Interstitial Time-Dose Corrections

The application of “Strandqvist-type” curves or tables to the adjustment of time and dose factors of external beam treatment has become standard practice in clinical radiation therapy. Most such adjustments, however, are based on reported observations involving time periods of weeks. Isoeffect doses in the shorter time range have been obtained by extrapolation and cannot be used with validity. Paterson (1), in reviewing clinical time-dose considerations, described an isoeffect curve for interstitial radium gleaned from clinical observations of “Manchester system” radium needle implants. Since few treatment centers have the opportunity to observe comparable numbers of cases, it is appropriate to present this information in a clinically useful form. The upper curve (Fig. 1) is a rearrangement of Paterson's curve on a “Strandqvist-type” log-log plot for comparison with other clinical time-dose information. Since in interstitial work, the dose rate is often fixed by the implantation prior to a final dose decision, the same information is plotted herein to facilitate selection of isoeffect times for various dose rates. “Manchester System” standard dose-rate, dose, and time are indicated by dotted lines for convenient reference. It is noted that the data from which Paterson's curve was drawn are not reported in detail. It may be assumed that the greatest numbers of cases and maximum validity of the curve cluster about the reference “standard dose.” Clinical approximations taken at distances from the reference standard are correspondingly less meaningful.