Cervix BT - 2016

Systematic dosimetric uncertainties

systematic underestimation of rectum D 2cm³ : rectum probe (iv. dosimetry) stays inside - rectum always fills with gas in between image acquisition and treatment -> D 2cm³ is systematically higher for each fraction

Systematic inter-/intra fraction variations for MRI- based cervix BT (Nesvacil et al. 2013, R&O 107): e.g. ∆ D90< +3%/fx => „observed“ local control @85 Gy 1% higher than model prediction

small targets, good coverage

large targets, less coverage

Systematically larger contours on CT vs. MRI => underestimation of D90 by CT contours (e.g. Viswanathan et al. 2007, IJROBP 68): e.g. i) ∆ D 90 =+10%/fx => 2% overestimation of local control @ 85 Gy ii) ∆ D 90 =+20%/fx => 3.5% overestimation of local control @ 85 Gy

e.g. i)

∆ D 2cm³ =+3%/fx => observed morbidity @75Gy 1% higher than model prediction =+5%/fx => observed morbidity @75 Gy is 2% higher than model prediction

ii) ∆ D 2cm³

“Can reduction of uncertainties in cervix cancer brachytherapy potentially improve clinical outcome?” Nesvacil et al. 2016, submitted to R&O