NSD Calculator (Ellis Formula)

Proposed by Frank Ellis in 1969, the NSD was the first widely adopted mathematical model to account for the three primary variables in radiotherapy: Total Dose, Number of Fractions, and Overall Treatment Time. It was based on observations of skin reaction tolerance and was the precursor to all modern radiobiological modeling.

The resulting unit of NSD is the ret (Rad Equivalent Therapy). An NSD of 1800 rets was considered the tolerance limit for normal connective tissue.