In intraoperative electron radiation therapy for breast cancer, attenuation plates are commonly used to protect organs at risk. These plates can be made of different materials, and the correct material (or combination of materials) has to be chosen in order to achieve the desired attenuation, while avoiding excessive backscattered radiation. The Monte Carlo method (BEAMnrcMP and DOSXYZnrcMP) has been used to characterize the electron beam generated by the setup (composed of a nondedicated linac and an applicator), and to simulate the percent depth dose (PDD) for plates of different materials. The beam has been characterized for nominal energies of 9 and 12 MeV. Several differently composed plates have been investigated: it was found, as expected, that the use of a plate presenting to the electron beam a high-Z material (i.e., lead) has to be avoided because of excessive backscatter (up to 52% compared to the PDD without plate). On the other hand, the use of a single low-Z material (i.e., aluminum) in the plate can lead to an insufficient attenuation of the beam. The two-layer plate (6 mm of Al plus 3 mm of Cu) used in S. Chiara Hospital has been found to attenuate the beam almost completely for both considered energies, causing negligible backscatter radiation. The spectrum at various depth and at the tissue-plate interface has also been investigated.