Biological effects evaluated as a function of patient thickness, beam quality, SSD, and treatments schedule

Continuing efforts are being made by clinical radiotherapists to evaluate radiation complications to normal tissue and organs by specific time-dose parameters. Currently, the NSD concept of Ellis is receiving wide application in the literature in the reporting of radiation complications and normal tissue tolerances. To afford an easy and broad application of the NSD concept to the evaluation of physiological, functional, or structural changes, the authors have evolved mathematical expressions for the calculations of NSD as a function of patient thickness, beam energy, SSD, and treatment schedule involving coplanar field arrangements whether the fields are treated alternately or simultaneously. Several interesting aspects evolving from the concepts of treatment planning in terms of the NSD or biological effects indicate that: for beam energies above 22 MeV, treatment is more ideally performed by treating only one field per day, since the depth of electronic equilibrium provides more effective sparing of superficial organs and tissues; large-field therapy, such as the total nodal irradiation of Hodgkin's disease, can be more effectively treated in terms of tissue sparing by higher energy beams than cobalt-60 or 4-MeV for practically all patient dimensions; a new concept of integral biological dose, the ''gram-ret,'' is proposed, which represents the quantitation of total biological effect; a series of tables with multiplication factors programmed on a digital computer is presented, which very quickly make available the NSD in any fractionated radiation treatment cycle to any plane of the body as a function of the beam energy, SSD, patient thickness, and continuous or split-course therapy schedule. (U.S.)