A quality assurance index for brachytherapy treatment plan verification

A method is described which provides an independent verification of a brachytherapy treatment plan. The method is applicable to any common geometric configuration and utilises a simple equation derived from a common form of non-linear regression. The basis for the index value is the relationship between the treatment time, prescribed dose, source strength and plan geometry. This relationship may be described mathematically as: Total Treatment Time proportional to (Prescribed Dose/Source Strength) x (a geometric term) with the geometric term incorporating three geometric components, namely the distance from source positions to points of dose normalisation d, the total length of the dwell positions L, and the number of source trains or catheters N. A general equation of the form GF = k (d)(-alpha) (L)(-beta) (N)(-gamma) is used to describe the plan geometry, where GF is what we have termed the geometric factor, k is a constant of proportionality and the exponents are derived from the non-linear regression process. The resulting index is simple to calculate prior to patient treatment and sensitive enough to identify significant error whilst being robust enough to allow for a normal degree of geometric distortion.