Delivery uncertainties and their consequences

Transcription

1 Planning CT CT after 5 weeks Beam stops at distal edge Beam overshoot Tony Lomax, Centre for Proton Radiotherapy, Paul Scherrer Institute, Switzerland

2 1. Density heterogeneities 2. Range uncertainty 3. Spatial uncertainty 4. Summary

3 Density heterogeneities 1. Degradation of the Bragg peak shape Water Bone ΔR hom Water Bone ΔR inhom

4 Density heterogeneities 1. Degradation of the Bragg peak shape Urie et al, PMB, 1986, 31;1-15

5 Density heterogeneities 1. Degradation of the Bragg peak shape Bragg peak degradation as function of voxel size Titt and Mohan, MD Anderson

6 Density heterogeneities 2. Sensitivity to set-up errors Soft tissue Bone ΔR inhom Positioning error Underdosed area

7 Density heterogeneities Uncorrected 2. Sensitivity to set-up errors Corrected Even when daily imaging is used to correct patient positioning, there are inevitably still residual positioning errors Stefania Comi, PSI/CNAO

8 Density heterogeneities 2. Sensitivity to set-up errors % Nominal 3 field spot scanned proton plan Dose differences after recalculation in repeated and corrected CT Alessandra Bolsi, Stefania Comi, PSI

9 Density heterogeneities 2. Sensitivity to set-up errors Recalculated in patient CT Recalculated in water CT Even after image guided re-positioning, density heterogeneities can somewhat degrade the delivered dose due to residual positioning errors Alessandra Bolsi, Stefania Comi, PSI

10 1. Density heterogeneities 2. Range uncertainty 3. Spatial errors 4. Summary

11 Range uncertainty Range uncertainty The advantage of protons is that they stop. The disadvantage of protons is that we don t always know where 10% range error

12 Range uncertainty Range uncertainty for IMPT plans Patched IMPT plan +5% CT -5% CT Lomax AJ (2007) in Proton and charged particle Radiotherapy, Lippincott, Williams and Wilkins

13 Range uncertainty Sources of range uncertainties Limitations of CT data (beam hardening, noise, resolution etc) [Σ ~ 1%] Uncertainty in energy dependent RBE [Σ ~ 2%] Calibration of CT to stopping power [Σ ~ 1-2%] CT artifacts [Σ] Variations in patient anatomy [Σ,σ] Variations in proton beam energy [σ] Variations in patient positioning [σ]

14 Range uncertainty 1. CT artefacts Many patients referred for RT post-operatively and with metal (titanium) stabilisation How accurately can we calculate proton ranges in such CT data sets?

15 Range uncertainty KVCT 1. CT artefacts Stopping power profiles PTV MVCT Stopping power KV SP MV SP X (voxels) kv-ct artifacts Prosthesis

16 Range uncertainty Set high HU values to stopping power of titanium (>=3095) in CT calibration curve Correct artifacts by delineating artifact areas and setting HU values to average values for fat and soft tissues Fat region Nominal kv-ct Soft tissue region Corrected kv-ct Francesca Albertini, PSI

17 Range uncertainty KVCT Effect of correcting artefacts MVCT Stopping power KV SP (uncorrected) MV CT kv SP (corrected) X (voxels) Francesca Albertini, PSI

18 Range uncertainty 2. Tumour shrinkage Initial Planning CT GTV 115 cc 5 weeks later GTV 39 cc S. Mori, G. Chen, MGH, Boston

19 Range uncertainty 2. Tumour shrinkage Planning CT CT after 5 weeks Beam stops at distal edge Beam overshoot S. Mori, G. Chen, MGH, Boston

20 Range uncertainty 3 field IMPT plan to an 8 year old boy 3. Patient weight changes During treatment, 1.5kg weight gain was observed Planning CT New CT Note, sparing of spinal cord in middle of PTV Francesca Albertini and Alessandra Bolsi (PSI) Max range differences: SC 0.8cm CTV 1.5cm

21 1. Density heterogeneities 2. Range errors 3. Spatial errors 4. Summary

22 Spatial errors Pencil beam scanning is very sensitive to organ motion Assume σ = 0.5cm For this example, dose errors of ~20% can result from motion (positioning) errors of 2.5mm 1.8σ 1.3σ Phillips and Pedroni, PMB 1991

23 Spatial errors 1. Effect of random uncertainty in position of delivered spots: Regular spot distribution Pencil beam positions varied randomly with SD s of 1, 0.5 and 0.25mm orthogonal to beam direction Spot weights Dose volume histograms 80 Dose distribution Volume (%) Nominal 0.25mm Positional uncertainty (sigma) 0.5mm positional uncertainty (sigma) 1mm positional uncertainty (sigma) Dose (%)

24 Spatial errors 1. Effect of random uncertainty in position of delivered spots: Clinical spot distribution Pencil beam positions varied randomly with SD s of 1, 0.5 and 0.25mm orthogonal to beam direction Spot weights Volume (%) Nominal 0.25mm Positional uncertainty (sigma) 0.5mm positional uncertainty (sigma) 1mm positional uncertainty (sigma) 20 Dose distribution Dose (%)

25 Spatial errors 1.Effect of random uncertainty in position of delivered spots: Clinical cases. % pf PTV % des with PTV dose innerhalb differences ±1,0% Differenz within Dosis +/- 1% 12 SFUD fields from 5 different patients Spot Scanning ohne IMPT (Toleranz 1,0%) Homogenous box field SD of Standard positional Abweichung error [mm] (mm) % pf PTV % des with PTV dose innerhalb differences ±1,0% Differenz within Dosis +/- 1% 16 IMPT fields from 5 different patients Spot Scanning mit IMPT (Toleranz 1,0%) Homogenous box field SD of Standard positional Abweichung error [mm] (mm) Semester project Lisa Pedrozzi 2007

26 Spatial errors 2. Organ motion and the interplay effect A scanned beam in a static patient Martin von Siebenthal, Phillipe Cattin, Gabor Szekely, Tony Lomax, ETH, Zurich and PSI, Villigen

27 Spatial errors 2. Organ motion and the interplay effect but real patients move. Martin von Siebenthal, Phillipe Cattin, Gabor Szekely, Tony Lomax, ETH, Zurich and PSI, Villigen 4D-CT derived from 4D-MRI

28 Spatial errors 2. Organ motion and the interplay effect Nominal (static) dose Calculated with real motion from 4D-MRI of volunteer

29 Spatial errors 2. Organ motion and the interplay effect Motion patient 1 Amplitude ~ 11mm Motion patient 2 Amplitude ~ 8mm Volume (%) Volume (%) Dose (%) Dose (%)

30 Spatial errors 2. Organ motion and the interplay effect % pf CTV with dose differences within +/- 3% Homogenous box field Amplitude of motion (cm) Semester project Nadine Graedel 2009

31 Spatial errors 2. Organ motion and the interplay effect Can gating help? 4D dose calculation applied to cylindrical target in presence of real motion (4D-MRI of volunteer) in liver Irregularities in breathing and amplitude over duration of treatment taken into account T av = 4.7s, A av = 10.9mm Gating signal taken from diaphragm wall motion ( ideal gating) Calculations performed for static, 100, 50 and 30% duty cycles Volume (%) Static 100% 50% 30% Dose (%) PhD thesis, Martin von Siebenthal, PSI and ETHZ

32 Spatial errors 2. Organ motion and drift Which is worst - interplay or drift? 4D-MRI analysis of liver motion Volume (%) Static Interplay Drift (30 min) Dose (%) DVH s of CTV for static, interplay and drift motions (volunteer 1) PhD thesis, Martin von Siebenthal, PSI and ETHZ Motion (gated and ungated) and drift in volunteer 4 (single point in liver)

33 Spatial errors 2. Organ motion and density heterogeneities Parallel opposed photons Single field photons Single field protons Images courtesy of Thomas Bortfeld, MGH, Boston

34 Spatial errors 2. Organ motion and density heterogeneities Parallel opposed photons Single field photons Single field protons Images courtesy of Thomas Bortfeld, MGH, Boston

35 Summary The power of proton therapy lies in the sharp Bragg peak but this can be substantially more sensitive to delivery uncertainties than conventional radiotherapy Density heterogeneities Range uncertainty Spatial uncertainties Selection of field directions Accurate dose calculations CT calibration Correction of artifacts Patient monitoring Robust planning Accurate patient positioning Multiple (SFUD!) field plans Organ motion management and patient monitoring