Prostate cancer brachytherapy:: Is real-time ultrasound-based dosimetry predictive of subsequent CT-based dose distribution calculation?: A study of 450 patients by the Institut Curie/Hospital Cochin (Paris) Group

Purpose: Real-time ultrasound (US)-based dosimetry performed during 1251 loose seed implantation provides the radiation oncologist with an estimation of the dose distribution at seed insertion. However, for a number of reasons, this distribution may not reflect the real (reference) dosimetry as determined by subsequent CT, usually performed 1-2 months after implantation. The present study compared the two dosimetry data sets (US and CT) to evaluate how predictive extemporaneous US-based dosimetry can be of the real dose distribution. Methods and Materials: A total of 450 patients with prostate cancer were treated with loose 1251 seed implantation between June 1999 and October 2002 by the Institut Curie/Hospital Cochin (Paris) Group. The mean patient age was 65 years. Most patients (74%) had Stage T1c; the stage did not exceed T2b for the others. All patients had a prostate-specific antigen level of <15 ng/mL and was <10 ng/mL for 72%; 84% had a Gleason score of :56 and did not exceed 7 for the others; and 56% were treated with neoadjuvant hormonal therapy for a mean of 4.3 months. All patients were treated with loose seed implantation. Real-time US-based dosimetry was performed intraoperatively for all patients. CT-based dosimetry was performed 2 months after implantation, using the VariSeed software. The minimal dose to 90% of the outlined volume (D-90) and percentage of volume receiving at least 100% of the prescribed dose (V-100) were calculated with the two methods and compared for all patients. Results: On CT-based dosimetry, the D-90 was found to be greater than or equal to145 Gy (range, 115-240 Gy) in all patients except one. A large majority (86%) of patients showed a CT-based V-100 of >95%, and 48% had a V-100 of >98%. The mean CT-based D-90/US-based D-90 ratio was 1.0 (range, 0.66-1.33). For 89% of the patients, the difference between the two values was <20% and for 62% was <10%. The mean CT-based V-100/US-based V-100 ratio was 0.98 (range, 0-1.02), with 89% of patients showing a difference of <5%. Conclusion: Our results indicate that the D-90 and V-100 values obtained intraoperatively with our real-time US-based dosimetry are in reasonable agreement with the subsequent values obtained with CT-based dosimetry performed 2 months after implantation. Recent innovations in our dose planning software allowed better control of the longitudinal seed position and could still improve the correlation between real-time US-based dosimetry and the subsequent CT-based dose distribution. (C) 2004 Elsevier Inc.