Answer to Question #5406 Submitted to "Ask the Experts"

Dentists and radiologists involved in diagnostic procedures routinely use radiation shielding to limit ionizing radiation. In keeping with the principle of ALARA (as low as reasonably achievable), why don't radiation therapists, who deliver doses orders of magnitude greater than the diagnosticians, routinely employ shielding? For example, a woman treated for breast cancer will not generally have her contralateral (opposite) breast shielded during external beam radiation—even if she asks. There is good data on scatter dose to the opposite breast and studies of the risk of radiation-induced breast cancers among women receiving radiation therapy and data on the effectiveness of shielding. Isn't this a breech of the ALARA principle?

The question concerns shielding patient anatomy located outside the diagnostic-imaging or radiation-treatment field. Specific patient anatomy is shielded with lead or lead-equivalent aprons during certain diagnostic radiographic procedures. The question asks why, according to ALARA, the same procedure is not being done for radiation-therapy procedures (we will assume the question refers specifically to external beam radiotherapy).

Patient shielding with lead aprons is used in diagnostic procedures when the machine head is located in proximity to the patient, e.g., dental films and in other cases where the imaging field is large and may include radiosensitive organs such as gonads. Different amounts of shielding are required at different photon energies. Diagnostic radiology procedures use energies in the kilovoltage (kV) range. At these energies, photoelectric interactions are most probable and the lead apron provides adequate shielding to patient anatomy. Further, to provide less than 2 percent transmission, only a thickness of approximately 0.5 mm of lead or lead-equivalent material is required. This thickness translates into an acceptable total weight that the apron places on the patient.

Therapeutic procedures use higher energies in the megavoltage (MV) range. At these energies, the radiation is more penetrating. To provide a 3 percent transmission, a thickness of approximately 6.5 cm of lead would be required for a therapy beam with a typical half-value layer of 1.3 cm. This thickness translates into an unacceptable weight that would be placed on the patient.

However, the main reason for the difference in patient shielding is field design. Therapeutic treatments are typically planned to conform to the volume of tissue needing irradiation. Therefore, shielding of the portion of the body outside the treatment field is essentially carried out by the machine collimators that define the field edges. The main contribution to dose deposited outside the treatment field is scatter, occurring within the patient, for which no noninvasive shielding attempts can be made. Transmission through the collimators is required to be less than 0.5 percent, as defined by machine specifications. The collimators reduce the dose to very low levels as compared with the dose due to scatter within the patient. The required thickness of a lead shield to be placed on a patient is unacceptably high, and using additional shielding materials is not practical. This is not a breech of the ALARA principle because efforts that are needed and practical are made in radiation therapy to minimize unwanted dose. Procedures to reduce doses to normal structures are routine, such as not using a wedge on a medial field to reduce the dose to the contralateral breast.

Recently, a portable, supported shield for the contralateral breast for use with medial tangential fields was developed at the Cleveland Clinic (Sohn et al. 1999). This shield reduces the scattered dose for the combined medial and lateral fields from approximately 4.8 Gy for a 50 Gy treatment course to about 2 Gy.

The significance of the dose to the contralateral breast has been studied by several investigators. Boice (Boice et al. 1992) and Roychoudhuri (Roychoudhuri et al. 2004) considered large historical series with long follow-up and found that breast patients receiving radiation had a relative risk of cancer in the contralateral breast of 1.34 compared to patients not receiving radiation, but with weak correlations. On the other hand, Unnithian and Macklis (2000) in a similar study found no significant difference, as did Obedian (Obedian et al. 2000). Brenner (Brenner et al. 2000) reviewed 32,000 cases in the surveillance, epidemiology, and end results tumor registry and found no significant increase in risk for second cancers in the contralateral breast for patients receiving radiation. Given the follow-up time for all of these studies, the treatment techniques were not current and still used wedges and blocks that produced greater scatter doses than modern techniques. Taken as a whole, there seems to be little reason for concern about second malignancies in the contralateral breast.


Leah Schubert and Bruce Thomadsen, CHP

References
Boice J, Harvey E, Blettner M, Stovall M, Flannery J. Cancer in the contralateral breast after radiotherapy for breast cancer. The New England J Med 326: 781-785; 1992.

Brenner DJ, Schiff PB, Zablotska LB. Adjuvant radiotherapy for DCIS. LANCET 355: 2071; 2000.

Obedian E, Fischer D, Haffty B. Second malignancies after treatment of early-stage breast cancer: Lumpectomy and radiation therapy versus mastectomy. J Clinical Oncol 18: 2406-2412; 2000.

Roychoudhuri R, Evans H, Robinson D, Moller H. Radiation-induced malignancies following radiotherapy for breast cancer. Br J Cancer 91:868-72; 2004.

Sohn J, Macklis R, Suh J, Kupelian P. A mobile shield to reduce scatter radiation to the contralateral breast during radiotherapy for breast cancer: Preclinical results. International Journal of Radiation Oncology-Biology-Physics. 43: 1037-1041; 1999.

Unnithian J, Macklis R. Breast cancer radiotherapy: Safe for all? J Clinical Oncol 18: 4000-4001; 2000.