Dose to contralateral breast: a comparison of four primary breast irradiation techniques.

PURPOSE: Contralateral breast dose from primary breast irradiation has been implicated in the risk of second breast malignancies. It has been previously shown that the use of half-beam blocking can increase the opposite breast dose by a factor of five. This study evaluates four different breast treatment techniques to compare the radiation dose to the contralateral breast. METHODS AND MATERIALS: Dose measurements were made using thermoluminescent dosimeters (TLD) capsules, which were placed in the Rando phantom in the following locations in the contralateral breast: seven along the central axis plane, on at 5 cm superior to, and one 5 cm inferior to the central axis plane. One TLD capsule was placed in the midcenter of the treated breast. The following radiation techniques were used: (a) half-beam with a custom block (HB+CB), (b) half-beam using asymmetric collimator jaw (HB/AJ), (c) half-beam using asymmetric collimator jaw with custom block (H/AJ+CB), and (d) isocentric technique with nondivergent posterior borders [Joint Center for Radiation Therapy (JCRT) techique]. For each technique, isodose distributions for the Rando phantom were optimized using (a) 15 degree medial and lateral compensating wedges, and (b) a single 30 degree lateral compensating wedge. The phantom was treated with 6 MV photons. Each technique was repeated six times, and the TLD readings were averaged.
RESULTS: The custom cerrobend half-beam block technique gives the highest contralateral breast dose, regardless of wedge. The remaining techniques give results in a similar range, with the asymmetric jaw with no medial wedge technique giving the lowest total dose (p = not significant). The use of a medial wedge increases the opposite breast dose for all techniques. The asymmetric half-beam technique gives significantly less dose than the cerrobend half-beam technique, due to decreased transmission through the asymmetric collimators. The asymmetric jaw vs JCRT technique results in similar contralateral breast dose.
CONCLUSIONS: As expected, dose to the contralateral breast increases with the use of a medial wedge. Cerrobend half-beam blocking gives the highest opposite breast dose. The lowest contralateral breast dose is with the asymmetric jaw with no medial wedge and no block. The asymmetric jaw technique with block yields equivalent contralateral breast doses to the JCRT technique.