The two types of correction of absorbed dose estimates for internal emitters

BACKGROUND. Two types of correction for absorbed dose ((D) over bar) estimates are described for clinical applications of internal emitters. The first is appropriate for legal and scientific reasons involving phantom-based estimates; the second is patient-specific and primarily intended for radio immunotherapy (RIT). METHODS. The Medical Internal Radiation Dose (MIRD) relationship ((D) over bar) = S (A) over tilde is used, where S is a geometric matrix factor and (A) over tilde is the integral of source organ activities. Internal consistency of the data and the size of organ systems in the humanoid phantom must be assured in both types of estimation. RESULTS. The first dose estimate correction (1) is one whereby computations refer to one or another standard (e.g., MIRD-type) phantom. In this case the S value remains as given, but the measured patient (A) over tilde data must be standardized. The correction factor is the phantom's ratio of organ mass to whole-body mass divided by the same ratio for the volunteer or patient. The second dose estimate correction (II) is patient-specific. While the (A) over tilde value is unchanged for this application, a correction term is provided for the phantom-derived S matrix. The dominant (nonpenetrating radiation) component of this correction factor can be obtained via the ratio of the patient to phantom organ masses. In both corrections, we recommend that true organ sizes, necessary in each method of estimation, be determined in a separate sequence of anatomic images. CONCLUSIONS. In both dose estimation corrections, true sizes of the patient's or volunteer's internal organs must be obtained. Correction due to organ mass size can be severalfold and is probably the dominant uncertainty in the internal emitter absorbed dose calculation process. Cancer 2002;94:1231-4. (C) 2002 American Cancer Society.