Postoperative radiotherapy for glioma has been shown benefit to improve survival with increased radiation doses. Dose from external beam radiotherapy is limited by normal brain toxicity. Brachytherapy can deliver higher radiation doses contained within short distance near the implants. However, traditional solid source implants require a second invasive procedure, and often fail to tailor the dose distribution to the target volume. One novel approach is to use radiolabeled target molecules to penetrate and deliver localized radiation to the tumor cells. TM-601 is a peptide derived from the venom of the scorpion that specifically binds to malignant brain tumors. This study reports dosimetry results of a recent Phase I/II study, in which 10 mCi of I-131-TM-601 peptide was directly infused into the resected tumor cavity. TM-601, or Chlorotoxin, (TransMolecular, Inc. Birmingham, AL) is a, 36 amino acid, peptide derived from the venom of the scorpion Leiurus Quinquestriatus. 18 patients with recurrent high-grade glioma received single injection of 10 mCi, 0.25–1.00 mg I-131-TM-601 into surgically created resection tumor cavities. At immediate and up to 168 hr post injection, sequential 5 whole body gamma camera images were acquired for normal organ I-131 quantification and 5 SPECT images were acquired for tumor cavity wall I-131 quantification. Attenuation correction for geometric-mean quantification was determined using Co-57 transmission scan. Peak tissue uptake of I-131-TM-601 was expressed as % injection dose (%ID) and clearance half time was fitted with mono-exponential curve. Radiation doses to normal organs were based on Medical Internal Radiation Dose (MIRD) formalism. Radiation dose to bone marrow was determined for electron radiation from I-131 in the blood and photon radiation from body. SPECT images were registered with CT/MRI using external fiducial markers to determine I-131-TM-601 distribution in 2-cm tumor cavity wall. I-131 distribution in cavity wall were converted to dose rate distribution using 3-D dose convolution at 5 time points. I-131-TM-601 quickly penetrated through tumor cavity wall. At 24 hour post injection, 79% (mean, range 44–99%) of injected I-131-TM-601 left tumor cavity. I-131-TM-601 was cleared out of body through urinary tract. Apart from tumor cavity, bladder, kidneys, stomach/spleen, and thyroid were visualized above body background in gamma camera images. Mean peak %ID was 2 (range 0.5–5) % for stomach/spleen, 2 (range 0.5–6.5) % for kidneys, 0.2 (range 0.1–1.1) % for thyroid, and 3 (range 1–5) % for normal half of the brain. Mean peak blood concentration was 0.2 (range 0.1–0.4) uCi/ml. Mean biological half-life, Tbio1/2, was 47 (range 31–81) hr for body, 25 (range 17–40) hr for stomach/spleen, 26 (18–39) hr for kidneys, and 42 (range 21–80) hr for normal half of the brain. 12 patients had Tbio1/2 of 35–2600 hr for thyroid, the remaining 6 patients showed no clearance in thyroid uptake within 168 hr observation. The mean beta phase clearance was 29 (range 14–41) hr for blood. Mean radiation dose (cGy/mCi) was 0.4 (range 0.2–0.7) for whole body, 0.7 (range 0.2–1.3) for stomach, 0.9 (range 0.3–1.8) for kidneys, 5.3 (range 0.8–22.9) for thyroid, 0.3 (range 0.1–0.4) for bone marrow, and 1.1 (range 0.3–2.4) for normal half of the brain. Radiation doses in 2-cm cavity wall were non-uniform. Mean radiation doses to 2-cm tumor cavity wall were ranged 25–260 cGy/mCi. The ratio of mean tumor wall dose-to-normal half of the brain dose ranged 60–260. I-131-TM-601 has a high tumor wall dose-to-normal tissue dose index. Dosimetric data suggest that large amount of radiation dose can be delivered to kill residual tumor cells without normal tissue toxicity. These promising results suggest potential in improving patient outcome using this novel target molecule