Planning CT CT after 5 weeks Beam stops at distal edge Beam overshoot Tony Lomax, Centre for Proton Radiotherapy, Paul Scherrer Institute, Switzerland
1. Density heterogeneities 2. Range uncertainty 3. Spatial uncertainty 4. Summary
Density heterogeneities 1. Degradation of the Bragg peak shape Water Bone ΔR hom Water Bone ΔR inhom
Density heterogeneities 1. Degradation of the Bragg peak shape Urie et al, PMB, 1986, 31;1-15
Density heterogeneities 1. Degradation of the Bragg peak shape Bragg peak degradation as function of voxel size Titt and Mohan, MD Anderson
Density heterogeneities 2. Sensitivity to set-up errors Soft tissue Bone ΔR inhom Positioning error Underdosed area
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
Density heterogeneities 2. Sensitivity to set-up errors % +17 +10-10 -25 Nominal 3 field spot scanned proton plan Dose differences after recalculation in repeated and corrected CT Alessandra Bolsi, Stefania Comi, PSI
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
1. Density heterogeneities 2. Range uncertainty 3. Spatial errors 4. Summary
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
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
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]
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?
Range uncertainty KVCT 1. CT artefacts Stopping power profiles 3.5 3 2.5 PTV MVCT Stopping power 2 1.5 1 KV SP MV SP 0.5 0 0 20 40 60 80 100 120 140 160 180 200 220 240 X (voxels) kv-ct artifacts Prosthesis
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
Range uncertainty KVCT Effect of correcting artefacts 3.5 3 2.5 MVCT Stopping power 2 1.5 1 KV SP (uncorrected) MV CT kv SP (corrected) 0.5 0 0 20 40 60 80 100 120 140 160 180 200 220 240 X (voxels) Francesca Albertini, PSI
Range uncertainty 2. Tumour shrinkage Initial Planning CT GTV 115 cc 5 weeks later GTV 39 cc S. Mori, G. Chen, MGH, Boston
Range uncertainty 2. Tumour shrinkage Planning CT CT after 5 weeks Beam stops at distal edge Beam overshoot S. Mori, G. Chen, MGH, Boston
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
1. Density heterogeneities 2. Range errors 3. Spatial errors 4. Summary
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
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 120 100 Dose volume histograms 80 Dose distribution Volume (%) 60 40 20 0 Nominal 0.25mm Positional uncertainty (sigma) 0.5mm positional uncertainty (sigma) 1mm positional uncertainty (sigma) 80 85 90 95 100 105 Dose (%)
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 (%) 120 100 80 60 40 Nominal 0.25mm Positional uncertainty (sigma) 0.5mm positional uncertainty (sigma) 1mm positional uncertainty (sigma) 20 Dose distribution 0 80 85 90 95 100 105 110 115 Dose (%)
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%) 110 100 90 80 70 60 50 40 30 20 Homogenous box field 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 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%) 110 100 90 80 70 60 50 40 30 20 Homogenous box field 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 SD of Standard positional Abweichung error [mm] (mm) Semester project Lisa Pedrozzi 2007
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
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
Spatial errors 2. Organ motion and the interplay effect Nominal (static) dose Calculated with real motion from 4D-MRI of volunteer
Spatial errors 2. Organ motion and the interplay effect Motion patient 1 Amplitude ~ 11mm Motion patient 2 Amplitude ~ 8mm 100 80 100 80 Volume (%) 60 40 20 Volume (%) 60 40 20 0 70 80 90 100 110 120 Dose (%) 0 70 80 90 100 110 120 Dose (%)
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
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 (%) 100 80 60 40 20 Static 100% 50% 30% 0 70 80 90 100 110 120 Dose (%) PhD thesis, Martin von Siebenthal, PSI and ETHZ
Spatial errors 2. Organ motion and drift Which is worst - interplay or drift? 4D-MRI analysis of liver motion 100 80 Volume (%) 60 40 20 Static Interplay Drift (30 min) 40 50 60 70 80 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)
Spatial errors 2. Organ motion and density heterogeneities Parallel opposed photons Single field photons Single field protons Images courtesy of Thomas Bortfeld, MGH, Boston
Spatial errors 2. Organ motion and density heterogeneities Parallel opposed photons Single field photons Single field protons Images courtesy of Thomas Bortfeld, MGH, Boston
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