- Email: [email protected]
A Dosimetric Planning Feasibility Study for Spot-Scanned Stereotactic Body Proton Therapy (SBPT) for Unresectable Pancreatic Tumors
Volume 90 Number 1S Supplement 2014
Poster Viewing Abstracts S917
weight throughout the treatment, and potentially adaptive re-planning, may be necessary to maintain adequate target coverage in IMPT for cervical cancer. Author Disclosure: D. Wang: A. Employee; University of Iowa Hospitals and Clinics. S. Bhatia: A. Employee; University of Iowa Hospitals and Clinics. E. Dinges: A. Employee; University of Iowa Hospitals and Clinics. B. Gross: A. Employee; University of Iowa Hospitals and Clinics. J. Buatti: A. Employee; University of Iowa Hospitals and Clinics. S. McGuire: A. Employee; University of Iowa Hospitals and Clinics.
3793 Evaluation of the Range Shifter Model in a Commercial Proton Therapy Planning System W. Matysiak, D. Yeung, R. Slopsema, and Z. Li; University of Florida Proton Therapy Institute, Jacksonville, FL Purpose/Objective(s): Existing proton therapy pencil beam scanning (PBS) systems have limitations on the minimum range in patient that can be treated. The design limitation arises from practical considerations such as beam current intensity, layer spacing and delivery time. A range shifter (RS), which is a slab of stopping material located between the nozzle and the patient, is used to reduce the residual range of the incoming beam so that the treatment ranges can be extended to shallower depths. The treatment planning system (TPS) has to accurately calculate the beam spot size entering the patient, given the proton energy, for arbitrary positions and thicknesses of the RS inserted in the beam path. The commercial TPS investigated in this study implements a model of the beam widening due to the RS by incorporating the scattering properties of the RS material into the, so called, V parameter. This parameter is derived from a pair of measurements: with and without the RS, for a series of clinically relevant proton energies. The V parameter is then used to calculate the phase space parameters of the beam after traversing the RS. This work evaluates the RS model implemented in Eclipse, by comparing the spot sizes in air after traversing the RS located at different positions along the beam axis, versus simulations from Geant4 Monte Carlo as well as analytical calculations using the Fermi-Eyges theory with Highland approximation of multiple Coulomb scattering. Materials/Methods: Spot sizes in air around the isocenter were extracted from PBS plans calculated by Eclipse with the RS located at various positions with respect to the isocenter and selected proton energies. The spot sizes calculated by the TPS were compared with those calculated using Fermi-Eyges theory as well as with results from Monte Carlo simulations for the corresponding RS positions and proton energies.
Scientific Abstract 3794; Table
Results: For range shifter positions that are close to that used in the measurements during commissioning, the differences between the TPS calculations and both: Monte Carlo simulations as well as analytical calculations, were small throughout the entire range of clinically relevant proton energies. However, for RS positions that deviate significantly (15 cm or more) from the original measurements, the differences between the TPS calculated spot size and Monte Carlo simulations, and that from analytic calculations, were considerable. The errors were worst for lower energies with as much as 30% in the worst case. Conclusions: The RS model implemented in the TPS under investigation introduces considerable inaccuracies to proton plans. The inaccuracies are most prominent for lower clinically relevant proton energies and when the RS is positioned away from the location used in beam configuration commissioning. Author Disclosure: W. Matysiak: None. D. Yeung: None. R. Slopsema: None. Z. Li: None.
3794 A Dosimetric Planning Feasibility Study for Spot-Scanned Stereotactic Body Proton Therapy (SBPT) for Unresectable Pancreatic Tumors T.T. Sio,1 E.J. Tryggestad,1 J.B. Ashman,2 C.J. Beltran,1 W.S. Harmsen,3 K.A. Hoeft,1 S.K. Wurgler,1 and R.C. Miller1; 1Department of Radiation Oncology, Mayo Clinic, Rochester, MN, 2Department of Radiation Oncology, Mayo Clinic, Scottsdale, AZ, 3Department of Biomedical Statistics and Informatics, Mayo Clinic, Rochester, MN Purpose/Objective(s): Few definitive treatment options exist for locally advanced pancreatic malignancies. We explored the planning feasibility and dosimetric advantage of SBPT vs. intensity modulated radiation therapy (IMRT). Materials/Methods: Ten patients previously completed photon therapy were retrospectively selected. In this hypothetical SBPT protocol, the GTV included only gross tumor on CT imaging with no elective nodal treatment. An ITV was created and breath-hold treatment scenario was assumed. Multiple protons plans were created using a posterior 3-field technique. Two beam lines were investigated, one with a nominal spot size (1s) at relevant depths of 4 mm and the other with 6 mm; both single and multifield optimizations (SFO vs. MFO) were employed. For “robustness” considerations, proton planning was further stratified for multiple scanning target volumes, i.e., CTV+3, +5, and +7 mm isotropically. For comparison, 9-field IMRT plans were generated at CTV+5 mm. All plans were normalized to at least 98% of CTV covered by 40 Gy(E), in 5 fractions; a
Dosimetric comparison of IMRT versus SBPT in pancreatic tumor treatment planning
Nomenclature (Note) Integral Dose (Body), V5Gy [cc] Volume [cc] receiving 5 Gy or more CTV, D99.9% [Gy] 99.9% of volume receiving this dose [Gy] or more Stomach, V20Gy [cc] Volume [cc] receiving 20 Gy or more Small Bowel, V20Gy [cc] Volume [cc] receiving 20 Gy or more Spinal Cord, D0.01cc [Gy] 0.01 cc of volume receiving this dose [Gy] or more Duodenum outside CTV, V38Gy Duodenal volume outside CTV [cc] [cc] receiving 38 Gy or more Liver, DC1250cc [Gy] 1250 cc of volume receiving this dose [Gy] or less Left Kidney, DC150cc [Gy] 150 cc of volume receiving this dose [Gy] or less Right Kidney, DC150cc [Gy] 150 cc of volume receiving this dose [Gy] or less
IMRT (9-field) - Mean (SE)
Group 1: MFO/6-mm Spot - Mean (SE)
Group 2: MFO/4-mm Spot - Mean (SE)
Group 3: SFO/6-mm Spot - Mean (SE)
Group 4: SFO/4-mm Spot - Mean (SE)
P values (Proton)
3285 (471)
1777 (221)
1590 (221)
1723 (228)
1560 (218)
<.0001
39.8 (0.0)
39.8 (0.0)
39.7 (0.0)
39.7 (0.1)
39.6 (0.1)
.01
39.2 (18.0)
7.3 (3.7)
5.4 (3.1)
6.6 (3.6)
5.1 (3.0)
.008
9.2 (5.9)
7.1 (4.9)
5.8 (4.7)
7.0 (5.2)
5.8 (4.8)
.01
17.9 (0.9)
18.8 (1.6)
18.4 (1.6)
16.0 (1.5)
16.0 (1.5)
.0002
6.2 (2.2)
7.8 (2.2)
7.1 (2.1)
5.8 (1.8)
6.0 (1.7)
.0015
13.9 (3.4)
11.3 (3.4)
8.7 (2.7)
10.3 (3.1)
7.9 (2.5)
.0042
9.6 (0.9)
8.5 (1.0)
8.0 (1.0)
7.5 (1.1)
7.2 (1.2)
.0069
11.8 (0.8)
9.0 (1.4)
8.8 (1.4)
10.5 (1.8)
10.2 (1.8)
.0081
S918
International Journal of Radiation Oncology Biology Physics
minimum dose of at least 39.2 GyE was achieved in 95% of the plans. Multicomparative and 2-sample t-tests were used. Results: The median GTV and CTV volumes were 43.9 and 56.9 cc, respectively. With median minimum separation from GTV of 0.15 cm (range, 0.0-1.6 cm), the duodenum was the most difficult organ-at-risk (OAR) for meeting dosimetric constraints, especially in the CTV+7 mm cases. For CTV+5 mm planning, selected IMRT and SBPT dosimetric values are shown in Table; P-values were calculated across all 4 proton groups. The integral dose was significantly better for proton vs. IMRT plans (V5Gy, 1663 vs. 3285 cc, P Z 0.0064), as for multiple other OARs. While most proton plans were acceptable, 4-mm spot plans gave slightly less dose to the OAR structures owing to a smaller spot size, but the binary differences (vs. 6-mm spot) were not statistically significant in most cases. Conclusions: Benchmarked against IMRT planning, pancreatic SBPT offers comparable duodenal sparing with significant improvements in most OAR dosimetric metrics considered, which represents an attractive therapeutic option. Under static proton planning considerations, MFO vs. SFO differences were generally not statistically significant. As expected, smaller spot size trended for better OAR sparing. Further proton plan robustness evaluations are warranted to determine the optimal combination of spot size, optimization technique, and target volume for spot placement in spotscanned SBPT. Author Disclosure: T.T. Sio: None. E.J. Tryggestad: None. J.B. Ashman: None. C.J. Beltran: None. W.S. Harmsen: None. K.A. Hoeft: None. S.K. Wurgler: None. R.C. Miller: None.
standard DVH metrics, we introduce Area-Outside-Left (AOL) and Area-Outside-Right (AOR) to summarize deficiencies observed in the perturbed plans. AOL/AOR is proportional to the area of the aggregated 1s DVH span for a given type of plan perturbation falling left/right of the target 1s span for the corresponding base plan. A dose threshold was also incorporated into the calculation, below and above which Area-Outside does not contribute to AOL and AOR, respectively. Results: The table provides an abbreviated summary of our findings. In row 1 we compute equal-weighted summations of AOL and AOR over all perturbations (SAOL+SAOR). With no other considerations, SFO-PRM is the “best” option for CTV+5mm and CTV+7mm plans, the latter having the lowest observed score. CTV+3mm plans tend to under-dose the CTV, especially, e.g., perturbations SP+3% and z+3mm; MFO SP-3% plans for CTV+5mm and (especially) CTV+7mm produce significant hot spots in the CTV. Applying dose thresholds of 95%Rx and 105%Rx to the AOL/AOR calculation (row 2) improved sensitivity to these underdose-overdose issues. Conclusions: Considering duodenal dose-volumes irradiated by CTV+7mm plans, CTV+5mm-SFO-PRM may be the best compromise between robustness and OAR sparing. Further investigation of plan robustness to intra-breath-hold motion is warranted and is in progress. Author Disclosure: E.J. Tryggestad: None. T.T. Sio: None. J.B. Ashman: None. C.J. Beltran: None. T.J. Whitaker: None. K.A. Hoeft: None. S.K. Wurgler: None. R.C. Miller: None.
3795 Aggregated Plan Robustness Analyses for Spot-Scanned Pancreatic Stereotactic Body Proton Therapy E.J. Tryggestad,1 T.T. Sio,1 J.B. Ashman,2 C.J. Beltran,1 T.J. Whitaker,1 K.A. Hoeft,1 S.K. Wurgler,1 and R.C. Miller1; 1Mayo Clinic, Rochester, MN, 2Mayo Clinic, Phoenix, AZ Purpose/Objective(s): We have previously established the dosimetric feasibility of spot-scanned stereotactic body proton therapy (SBPT) for unresectable pancreatic cancer using IMRT as a benchmark. We expand our prior study by performing a proton plan robustness analysis using a novel population-based tool. Materials/Methods: 120 “base” plans were generated for 10 patients with locally advanced pancreatic cancer who previously completed conventional radiation therapy. Variables tested were single- vs. multifield optimization (SFO vs. MFO); 4 vs. 6 mm (s) spot size (VAC vs. PRM); and isotropic CTV margins of 3, 5 and 7 mm for scanning target volumes (STV Z CTV+3mm vs. CTV+5mm vs. CTV+7mm). A hypo-fractionated regimen of 40 GyE in 5 fractions; breath-holding for motion management; and 3 posterior fields to minimize bowel gas and motion variability were hypothetically assumed. Base plans were normalized such that CTV V100% Z 98%. Eight perturbed plans isocenter shifts of 3 mm in x, y, z; 3% systematic range (SP) uncertainties - were generated from each base plan. 1,080 proton plans were evaluated in total. Custom DVH aggregation tools were developed to compare planning results. In addition to evaluating Scientific Abstract 3795; Table
3796 Modeling the Dosimetric Effects of Repainting in RespiratoryGated Spot Scanning Proton Treatment Plans J.E. Johnson, C. Beltran, M.G. Herman, and J.J. Kruse; Mayo Clinic, Rochester, MN Purpose/Objective(s): Interplay between target motion and spot scanning proton delivery can lead to significant degradation in plan quality. The combination of repainting and respiratory gating was investigated as a strategy to mitigate these effects. Materials/Methods: An analytic routine modeled three-dimensional dose distributions of pencil-beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15-cm deep, 3-cm radius spherical CTV with a 1-cm isotropic ITV expansion. The interplay effect was studied in subsequent calculations by modeling proton delivery from a clinical synchrotron based spot scanning system and respiratory target motion, patterned from surrogate breathing traces from clinical 4DCT scans and normalized to nominal 1 cm amplitude. Motion both parallel and orthogonal to the raster direction of the beam was investigated. Repainting was modeled by setting a maximum MU to be delivered in each visit of the beam to a spot. The beam rastered over all prescribed spots in an energy layer, either delivering the prescribed MU or the maximum allowed MU. The beam rescanned over any spots in that layer with remaining MU until the prescribed MU was reached for all spots; then the system switched to the next energy
Abbreviated summary of robustness analysis for pancreatic SBPT planning study (N_Pts Z 10) STV Z CTV+3mm
Structure CTV
Duodenum
DVH Metric
Perturbation
SAOL+SAOR Norm’zd. SAOL(D<95%)+SAOR(D>105%) Norm’zd. Mean D D Min. in cGy (1s) Mean D D Min. in cGy (1s) Mean D D Max. in cGy (1s) Mean V 20 Gy in cc (1s) Mean V 38 Gy in cc (1s)
all comb’d. all comb’d. S.P. + 3% z + 3 mm S.P. - 3% base base
SFO-PRM
STV Z CTV+5mm
STV Z CTV+7mm
SFO-PRM
MFO-VAC
MFO-PRM
SFO-PRM
MFO-VAC
MFO-PRM
1.31 14.5
1.07 3.67
1.18 3.83
1.26 5.71
1.00 1.00
1.09 7.58
1.12 6.67
-634 (196) -327 (144) -48.7 (19.7) 24.3 (17.2) 7.40 (10.5)
-187 (155) -111 (122) -56.9 (20.4) 28.6 (19.3) 9.76 (12.4)
-131 (130) -51.2 (65.2) 92.2 (116) 25.6 (18.3) 11.1 (13.2)
-181 (108) -49.1 (75.4) 126 (158) 28.5 (19.2) 11.8 (13.5)
-3.66 (64.8) -25.4 (32.2) -54.3 (34.4) 34.8 (21.8) 14.6 (14.1)
-76.1 (89.0) -15.9 (20.0) 136 (257) 30.3 (20.4) 15.3 (14.6)
-85.2 (77.5) -30.7 (41.3) 132 (201) 33.1 (20.7) 15.1 (14.7)
Poster Viewing Abstracts S917
weight throughout the treatment, and potentially adaptive re-planning, may be necessary to maintain adequate target coverage in IMPT for cervical cancer. Author Disclosure: D. Wang: A. Employee; University of Iowa Hospitals and Clinics. S. Bhatia: A. Employee; University of Iowa Hospitals and Clinics. E. Dinges: A. Employee; University of Iowa Hospitals and Clinics. B. Gross: A. Employee; University of Iowa Hospitals and Clinics. J. Buatti: A. Employee; University of Iowa Hospitals and Clinics. S. McGuire: A. Employee; University of Iowa Hospitals and Clinics.
3793 Evaluation of the Range Shifter Model in a Commercial Proton Therapy Planning System W. Matysiak, D. Yeung, R. Slopsema, and Z. Li; University of Florida Proton Therapy Institute, Jacksonville, FL Purpose/Objective(s): Existing proton therapy pencil beam scanning (PBS) systems have limitations on the minimum range in patient that can be treated. The design limitation arises from practical considerations such as beam current intensity, layer spacing and delivery time. A range shifter (RS), which is a slab of stopping material located between the nozzle and the patient, is used to reduce the residual range of the incoming beam so that the treatment ranges can be extended to shallower depths. The treatment planning system (TPS) has to accurately calculate the beam spot size entering the patient, given the proton energy, for arbitrary positions and thicknesses of the RS inserted in the beam path. The commercial TPS investigated in this study implements a model of the beam widening due to the RS by incorporating the scattering properties of the RS material into the, so called, V parameter. This parameter is derived from a pair of measurements: with and without the RS, for a series of clinically relevant proton energies. The V parameter is then used to calculate the phase space parameters of the beam after traversing the RS. This work evaluates the RS model implemented in Eclipse, by comparing the spot sizes in air after traversing the RS located at different positions along the beam axis, versus simulations from Geant4 Monte Carlo as well as analytical calculations using the Fermi-Eyges theory with Highland approximation of multiple Coulomb scattering. Materials/Methods: Spot sizes in air around the isocenter were extracted from PBS plans calculated by Eclipse with the RS located at various positions with respect to the isocenter and selected proton energies. The spot sizes calculated by the TPS were compared with those calculated using Fermi-Eyges theory as well as with results from Monte Carlo simulations for the corresponding RS positions and proton energies.
Scientific Abstract 3794; Table
Results: For range shifter positions that are close to that used in the measurements during commissioning, the differences between the TPS calculations and both: Monte Carlo simulations as well as analytical calculations, were small throughout the entire range of clinically relevant proton energies. However, for RS positions that deviate significantly (15 cm or more) from the original measurements, the differences between the TPS calculated spot size and Monte Carlo simulations, and that from analytic calculations, were considerable. The errors were worst for lower energies with as much as 30% in the worst case. Conclusions: The RS model implemented in the TPS under investigation introduces considerable inaccuracies to proton plans. The inaccuracies are most prominent for lower clinically relevant proton energies and when the RS is positioned away from the location used in beam configuration commissioning. Author Disclosure: W. Matysiak: None. D. Yeung: None. R. Slopsema: None. Z. Li: None.
3794 A Dosimetric Planning Feasibility Study for Spot-Scanned Stereotactic Body Proton Therapy (SBPT) for Unresectable Pancreatic Tumors T.T. Sio,1 E.J. Tryggestad,1 J.B. Ashman,2 C.J. Beltran,1 W.S. Harmsen,3 K.A. Hoeft,1 S.K. Wurgler,1 and R.C. Miller1; 1Department of Radiation Oncology, Mayo Clinic, Rochester, MN, 2Department of Radiation Oncology, Mayo Clinic, Scottsdale, AZ, 3Department of Biomedical Statistics and Informatics, Mayo Clinic, Rochester, MN Purpose/Objective(s): Few definitive treatment options exist for locally advanced pancreatic malignancies. We explored the planning feasibility and dosimetric advantage of SBPT vs. intensity modulated radiation therapy (IMRT). Materials/Methods: Ten patients previously completed photon therapy were retrospectively selected. In this hypothetical SBPT protocol, the GTV included only gross tumor on CT imaging with no elective nodal treatment. An ITV was created and breath-hold treatment scenario was assumed. Multiple protons plans were created using a posterior 3-field technique. Two beam lines were investigated, one with a nominal spot size (1s) at relevant depths of 4 mm and the other with 6 mm; both single and multifield optimizations (SFO vs. MFO) were employed. For “robustness” considerations, proton planning was further stratified for multiple scanning target volumes, i.e., CTV+3, +5, and +7 mm isotropically. For comparison, 9-field IMRT plans were generated at CTV+5 mm. All plans were normalized to at least 98% of CTV covered by 40 Gy(E), in 5 fractions; a
Dosimetric comparison of IMRT versus SBPT in pancreatic tumor treatment planning
Nomenclature (Note) Integral Dose (Body), V5Gy [cc] Volume [cc] receiving 5 Gy or more CTV, D99.9% [Gy] 99.9% of volume receiving this dose [Gy] or more Stomach, V20Gy [cc] Volume [cc] receiving 20 Gy or more Small Bowel, V20Gy [cc] Volume [cc] receiving 20 Gy or more Spinal Cord, D0.01cc [Gy] 0.01 cc of volume receiving this dose [Gy] or more Duodenum outside CTV, V38Gy Duodenal volume outside CTV [cc] [cc] receiving 38 Gy or more Liver, DC1250cc [Gy] 1250 cc of volume receiving this dose [Gy] or less Left Kidney, DC150cc [Gy] 150 cc of volume receiving this dose [Gy] or less Right Kidney, DC150cc [Gy] 150 cc of volume receiving this dose [Gy] or less
IMRT (9-field) - Mean (SE)
Group 1: MFO/6-mm Spot - Mean (SE)
Group 2: MFO/4-mm Spot - Mean (SE)
Group 3: SFO/6-mm Spot - Mean (SE)
Group 4: SFO/4-mm Spot - Mean (SE)
P values (Proton)
3285 (471)
1777 (221)
1590 (221)
1723 (228)
1560 (218)
<.0001
39.8 (0.0)
39.8 (0.0)
39.7 (0.0)
39.7 (0.1)
39.6 (0.1)
.01
39.2 (18.0)
7.3 (3.7)
5.4 (3.1)
6.6 (3.6)
5.1 (3.0)
.008
9.2 (5.9)
7.1 (4.9)
5.8 (4.7)
7.0 (5.2)
5.8 (4.8)
.01
17.9 (0.9)
18.8 (1.6)
18.4 (1.6)
16.0 (1.5)
16.0 (1.5)
.0002
6.2 (2.2)
7.8 (2.2)
7.1 (2.1)
5.8 (1.8)
6.0 (1.7)
.0015
13.9 (3.4)
11.3 (3.4)
8.7 (2.7)
10.3 (3.1)
7.9 (2.5)
.0042
9.6 (0.9)
8.5 (1.0)
8.0 (1.0)
7.5 (1.1)
7.2 (1.2)
.0069
11.8 (0.8)
9.0 (1.4)
8.8 (1.4)
10.5 (1.8)
10.2 (1.8)
.0081
S918
International Journal of Radiation Oncology Biology Physics
minimum dose of at least 39.2 GyE was achieved in 95% of the plans. Multicomparative and 2-sample t-tests were used. Results: The median GTV and CTV volumes were 43.9 and 56.9 cc, respectively. With median minimum separation from GTV of 0.15 cm (range, 0.0-1.6 cm), the duodenum was the most difficult organ-at-risk (OAR) for meeting dosimetric constraints, especially in the CTV+7 mm cases. For CTV+5 mm planning, selected IMRT and SBPT dosimetric values are shown in Table; P-values were calculated across all 4 proton groups. The integral dose was significantly better for proton vs. IMRT plans (V5Gy, 1663 vs. 3285 cc, P Z 0.0064), as for multiple other OARs. While most proton plans were acceptable, 4-mm spot plans gave slightly less dose to the OAR structures owing to a smaller spot size, but the binary differences (vs. 6-mm spot) were not statistically significant in most cases. Conclusions: Benchmarked against IMRT planning, pancreatic SBPT offers comparable duodenal sparing with significant improvements in most OAR dosimetric metrics considered, which represents an attractive therapeutic option. Under static proton planning considerations, MFO vs. SFO differences were generally not statistically significant. As expected, smaller spot size trended for better OAR sparing. Further proton plan robustness evaluations are warranted to determine the optimal combination of spot size, optimization technique, and target volume for spot placement in spotscanned SBPT. Author Disclosure: T.T. Sio: None. E.J. Tryggestad: None. J.B. Ashman: None. C.J. Beltran: None. W.S. Harmsen: None. K.A. Hoeft: None. S.K. Wurgler: None. R.C. Miller: None.
standard DVH metrics, we introduce Area-Outside-Left (AOL) and Area-Outside-Right (AOR) to summarize deficiencies observed in the perturbed plans. AOL/AOR is proportional to the area of the aggregated 1s DVH span for a given type of plan perturbation falling left/right of the target 1s span for the corresponding base plan. A dose threshold was also incorporated into the calculation, below and above which Area-Outside does not contribute to AOL and AOR, respectively. Results: The table provides an abbreviated summary of our findings. In row 1 we compute equal-weighted summations of AOL and AOR over all perturbations (SAOL+SAOR). With no other considerations, SFO-PRM is the “best” option for CTV+5mm and CTV+7mm plans, the latter having the lowest observed score. CTV+3mm plans tend to under-dose the CTV, especially, e.g., perturbations SP+3% and z+3mm; MFO SP-3% plans for CTV+5mm and (especially) CTV+7mm produce significant hot spots in the CTV. Applying dose thresholds of 95%Rx and 105%Rx to the AOL/AOR calculation (row 2) improved sensitivity to these underdose-overdose issues. Conclusions: Considering duodenal dose-volumes irradiated by CTV+7mm plans, CTV+5mm-SFO-PRM may be the best compromise between robustness and OAR sparing. Further investigation of plan robustness to intra-breath-hold motion is warranted and is in progress. Author Disclosure: E.J. Tryggestad: None. T.T. Sio: None. J.B. Ashman: None. C.J. Beltran: None. T.J. Whitaker: None. K.A. Hoeft: None. S.K. Wurgler: None. R.C. Miller: None.
3795 Aggregated Plan Robustness Analyses for Spot-Scanned Pancreatic Stereotactic Body Proton Therapy E.J. Tryggestad,1 T.T. Sio,1 J.B. Ashman,2 C.J. Beltran,1 T.J. Whitaker,1 K.A. Hoeft,1 S.K. Wurgler,1 and R.C. Miller1; 1Mayo Clinic, Rochester, MN, 2Mayo Clinic, Phoenix, AZ Purpose/Objective(s): We have previously established the dosimetric feasibility of spot-scanned stereotactic body proton therapy (SBPT) for unresectable pancreatic cancer using IMRT as a benchmark. We expand our prior study by performing a proton plan robustness analysis using a novel population-based tool. Materials/Methods: 120 “base” plans were generated for 10 patients with locally advanced pancreatic cancer who previously completed conventional radiation therapy. Variables tested were single- vs. multifield optimization (SFO vs. MFO); 4 vs. 6 mm (s) spot size (VAC vs. PRM); and isotropic CTV margins of 3, 5 and 7 mm for scanning target volumes (STV Z CTV+3mm vs. CTV+5mm vs. CTV+7mm). A hypo-fractionated regimen of 40 GyE in 5 fractions; breath-holding for motion management; and 3 posterior fields to minimize bowel gas and motion variability were hypothetically assumed. Base plans were normalized such that CTV V100% Z 98%. Eight perturbed plans isocenter shifts of 3 mm in x, y, z; 3% systematic range (SP) uncertainties - were generated from each base plan. 1,080 proton plans were evaluated in total. Custom DVH aggregation tools were developed to compare planning results. In addition to evaluating Scientific Abstract 3795; Table
3796 Modeling the Dosimetric Effects of Repainting in RespiratoryGated Spot Scanning Proton Treatment Plans J.E. Johnson, C. Beltran, M.G. Herman, and J.J. Kruse; Mayo Clinic, Rochester, MN Purpose/Objective(s): Interplay between target motion and spot scanning proton delivery can lead to significant degradation in plan quality. The combination of repainting and respiratory gating was investigated as a strategy to mitigate these effects. Materials/Methods: An analytic routine modeled three-dimensional dose distributions of pencil-beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15-cm deep, 3-cm radius spherical CTV with a 1-cm isotropic ITV expansion. The interplay effect was studied in subsequent calculations by modeling proton delivery from a clinical synchrotron based spot scanning system and respiratory target motion, patterned from surrogate breathing traces from clinical 4DCT scans and normalized to nominal 1 cm amplitude. Motion both parallel and orthogonal to the raster direction of the beam was investigated. Repainting was modeled by setting a maximum MU to be delivered in each visit of the beam to a spot. The beam rastered over all prescribed spots in an energy layer, either delivering the prescribed MU or the maximum allowed MU. The beam rescanned over any spots in that layer with remaining MU until the prescribed MU was reached for all spots; then the system switched to the next energy
Abbreviated summary of robustness analysis for pancreatic SBPT planning study (N_Pts Z 10) STV Z CTV+3mm
Structure CTV
Duodenum
DVH Metric
Perturbation
SAOL+SAOR Norm’zd. SAOL(D<95%)+SAOR(D>105%) Norm’zd. Mean D D Min. in cGy (1s) Mean D D Min. in cGy (1s) Mean D D Max. in cGy (1s) Mean V 20 Gy in cc (1s) Mean V 38 Gy in cc (1s)
all comb’d. all comb’d. S.P. + 3% z + 3 mm S.P. - 3% base base
SFO-PRM
STV Z CTV+5mm
STV Z CTV+7mm
SFO-PRM
MFO-VAC
MFO-PRM
SFO-PRM
MFO-VAC
MFO-PRM
1.31 14.5
1.07 3.67
1.18 3.83
1.26 5.71
1.00 1.00
1.09 7.58
1.12 6.67
-634 (196) -327 (144) -48.7 (19.7) 24.3 (17.2) 7.40 (10.5)
-187 (155) -111 (122) -56.9 (20.4) 28.6 (19.3) 9.76 (12.4)
-131 (130) -51.2 (65.2) 92.2 (116) 25.6 (18.3) 11.1 (13.2)
-181 (108) -49.1 (75.4) 126 (158) 28.5 (19.2) 11.8 (13.5)
-3.66 (64.8) -25.4 (32.2) -54.3 (34.4) 34.8 (21.8) 14.6 (14.1)
-76.1 (89.0) -15.9 (20.0) 136 (257) 30.3 (20.4) 15.3 (14.6)
-85.2 (77.5) -30.7 (41.3) 132 (201) 33.1 (20.7) 15.1 (14.7)