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

Abstract 

Purpose

Real-time ultrasound (US)-based dosimetry performed during ¹²⁵I 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 ¹²⁵I 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 ≤6 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 (D₉₀) and percentage of volume receiving at least 100% of the prescribed dose (V₁₀₀) were calculated with the two methods and compared for all patients.

Results

On CT-based dosimetry, the D₉₀ was found to be ≥145 Gy (range, 115–240 Gy) in all patients except one. A large majority (86%) of patients showed a CT-based V₁₀₀ of >95%, and 48% had a V₁₀₀ of >98%. The mean CT-based D₉₀/US-based D₉₀ 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 V₁₀₀/US-based V₁₀₀ ratio was 0.98 (range, 0–1.02), with 89% of patients showing a difference of <5%.

Conclusion

Our results indicate that the D₉₀ and V₁₀₀ 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.

Keywords:  Prostate cancer, Brachytherapy, Dosimetry, Intraoperative planning