A spherical tumor 3 cm in diameter consisting of prostate cells may result in a

Question

A spherical tumor 3 cm in diameter consisting of prostate cells may result in a PSA blood level of about 20 nanograms/milliliter of blood. The test can only detect the presence of PSA in the blood if it is greater than about 0.1 ng/ml. An MRI can detect a cancerous tumor down to about a diameter of 1 mm. Which is a more sensitive test for the return of prostate cancer, the blood test or an MRI? To decide this, estimate how big a tumor could be and still produce a PSA level less than 0.1 ng/ml. diameter <__________ mm

Would a tumor that produced this (undetectable) amount of PSA be visible on an MRI? No or Yes Which test would be able to see the smaller tumor? The MRI scan or The PSA blood test Explain your reasoning.

Explanation / Answer

Simple math:

30 mm (3 cm) of tumor produces a PSA blood level of 20 ng/ml. If we calculate arithmatically we see that 1 mm of tumor will still produce a PSA blood level of 0.666 ng//ml which is way above the detectable limit of the diagnostic assay. Therefore PSA blood test is more sensitive than MRI scan because it can even detect the presence of the tumor below 1mm in diameter which the MRI will fail to detect.

Thank you.

PS: If you need more clarification please feel free to comment on the answer.

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