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Year : 2012  |  Volume : 27  |  Issue : 2  |  Page : 101-104

Updated anatomical data and mathematical models for embryo/fetus dosimetry

Department of Health Research, National Institute of Medical Statistics (ICMR), Ansari Nagar, New Delhi, India

Correspondence Address:
Suresh Mehta
National Institute of Medical Statistics (ICMR), Ansari Nagar, New Delhi - 110 029
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-3919.110693

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Purpose of the Study: It is proposed to fill in the gaps in the existing data matrix of mass/volume of uterus, its contents as well as mass of fetal organs by mathematical techniques down to 6 week gestation and relate this dynamic target mass during in-utero growth to recently revised Medical Internal Radiation Dose (MIRD) 21 schema. Materials and Methods: The existing data is subjected to numerical interpolations using a standard 4 degree polynomial for certain set of variables. Interpolations of mass, volume, etc., of various components of the uterus (placenta, embryo/fetus, brain, uterine wall, etc.) at weekly/biweekly intervals have been carried out. Subsequently, the step wise regression starting with three predictors - placental mass (W p ), total fetal mass (W f ) and greatest length (H) for the augmented data set led to identification of "H" and "W f" as the most significant predictors for 10 fetal organ masses W i using standard software "MS Excel." Results and Discussion: Further analysis utilizing allometric equations reveal that there is strong evidence in favor of W f compared to H for predicting (P < 0.001) the individual organ mass "W i". The prediction of W i -liver, heart, thymus, pancreas, and thyroid fall under the linear case of prediction (predictor is ln [W f ]); whereas the brain, lung, kidney, spleen, crown-heel length, etc., fall under linear-quadratic case (where ln (W f ) plus [ln (W f )] 2 ) are the predictors) respectively. The estimates indicate a rapid decline of "brain mass/total mass" ratio from 80% to 39% during 7-9 weeks. Information on specific absorbed fraction Φ (=φ/m T ) is required to arrive at the dose estimates (φ being the absorbed fraction). The very small target mass m T -few milligrams (for 90% of organs) to a maximum 11 g for brain during early pregnancy; the fetal thyroid, with its mass variation of about 300% during 10-13 weeks can impact Φ. Reported standardized doses are presented and variation of Φ with source-target distance for individual specific scaling of Φ is discussed. Conclusion: Time dependent mass m (t) of the target and consequently Φ(t) [=φ(t)/m T (t)] of the revised MIRD dose expression can be of relevance in fetal dosimetry when source-target distances are in reasonable limits.

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