ESTRO 2022 - Abstract Book

S1415

Abstract book

ESTRO 2022

Figure 1: PTV sizes versus pass rates of the global 1%/1mm with 10% (A), 20% (B), 50% (C) dose thresholds and local 1%/1mm with 10% dose threshold (D) Conclusion The complimentary global 1%/1mm and local 1%/1mm with 50% and 10% dose thresholds respectively are recommended for a reduced PTV bias dose evaluation. Future brain sCT studies should also report lesions volumes for valuable image quality analyses.

PO-1624 An empirical approach for metal implant classification using 2D dual energy radiography

J. Edmund 1,2 , U. Bjelkengren 1

1 Gentofte and Herlev Hospital, University of Copenhagen, Radiotherapy Research Unit, Department of Oncology, 2730 Herlev, Denmark; 2 Niels Bohr Institute , University of Copenhagen, 2100 Copenhagen, Denmark Purpose or Objective The composition of metal implants (MI) is often unknown, leading to wrongful material assignments in radiotherapy (RT) dose planning. CT numbers of MI can be ambiguous due to the combined effects of photoelectric and Compton interactions at low scanning energies (keV) and the high charge (Z) of metals. Further, CT numbers can be incorrect due to beam hardening and photon starvation modelling effects. To address these concerns, we investigate whether 2D dual energy radiographs, simulating 2D CT topogram projections, can provide a bulk effective charge (Z eff ) classification of MI. Materials and Methods We included bone and metals inserts (Cirs and Gammex Inc.) with known charges for modelling covering Z/Z eff =10 (Inner bone) to 29 (Copper). Z eff = √ ( ∑ i w i Z i 2.98 ) 1/2.98 where w i is the fraction of total electrons of material i. 4 unknown materials were evaluated; the head of a hip implant (HipHead) and proximal femoral rod (rod) from Corail Hip® systems, teeth with dental implants (teeth) and a stainless steel alloy (SS_unknw) insert (Gammex Inc.). Radiographs with (I) and without (I 0 ) materials were obtained using the on-board imaging system on a Varian TrueBeam v2.7 (Varian Medical Systems) at energies (E)= 70, 80, 100, 120 and 140kV and constant mAs=0.5. Radiographs E>80kV were also acquired with a Ti filter (E Ti ). Projections p(E)=ln[I 0 (E)/I(E)] were generated and ratios pR=p(E High )/p(E Low ) were created where E High are all E>E Low . Average pR values were extracted for each material. A mono exponential empirical model Z eff =exp[a 0 +a 1 · pR] was fitted for each dual energy pair to search for an optimal combination. Further, materials with a larger Z and a quadratic pR term were investigated but did not improve the model predictions. Thus a simpler model in a more relevant Z interval was chosen. p(E) and a 0 values < 0 were disregarded as too noisy (I>I 0 ) or non-physical (Z<1). Only E combinations proving a reasonable fit (R 2 >0.8) were used for Z eff prediction. Results pR with combinations E Low -(E High ) =70-(100/120/120 Ti /140 Ti ), 80-(100/120/120 Ti /140/140 Ti ), 100-(140/140 Ti ), 100 Ti - (120/140/140 Ti ), 120 Ti -(140/140 Ti ) kV all had similar fits (see figure).

The corresponding estimated Z eff intervals were 22-24 for rod (Ti+SS alloys, Z:20-30), 21-27 for teeth (bone+Au+Sn+Cu, Z:14-45), 28-36 for SS_unknw (Cr+F+Ni+Cu+Mo, Z:24-42) and 30-41 for HipHead (Ti-Al+Co/Cr alloys, Z<27) ± 15-48% range uncertainty. Known model materials could be predicted with 4-21% uncertainty (see table). All estimated Z eff values of the unknown materials were within a theoretical Z interval except for HipHead which was estimated higher but within the uncertainty range.