The historical vignette lends some perspective on the development of computed tomography (CT) dosimetry. The measurement of CT dose and mapping of the dose distribution was primarily done using thermoluminescent dosimetry which was tedious and had relatively low spatial resolution. Robert L. Dixon and K. E. Ekstrand independently introduced surface dose mapping using a slower radiation therapy verification film, digitized using a scanning densitometer for various scanners of the day. A long period of quiet acceptance prevailed, during which time the mathematical theory behind the pencil chamber and subscripted computed tomography dose index (CTDI) methodology was forgotten – and some began to believe that they were making an actual “dose” measurement with the pencil chamber. The divisor of the CTDI integral becomes nT which is the active detector length as projected back to scanner isocenter and represents the total available scan width for reconstruction.