Discussion topic:
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)?
Discussion post:
Proper shielding is an important part of electron treatments. Shielding ensures minimal healthy tissue is affected by the treatment beam. The two shielding methods for electrons are internal and external shielding. Each method affects dose differently but both work to protect healthy tissue. While shielding benefits the patient it is also important to consider the advantages and disadvantages of each method. In the following discussion, we will examine the pros and cons of electron shielding and discuss what can be done to reduce the undesirable side effects of each shielding approach.
External electron beam shielding is simple in approach and consists of any lead or lead alloy placed between the electron beam and patient to shape the field and reduce patient dose. Lead masks used to treat skin cancers of the head and face are one example. Also, lead sheets with small cutouts can be placed on patient extremities in situations where a small block placed in the treatment cone would be difficult to calculate. The benefits of this method are straight forward; reduce dose to unwanted areas and shape the field. Drawbacks to external electron shielding can be limitations of the weight of the shield. A Cerrobend mask of adequate thickness can be quite heavy. When making an external shield, thickness is important and the shield should be thick enough so that any small inconsistencies in thickness will not result in excessive dose to the patient.1 Another cause for concern is misplacement of the external shield. If adequate margin isn’t given to account for setup variation or if the setup in not accurate to PTV can be under dosed.2 In these cases skin marking and documentation of patient setup are important tools to ensure accuracy.1,2
Internal shielding is another way to reduce patient dose. Internal shields protect healthy tissue by preventing electrons from penetrating the patient too deeply. Examples of internal shields are lead nasal plugs, eye shields, and buccal shields placed behind the lips within the oral cavity. Drawbacks to lead internal shielding are electron backscatter factor and patient comfort. Electron backscatter can be reduced by coating the lead shield with a low Z material such as wax, aluminum, acrylic, or polystyrene.1 Internal shielding can be uncomfortable especially as treatment progresses and skin side effects are experienced. However, the benefit of the shields make the discomfort worth the sacrifice.1,2
References
1. Khan FM, Gibbons JP. Classical radiation therapy. In: Khan FM, Gibbons JP, ed. The Physics of Radiation Oncology. 5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014:288-295.
2. Burch, Doug. Electron Shielding. [Clinical lecture]. Orlando, FL: UF Cancer Center Medical Dosimetry Program; 2017.
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)?
Discussion post:
Proper shielding is an important part of electron treatments. Shielding ensures minimal healthy tissue is affected by the treatment beam. The two shielding methods for electrons are internal and external shielding. Each method affects dose differently but both work to protect healthy tissue. While shielding benefits the patient it is also important to consider the advantages and disadvantages of each method. In the following discussion, we will examine the pros and cons of electron shielding and discuss what can be done to reduce the undesirable side effects of each shielding approach.
External electron beam shielding is simple in approach and consists of any lead or lead alloy placed between the electron beam and patient to shape the field and reduce patient dose. Lead masks used to treat skin cancers of the head and face are one example. Also, lead sheets with small cutouts can be placed on patient extremities in situations where a small block placed in the treatment cone would be difficult to calculate. The benefits of this method are straight forward; reduce dose to unwanted areas and shape the field. Drawbacks to external electron shielding can be limitations of the weight of the shield. A Cerrobend mask of adequate thickness can be quite heavy. When making an external shield, thickness is important and the shield should be thick enough so that any small inconsistencies in thickness will not result in excessive dose to the patient.1 Another cause for concern is misplacement of the external shield. If adequate margin isn’t given to account for setup variation or if the setup in not accurate to PTV can be under dosed.2 In these cases skin marking and documentation of patient setup are important tools to ensure accuracy.1,2
Internal shielding is another way to reduce patient dose. Internal shields protect healthy tissue by preventing electrons from penetrating the patient too deeply. Examples of internal shields are lead nasal plugs, eye shields, and buccal shields placed behind the lips within the oral cavity. Drawbacks to lead internal shielding are electron backscatter factor and patient comfort. Electron backscatter can be reduced by coating the lead shield with a low Z material such as wax, aluminum, acrylic, or polystyrene.1 Internal shielding can be uncomfortable especially as treatment progresses and skin side effects are experienced. However, the benefit of the shields make the discomfort worth the sacrifice.1,2
References
1. Khan FM, Gibbons JP. Classical radiation therapy. In: Khan FM, Gibbons JP, ed. The Physics of Radiation Oncology. 5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014:288-295.
2. Burch, Doug. Electron Shielding. [Clinical lecture]. Orlando, FL: UF Cancer Center Medical Dosimetry Program; 2017.