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

Background and purpose: To investigate prostate dose coverage and overdosag e in planned and realized permanent iodine seed prostate implants and to ex plore the use of the natural dose-volume histogram (NDVH) and the cumulativ e dose-volume histogram (CDVH) as tools to optimize prostate implants. Materials and methods: The optimal prescription dose (PD) or natural prescr iption dose (NPD) was derived from the NDVH. The mismatch between the NPD a nd the given PD was called the natural dose ratio (NDR). For an ideal impla nt 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 p atients. Prostate coverage was determined from the CDVH based on pre-implan t ultrasound or post-implant MRI for the planned and realized implants, res pectively. The use of the NDVH to further optimize the planned prostate imp lants was also explored. Results: 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, re alized and optimized seed distributions, respectively. The realized prostat ic implants showed smaller prostate coverage than the planned implants. The prostate volume fractions receiving 100% of the prescription dose were V-1 00 = 79 +/- 6% and V-100 = 97 +/- 3% for the realized and the planned impla nts, respectively. Conclusions: The NDVH and the CDVH proved to be valuable tools in plan eval uation. The NDVH and its derived parameter NDR quantify the risk of under o r 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 cov erage of V100 = 95% was not realized, mainly due to inadequate coverage of the base of the prostate. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.