Anne Stachowiak

The Safety of Lymphatic Mapping in Pregnant Breast Cancer Patients Using Tc‐99m Sulfur Colloid

Abstract:  This investigation was undertaken to assess the risk to the embryo/fetus associated with sentinel lymph node biopsy and lymphoscintigraphy of the breast performed in pregnant patients. Approximately 92.5 MBq (2.5 mCi) of filtered Tc‐99m sulfur colloid was injected peritumorally the day before surgery in two nonpregnant women with breast cancer. The whole‐body distribution of the radiopharmaceutical was evaluated using a gamma camera 1 hour after injection. We then calculated the absorbed dose to the embryo/fetus for three theoretical extreme scenarios of biodistribution and pharmacokinetics: 1) all of the injected radiopharmaceutical remains in the breast and is eliminated only by physical decay; 2) all of the injected radiopharmaceutical is instantaneously transported to the urinary bladder, where it remains and is eliminated only by physical decay; and 3) the injected radiopharmaceutical behaves as though it were administered intravenously, that is, it has the biodistribution and pharmacokinetics of Tc‐99m sulfur colloid injected for a liver/spleen or bone marrow scan. The fetal radiation absorbed dose was then estimated for two Tc‐99m dosages: 18.5 MBq (0.5 mCi) and 92.5 MBq (2.5 mCi). The Medical Internal Radiation Dosimetry (MIRD) program was used to estimate the absorbed doses to the embryo/fetus for the first two scenarios. Published data were used to calculate the doses for the third scenario. A single breast is not among the source organs in the MIRD program, so the heart was used as a surrogate in the first scenario. In the two breast cancer patients, whole‐body gamma‐camera images obtained 1 hour after radiopharmaceutical injection revealed no radioactivity except in the vicinity of the injection site. In the theoretical scenarios, with 92.5 MBq, the highest absorbed doses to the embryo/fetus were as follows: scenario 1, 7.74 × 10−2 mGy at 9 months of pregnancy; scenario 2, 4.26 mGy during early pregnancy; and scenario 3, 0.342 mGy at 9 months of pregnancy. The maximum absorbed dose to the fetus of 4.3 mGy calculated for the worst‐case scenario is well below the 50 mGy that is believed to be the threshold absorbed dose for adverse effects. Thus breast lymphoscintigraphy during pregnancy appears to present a very low risk to the embryo/fetus.