Experimental measurement with an anthropomorphic phantom of the proton dose distribution in the presence of metal implants

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1 E13195: Slightly better series/treatment with PRONE combination with vertical field. - This field is only possible in case the tumor is not too caudal (problem with energy). - It will increase the integral dose as it can be seen from the DVHs: To promote excellency in patient care and innovative proton treatment Experimental measurement with an anthropomorphic phantom of the proton dose distribution in the presence of metal implants Dietlicher I, Casiraghi M, Ares C, Bolsi A, Weber DC, Lomax AJ, Albertini F PSI,

2 Rationale 40 patients with Spinal Axis-Associated Chordomas/Chondrosarcomas ( ) 21 patients (50%) treated with titanium implants [12/21 have a local failure in 5y] No implant With titanium implant The presence of metal has been shown to be a risk factor for patient outcome Investigation of the artefacts effects on dose distribution Staab et al IJROBP 2011

3 Rationale: Dealing with metal implants - in practice Titanium stopping power is well known (<1%) BUT artefacts introduce range uncertainties Set high HU values (>=3095) to stopping power of titanium in CT calibration curve Correct artifacts by delineating artifact areas and setting HU values to average values for fat and soft tissues Nominal kv-ct Uncorrected Corrected kv-ct Is this policy effective?

4 Material and Methods: Use of an anthropomorphic phantom Phantom with titanium implants Correct for artifacts by delineating artifact areas and setting HU values to average values for fat and soft tissues. Nominal kv-ct CT not corrected corrected for artefacts for artefacts Calculate (and deliver) clinically relevant IMPT and SFUD proton plans

5 Material and Methods: GafChromic EBT2 1) Measure the 2D dose distribution with radiochromic films positioned along sagital planes, close to the implant (note: slice 2 is <1mm to the metal) 2) Digitization: - 24h after irradiation - transmission - average of 3 scans - red channel extraction 3) Linearization of the Response calibration curve 138 MeV SOBP 5 cm Dose (Gy) LG

6 Results: without artifacts correction WITHOUT artifacts correction, SFUD composite plan % of points satisfying the gamma criteria of 3%/3mm 94% 89% 97% Dietlicher et al PMB (submitted)

7 Results: with artifacts correction WITH artifacts correction, SFUD composite plan % of points satisfying the gamma criteria of 3%/3mm 100% 97% 99% Dietlicher et al PMB (submitted)

8 Results: Effect of the artifacts on the phantom Gamma analysis (3%, 3mm) reported on the phantom Biggest area of failure criteria close to the metal Slice 1 Slice 2 Slice 3 Area of failure γ- criteria 1cm 2 5cm 2 2.6cm 2 Dietlicher et al PMB (submitted)

9 Summary Our results indicate that when artifacts are corrected in the planning CT, the accuracy of the dose calculation is quite good. With some limitations (e.g. use multiple fields, avoid passage through the metal) patients with implants can therefore be treated with protons. The poor clinical outcome of these patients has to (probably) be related to some other factors (tumour more aggressive/more advanced stage )

10 Thank you