Tissue Weighting Factor and Its Clinical Relevance
S Akber*, Consulting Physicist, Lorain, OHSU-E-T-311 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Since 1977, ICRP published, revised and updated Tissue Weighting Factors (TWFs) in human tissues /organs. In computing TWFs, ICRP did not take into account the body weight, organ weight, and gender difference.The present TWFs are computed based on the ratio of organ weight to body weight of reference human being.
Methods: The TWFs in the present methodology is calculated as
Organ weight/ Body weight x electron density/kg divide by electron density/m3 x p (physical density kg/m3) =TWF.
Results: The proper way to interpret TWFs is that they are indication of organ risk which is based on the ratio of organ weight to body weight. ICRP TWFs are based on the radio sensitivity of the organ. If this is the case than how come 10 organs have the same TWFs of 0.05 meaning that they have same radio sensitivity. In these 10 organs, the organ weight varies from 20 g to 1800 g but the TWFs is the same? For example, the thyroid blood content is 3.6 mL and weighs only 20 g: whereas the TD50 is 80 Gy. Liver on the other hand, blood content is 250mL and weighs 1800 g and the TD50 is 40 Gy. The blood flow in thyroid is 50 mL/min and 350 mL/min for liver. Is the radiation sensitivity of the two organs are the same? The present methodology yields the TWFs for thyroid and liver as 0.0002 and 0.02589 respectively. It is interesting to note that TD50 yield a nice correlation with TWFs both in male and female unlike ICRP TWFs.
Conclusion: The present methodology provides a convenient way to compute the TWFs based on the ratio of organ weight/Body weight which also provides the radio sensitivity of each organ.