In nuclear medicine, unsealed source therapy is the predominant treatment modality. ere are examples of near-perfect simulation such as distribution of 123I metascan prior to administering 131I for ablation in thyroid cancer or the distribution of 99mTc-methylene diphosphonate (MDP) prior to administering 153Sm or 223Ra in prostate cancer (Silberstein et al., 2003). More recently with the advent of semiquantitative 68Ga PET imaging, simulations are being performed with 68Ga DOTAd-Phe(1)-Tyr(3)-octreotide (DOTATOC) for neuroendocrine tumors followed by treatment with a beta emitter such as 90Y or 177Lu DOTATOC (Baum and Kulkarni, 2012). e disconnection between radiation therapy using unsealed internal emitters compared with sealed source internal and external radiation therapy has been and is the lack of quantication. Most radiation oncology treatment plans have isodose curves planned on the images prior to the treatment administration, accurately identifying radiation-absorbed dose to the organs of interest to the nearest Gy. In nuclear medicine, however, dosimetry is vague, since many radiotracers have a multivariable systemic distribution. In the past, the lack of quantitative imaging techniques has led to
the omission of this very important question of “how much” of a cytotoxic agent are we going to give.