Dose homogeneity as a function of source strength in optimized treatment plans

Dose homogeneity can be one of the desired end points from a treatment planning perspective. The aim of this work was to study dose homogeneity in permanent implants with varying source strengths. The rationale for using lower activity seeds in implants is rooted in the clinical observation that they result in more homogeneous (uniform) dose distributions. Dose homogeneity was compared in implants using 3 different seed activities (0.28 mCi, 0.44 mCi and 1.4 mCi) and two rival optimized treatment plans, one of which incorporated dose homogeneity into the scoring (objective) function. It was found that 0.44 mCi seeds yield the greatest degree of dose uniformity when homogeneity is included in the scoring function and when the number of needles is held to an acceptable range. It is concluded that when uniformity is not incorporated into the scoring (objective) function, homogeneity is highest in implants performed with 0.28 mCi seeds as indicated by the smaller FWHM of the differential VDH and the higher volume of the target receiving a dose in the “dose window.” When dose uniformity is prioritized in the scoring function, 0.44 mCi sees yield the highest homogeneity. This is even superior to implants using 0.28 mCi seeds with the constraints on needle number. This suggests that a medium range activity (0.44 mCi) may result in more uniform dose distributions in treatment plans that seek to maximize homogeneity with approximately the same needle count. 0.44 mCi seeds require fewer seeds and needles thus making them viable alternatives to 0.28 mCi seeds from a standpoint of cost, time of procedure and quality of implant (target coverage, dose homogeneity, etc.)