A conformation number to quantify the degree of conformality in brachytherapy and external beam irradiation: Application to the prostate

Purpose : This article presents a method of quantitative assessment of the degree of conformality and its designation by a single numerical value. Methods and Materials : A conformation number is introduced to evaluate objectively the degree of conformality. A comparison is made between the conformation number as found for external beam treatment plans and ultrasonically guided 125I seed implants for localized prostate cancer. Results : The conformation number in case of a planning target volume irradiated with two opposed open beams, three open beams, and three beams with customized blocks amounted to 0.17, 0.39, and 0.65, respectively. The conformation number as found for ultrasonically guided permanent prostate implants using 125I seeds averaged 0.72. Conclusions : The conformation number is a convenient instrument for indicating the degree of conformality by a single numerical value. Treatemnts with a conformation number greater than 0.60 might be termed conformal radiotherapy.