DOS 523 - Week 3 Discussion
Initial Post: External and Internal Shielding for Electron Therapy
At Loyola University Health System, we are equipped to use or manufacture a variety of electron shielding devices, but the types of cases we typically see rarely require anything more exotic than a standard electron block tray. These trays are a type of external shielding. Many of our intact breast cases receive an electron boost to the lumpectomy site and surgical scar at the end of photon treatment. The boost covers the site plus an appropriate margin, and the beam is shaped by a custom-made block of Cerrobend metal with the shape of the treatment site cut out. To manufacture these blocks, the hole through which the electrons pass is not actually cut out; it is the negative space from a styrofoam block that was cut to the appropriate dimensions and placed into a tray and frame. Once the styrofoam cutout is in place, molten Cerrobend metal is poured into the tray and forms around the styrofoam. Once it has cooled, the tray can be separated and the styrofoam can be knocked out, leaving a block of Cerrobend mounted to a frame with a perfectly shaped hole that defines the treatment field. These Cerrobend blocks are mounted on the treatment head of the linac. The distance from the skin makes the penumbra somewhat fuzzier than if it were closer to the patient or even on the skin, but the weight of these blocks at large sizes would be impractical, uncomfortable, or even painful for patients, so being supported by the machine is a better option.1
External shielding directly on the skin can also be accomplished by placing sheets of lead around the area to be blocked, but this is not a common practice at Loyola.
At Loyola, the types of internal shielding we typically use consists of eye shields that are placed under the eyelid to protect the eye during treatment to the eyelid or near the eye, and wax-coated lead shields that are formed around the teeth and jaw inside the mouth to protect the mandible during electron delivery to the cheeks, lips, or other nearby areas.
Eyelid shields must necessarily be thin in order to fit between the eye and eyelid, so backscatter of electrons can be a problem. One technique that can help capture low energy electrons is to coat the shield with a thin layer of dental acrylic.1 Since this will not help with higher energy electrons, it is important to plan for the backscatter as part of the therapeutic dose. If space permits on a specific patient, 2 mm of aluminum may be placed in front of an eye shield to absorb backscattered electrons.
Inside the mouth, where there is usually more space, lead shielding can be covered with wax bolus to absorb the higher energy backscattered electrons that would typically result from energies used to penetrate the centimeter or two of lip/cheek. A centimeter of wax or 4 mm of aluminum are adequate for a 9 MeV beam that has passed through 2 cm of tissue. At Loyola, we opt for wax over aluminum in these cases because it is easy to work with.
One of our physicists, Sébastien Gros, described (oral communication, March 4, 2015) a shielding technique he observed being used at the University of California San Francisco while he was there. A patient was undergoing surgery for a sarcoma of the thigh, and the treatment plan included intraoperative radiation therapy (IORT). The team used a Mobetron electron device, which is essentially the same electron gun and accelerator from a CyberKnife machine. The electron cone for the Mobetron is circular, but the team wanted to block the field into a rectangle for their patient. They improvised a block system by using sheets of lead positioned inside the operative site while the patient was still mid-surgery. This certainly blurs the line between internal and external shielding.
- Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:286-292.