Advanced Brachytherapy Physics - Vienna 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

 D 2cm³

=+3%/fx => observed morbidity

e.g. i)

@75Gy 1% higher than model prediction

ii)

 D 2cm³

=+5%/fx => observed morbidity y s g er an mo e

@75 G i 2% hi h th d l

control @ 85 Gy

prediction

ii)  D 90

=+20%/fx => 3.5% overestimation of local control @ 85 Gy

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