The combined use of the natural and the cumulative dose-volume histograms in planning and evaluation of permanent prostatic seed implants.

Abstract

To investigate prostate dose coverage and overdosage in planned and realized permanent iodine seed prostate implants and to explore the use of the natural dose-volume histogram (NDVH) and the cumulative dose-volume histogram (CDVH) as tools to optimize prostate implants. The optimal prescription dose (PD) or natural prescription dose (NPD) was derived from the NDVH. The mismatch between the NPD and the given PD was called the natural dose ratio (NDR). For an ideal implant the NDR should be 1. The target is overdosed if NDR >1 and underdosed if NDR <1. The NDR and prostate coverage were evaluated in implants of nine patients. Prostate coverage was determined from the CDVH based on pre-implant ultrasound or post-implant MRI for the planned and realized implants, respectively. The use of the NDVH to further optimize the planned prostate implants was also explored. The mean values of the NDRs were 1.30+/-0.34 (range 0.76-1.79), 1.22+/-0.31 (0.76-1.74) and 1.22+/-0.12 (0.98-1.33) for the planned, realized and optimized seed distributions, respectively. The realized prostatic implants showed smaller prostate coverage than the planned implants. The prostate volume fractions receiving 100% of the prescription dose were V(100)=79+/-6% and V(100)=97+/-3% for the realized and the planned implants, respectively. The NDVH and the CDVH proved to be valuable tools in plan evaluation. The NDVH and its derived parameter NDR quantify the risk of under or overdosage for a given PD. The CDVH is valuable in evaluation of prostate coverage realized prostate. Our strategy to implant just the prostate and not the prostate plus a margin led to NDR values between 1.1 and 1.3 and a prostate coverage of V(100)=79+/-6% in the nine patients. The planned coverage of V(100)=95% was not realized, mainly due to inadequate coverage of the base of the prostate.