Give an example of when internal shielding is used with electron treatments. Give an example of when external shielding is used with electron treatments. Why would you pick one method over the other (i.e. what are the pros and cons of the different types of shielding)?
Field shaping may be required for some electron beams to avoid dose to surrounding healthy tissues or structures.[1] Often, lead cutouts are used to achieve the desired field shape and can either be placed on the skin or at the end of the treatment cone. Transmission of the cutouts should be less than 5% and this can typically be accomplished for beam energies less than 10 MeV with less than 5 mm of thickness of lead for the cutout, although it varies depending on beam energy. There are 2 types of shields, external and internal, and they are often covered in a wax bolus to reduce scatter. External shielding is shielding that is placed on the outside of the patient’s body. The thickness of the shield is extremely important as a shield too thin could result in increased transmission of the beam and a shield too thick could result in decreased patient comfort or compromised setup due to its weight.[1] An example of an external shield that I have used when practicing as a radiation therapist was an eye shield made of lead that was covered in red wax. The patient had a skin lesion on her right temple in close proximity to her right outer canthus. The shield was placed over her eye, contoured to the slope of her orbit, and secured with tape to avoid dose from the electron field to her right lens and eye.
Internal shielding is shielding that is placed inside a body cavity to protect the healthy structures that lay beyond the intended treatment volume.[1] One aspect to be aware of when using internal shields is the electron backscatter from the lead which will increase the dose to the tissue prior to the shield. Backscatter is higher for lower energy beams and can be decreased by covering the lead shield with a low-z material, such as wax. An example of an internal shield that I previously used as a radiation therapist were eye shields that were made of lead covered with acrylic and shaped like small suction cups so they would fit over the eye and under the eye lids. These internal eye shields were used for a patient getting total body electron radiation therapy for mycosis fungoides. The patient was positioned standing up on a rotating platform, a rotisserie, and held on to a bar above him. The patient would spin throughout the treatment.
Reference
Internal shielding is shielding that is placed inside a body cavity to protect the healthy structures that lay beyond the intended treatment volume.[1] One aspect to be aware of when using internal shields is the electron backscatter from the lead which will increase the dose to the tissue prior to the shield. Backscatter is higher for lower energy beams and can be decreased by covering the lead shield with a low-z material, such as wax. An example of an internal shield that I previously used as a radiation therapist were eye shields that were made of lead covered with acrylic and shaped like small suction cups so they would fit over the eye and under the eye lids. These internal eye shields were used for a patient getting total body electron radiation therapy for mycosis fungoides. The patient was positioned standing up on a rotating platform, a rotisserie, and held on to a bar above him. The patient would spin throughout the treatment.
Reference
- Khan FM. The Physics of Radiation Therapy.5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2014:286-289.