Ultrasound or stereotactic guided hook-wire localisation has been the standard-of-care for the pre-surgical localisation of impalpable breast lesions, which account for approximately a third of all breast cancer. Radioguided occult lesion localisation using I-125 seeds (ROLLIS) is a relatively new technique for guiding surgical excision of impalpable breast lesions, and is a promising alternative to the traditional hook-wire method. When combined with Tc-99m labelled colloid for sentinel node mapping in clinically indicated cases, there has been uncertainty regarding whether the downscatter of Tc-99m into the I-125 energy spectrum could adversely affect the intra-operative detection of the I-125 seed, especially pertaining to a peritumoral injection. Objective: To evaluate the percentage contribution of downscattered activity from Tc-99m into the I-125 energy spectrum in simulated intra-operative resections of an I-125 seed following different sentinel node injection techniques. Method: Two scenarios were simulated using breast phantoms with lean chicken breast. The first scenario, with a 2cm distance between the Tc-99m injection site and the I-125 seed, simulated a periareolar ipsiquadrant injection with the subdermal or intradermal technique. The second scenario simulated a peritumoral injection technique with the Tc-99m bolus and an I-125 seed at the same site. Count rates were acquired with a hand-held gamma probe, and the percentage contribution of downscattered Tc-99m gamma photons to the I-125 energy window was calculated. Results: In scenarios one and two, downscattered Tc-99m activity contributed 0.5% and 33% respectively to the detected count rate in the I-125 energy window. In both scenarios, the I-125 seed was successfully localised and removed using the gamma probe. Conclusion: There is no significant contribution of downscattered activity associated with a peritumoral injection of Tc-99m to adversely affect the accurate intra-operative localisation of an I-125 seed.