Use Of An Intra-Operative Dosimetry Technique Accurately Predicts 1 Month Post-Implant Dosimetry

One-month post-operative prostate seed implant dosimetry has been shown to predict long-term biochemical control rates and rectal morbidity. We sought to determine whether intra-operative treatment planning can accurately predict post-implant dosimetry. A comparative analysis of radiation doses as calculated intra-operatively and post-operatively was performed. From May 2000 to December 2008, 1395 patients with clinically localized prostate cancer were treated with either Iodine-125 monotherapy or a combination of Palladium-103 and 45 Gy of external beam radiotherapy. Implants were performed by 8 physicians at 6 different centers. Real-time intra-operative dosimetry was performed using the Variseed TM (Varian, Inc, Palo Alto, CA) treatment planning system. The prescription dose for I-125 implants was 160 Gy and for Pd-103 implants was 100 Gy. Our intra-operative goal was to attain the desired prostate dose while minimizing rectal and urethral doses. Mean total procedure time, from the onset of anesthesia to completion of the planning and implant, was 50 minutes (95% CI: 40-60). All patients underwent CT based dosimetry 1 month following the seed implant. The Pearson correlation was used to determine the relationship between intra-operative and post-operative parameters. The median intra-operative dose to ≥ 90% of the prostate (PD90) was 111% of the prescription dose (range 85 - 148%), and the median post-operative PD90 was 105% of prescription for the entire group (range 70 - 137%) (p < 0.001) with a Pearson correlation r-value of 0.95. The mean difference between the intra-operative and post-operative PD90 for all patients was 5.9% (95% CI: 5.4-6.4). The mean difference for I-125 was 4.4% (95% CI:3.55-5.25) and Pd-103 was 6.3% (95% CI:5.7-6.9) (p < 0.001). 0.8% of I-125 patients (4/477) had a post-operative PD90 of<140 Gy and 1% of Pd-103 patients (10/918) had a post-operative PD90 of<80 Gy. The post-operative PV100 was within 10% of the intra-operative PV100 for 92% of all patients. 1.2% (17/1395) of all patients had a post-operative rectal V100 of>1.0cc and 0.3% (4/1395) had an rectal V100 >2.0cc. The postoperative RV100 was within was within 0.5cc of the intra-operative RV100 for 94% of patients. Within a community group practice, utilizing a real-time intra-operative dosimetry seed implant technique, we consistently predicted the final post-implant dosimetry with near linear correlation. Nearly all patients obtained an optimal post-operative prostate D90 while maintaining minimal rectal doses.