Leonidas graduated from the Volta Institute of Naples in October, 1925, with highest honors and a gold Queen Victoria coin.

Two months after his nineteenth birthday, he emigrated to New York City and worked as a meter tester while studying at Cooper Union Night School of Electrical Engineering.

[2][3] In 1946, he developed the systematic dosimetry to distribute radioactive iodine to treat all locations of the metastases of a patient's thyroid cancer.

The explosive growth of radiation medicine enlarged his responsibilities to the Head of Physics at Memorial-Sloan Kettering Institute.

In 1948, he added to his publication of internal radiation dosimetry the supplementary biological considerations contributed by Edith Quimby.

[6] In 1953, he improved the "twin" scintillation low-level gamma-ray crystal spectrometry method to detect and locate elements that are naturally radioactive in the human body.

In radiology he pioneered the detection of minimal burdens of radioactivity in humans, studying their distribution and variation in tissues and the epidemiology of chronic low levels of radiation.

[13] The Center for Human Radiobiology, which now has the responsibility for all AEC-supported research on the effects of internally deposited radioisotopes, grew out of his effort.

[16] In radiology, he detected radium distribution and variation in tissues, and the epidemiology of chronic low levels of radiation[17] US Patent 2,795,703A - Isadore B. Berlman and Leonidas D. Marinelli, "Apparatus for counting fast neutrons in the presence of gamma rays", issued 1957, applied for 1954.