheelchair or on a trolley. For treatments of facial lesions, either a vacubag or sandbag can be utilised to support the patient’s head and assist with immobilisation.

For all treatments, pillows and foam wedges are commonly used whenever needed to enable the patient to maintain a comfortable position [2].

Beam Arrangement

For superficial and orthovoltage techniques, an applied field is used and the operator must attempt to achieve skin apposition where possible. The end of the applicator is angled so it’s parallel to the surface of the skin, therefore accurately establishing the required Focal Skin Distance (FSD).

Due to the short FSD’s used for superficial and orthovoltage treatments, even small variations in distance between the end of the applicator and the patient’s skin will have an important effect on the dose delivered to the patient.

It should be noted that:

  • Where the treated volume can be seen to protrude into the applicator, a stand in calculation must be applied.
  • Where the treatment volume is concave, a stand off calculation should be applied.

In both cases, it is necessary to apply a factor to the standard dose charts to take into account the effect of the inverse-square-law on the dose rate, as follows:

Stand-in will act to increase the dose rate at the surface of the treatment volume [3]:

New Dose Rate Formula (Stand In)

Stand-off will act to decrease the dose rate at the surface of the treatment volume:

New Dose Rate Formula (Stand Off)

NEXT: Treatment Setup & Accessories: Example Stand Off Calculation (2 min)