ESTRO 2024 - Abstract Book

S6020

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Key target and OAR dose constraints were evaluated and a pairwise comparison performed using a Wilcoxon signed rank test (significant p<0.01). Correlation between rectum and bladder volume and constraints was examined using Spearman’s rho (significant p<0.01).

Results:

Median (range) dose to 95% of CTVpsv_4000 was 4026cGy (3977-4136cGy), 4007cGy (3692-4182cGy) and 3942cGy (3718-4107cGy) for the session, verification and not-adapted plans respectively. Session plan CTVpsv_4000 dose was significantly higher than the verification and not-adapted plans (p<0.001), and verification plan dose higher that the not-adapted plans (p<0.001). A similar pattern was seen for dose to 95% of PTVpsv_3625, median (range) dose was 3626cGy (3349-3664cGy), 3553cGy (3093-3731cGy) and 3475cGy (3251-3702cGy) for the session, verification and not adapted plans. The difference in PTVpsv_3625 dose between session, verification and not-adapted plans was significant (p<0.001) as was the difference between verification and not-adapted plans (p<0.01). Median (range) rectal volume ≥3600cGy was 1.5cm 3 (0.3-2.0cm 3 ), 1.7cm 3 (0.0-7.9cm 3 ) and 1.84cm 3 (0.3-4.7cm 3 ) for the session, verification and not-adapted plans. Volume difference was not significant between session and verification (p0.02) nor verification and not-adapted plans (p0.51) but the latter was significantly larger than the session (p<0.01). No significant difference in bladder volume ≥3700cGy was seen between groups. Urethra percentage volume ≥4200cGy was significantly lower on the session plans compared to the verification and not-adapted plans (p<0.001) and between the verification and not-adapted plans (p<0.001). Median (range) volume ≥4200cGy was 10.0% (0.0-93.9%), 31.1% (0.0-96.7%) and 66% (0.0-96.6%) for the session, verification and not-adapted plans. Figure 1 summarises the group results.

Figure 2 presents patient specific results. Adaptive plans reduced rectal and bladder dose for some patients more than others. Patients’ rectal and bladder volumes on MRI session and MRI verification , and volume change between scans, was not correlated with rectal volume ≥3600cGy or bladder volume ≥3700cGy.