Comparison of day 0 and day 14 dosimetry for permanent prostate implants using stranded seeds

Purpose: To determine, using MRI-based dosimetry (Day 0 and Day 14), whether clinically significant changes in the dose to the prostate and critical adjacent structures occur between Day 0 and 14, and to determine to what degree any changes in dosimetry are due to swelling or its resolution. Methods and Materials: A total of 28 patients with a permanent prostate implant using ¹²⁵I rapid strands were evaluated at Days 0 and 14 by CT/MRI fusion. The minimal dose received by 90% of the target volume (prostate D₉₀), percentage of volume receiving 100% of prescribed minimal peripheral dose (prostate V₁₀₀), external sphincter D₉₀, and 4-cm ³ rectal volume dose were calculated. An acceptable prostate D ₉₀ was defined as D₉₀ >90% of prescription dose. Prostate volume changes were calculated and correlated with any dosimetry change. A paradoxic dosimetric result was defined as an improvement in D ₉₀, despite increased swelling; a decrease in D₉₀, despite decreased swelling; or a large change in D₉₀ (>30 Gy) in the absence of swelling. Results: The D₉₀ changed in 27 of 28 patients between Days 0 and 14. No relationship was found between a change in prostate volume and the change in D₉₀ (R² = 0.01). A paradoxic dosimetric result was noted in 11 of 28 patients. The rectal dose increased in 23 of 28 patients, with a >30-Gy change in 6. The external sphincter D ₉₀ increased in 19 of 28, with a >50-Gy increase in 6. Conclusion: The dose to the prostate changed between Days 0 and 14 in most patients, resulting in a change in clinical status (acceptable or unacceptable) in 12 of 28 patients. Profound increases in normal tissue doses may make dose and toxicity correlations using Day 0 dosimetry difficult. No relationship was found between the prostate volume change and D₉₀ change, and, in 11 patients, a paradoxic dosimetric result was noted. A differential z-axis shift of stranded seeds vs. prostate had a greater impact on final dosimetry and dose to critical adjacent tissues than did prostate swelling. These findings challenge the model that swelling is the principal cause of dosimetric changes after implantation. Stranded seeds may have contributed to this outcome. On the basis of these findings, a change in technique to avoid placement of stranded seeds inferior to the prostate apex has been adopted. These results may not apply to implants using single seeds within the prostate.