Impact of planning target volume margins and rectal distention on biochemical failure in image-guided radiotherapy of prostate cancer

Background and purpose

A previous study in our department demonstrated the negative impact on freedom from biochemical failure (FFBF) of using too narrow planning target volume (PTV) margins during prostate image-guided radiotherapy (IGRT). Here, we investigated the impact of appropriate PTV margins and rectal distention on FFBF.

Methods and materials

A total of 50 T1-T3N0M0 prostate cancer patients were treated with daily IGRT by implanted markers. In the first 25 patients, PTV margins were 3 mm laterolateral, 5 mm anterioposterior and 4 mm craniocaudal. The subsequent 25 patients were treated with isotropic margins of 6 mm. The rectal cross-sectional area (CSA) was determined on the planning CT. Median follow-up was 61 months.

Results

The overall 5-year FFBF was 83%. A 6 mm PTV margin was related to increased 5-year FFBF on univariate analysis (96% vs 74% with the tighter PTV margins, p = 0.04). The 5-year FFBF of patients with a rectal distention on the planning CT was worse compared to those with limited rectal filling (75% for CSA ? 9 cm² vs 89% for CSA < 9 cm², p = 0.02), which remained significant on multivariate analysis (p = 0.04).

Conclusion

This retrospective study illustrated the positive impact of PTV margin adaptation and addressed the importance of avoiding rectal distention at time of the planning CT.