8.4.4 - Shielding Of Organs At Risk

The methods to shield organs at risk differ for megavoltage and kilovoltage beams.

Shielding of Megavoltage Beams

To shield organs with megavoltage beams would require a significant thickness of lead, similar to that in the treatment head (eg. up to 7 cm). This is not usually possible to place on the patient; therefore shielding is usually done by changing the alignment of the beam and adjusting field shape and size.

Shielding of Kilovoltage Beams

Kilovoltage beams are attenuated more easily with thin sheets of lead. The most common sites shielded are mucosa / teeth and the eyes.
A cheek shield is placed in the oral vestibule between the cheek and the alveolar process of the mandible / maxilla. The shield is designed to attenuate at least 95% of the beam. Dose to tissues beyond the shield is therefore very low. Cheek shields are particularly useful when treating superficial lesions of the cheek or outer lip.
Eye shields may be either internal (between the eyelid and the eyeball) or external (outside the eyelid).
Care must be taken when using shielding to prevent excessive dose to tissue adjacent to the shield. Backscatter occurs with greater frequency for kilovoltage beams due to lower scattered energy and increased photoelectric effect in high atomic number materials. This can lead to increased dose in the adjacent mucosa. This is often compensated for by the use of a wax coating, which acts as a tissue equivalent material and absorbs most backscatter.