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Impact of Different Image-guided Protocols on Geometric Accuracy of Radiotherapy for Locally-advanced Lung Cancer
I. J. Radiation Oncology d Biology d Physics
S98
Volume 75, Number 3, Supplement, 2009
image quality is sufficient, the in-treatment image can be used for re-calculation the delivered dose distribution. However, increased scatter from the MV beam and nonuniformly spaced projections may degrade Hounsfield unit (HU) accuracy of the image. This study assesses the quality of CBCT acquired during VMAT, and potential improvements using a postprocessing correction algorithm. Materials/Methods: The CBCT projection images were acquired simultaneously with delivery of a VMAT prostate treatment (8 MV, 46 control points, 517 MU per fraction) to the Rando phantom pelvis using Elekta Synergy. A standard CBCT pelvis image was also acquired using the same imaging settings (120 kV, 40 mA, 16 ms per frame). The CBCT image quality was compared to a high-quality reference CT scan using regions of interest placed throughout the phantom to measure noise, nonuniformity (NU), contrast to noise ratio (CNR), and HU accuracy (DHUmax). A postprocessing algorithm to reduce low frequency shading artifacts (Marchant et al., Physics in Medicine and Biology 2008;53:5719) was applied to the CBCT images and its effect on image quality evaluated. Results: The CBCT image acquired during VMAT delivery (CBCT-V) was reconstructed from 750 projections, while the standard CBCT (CBCT-S) had 650 projections. The CBCT-V had the highest noise level (s = 28), followed by CBCT-S (s = 21) and reference CT (s = 9). Shading correction had little effect on noise levels. Uniformity was much poorer in CBCT-S (NU = 18%) and CBCT-V (NU = 13%) than the reference CT (NU = 1%). Shading correction greatly improved uniformity of both CBCTs so that it was similar to the reference CT. The CNR was poorest for CBCT-V (CNR = 14), compared to CBCT-S (CNR = 23) and reference CT (CNR = 50). Shading correction had little effect on CNR. The HU accuracy was poor for both CBCT-S (DHUmax = 22%) and CBCT-V (DHUmax = 14%). After shading correction, both CBCT images had much improved HU accuracy (DHUmax = 1%). Streak artifacts were more prominent in CBCT-V than CBCT-S, probably due to uneven spacing of projection angles. Conclusions: The CBCT images acquired during VMAT delivery have lower quality than standard CBCT. The drop in image quality was characterized by increased noise, decreased CNR, and increased streak artifacts. Uniformity and HU accuracy were unexpectedly improved slightly in the presence of additional MV scatter, although were still much worse than reference CT. A shading correction algorithm for CBCT improved both uniformity and HU accuracy, making the images more suitable for accurate assessment of delivered dose. Author Disclosure: T. Marchant, None; C. Rowbottom, None; C. Moore, None.
208
Impact of Different Image-guided Protocols on Geometric Accuracy of Radiotherapy for Locally-advanced Lung Cancer
J. A. Higgins1,2, A. Bezjak1,2, A. Hope1,2, T. Panzarella3, W. Li1, T. Craig1,2, A. Brade1,2, A. Sun1,2, J. Cho1,2, J. Bissonnette1,2 Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada, 2University of Toronto, Toronto, ON, Canada, 3Department of Biostatistics, Toronto, ON, Canada
1
Purpose/Objective(s): Daily volumetric image-guidance (IG) improves geometric accuracy for lung cancer radiotherapy but can be perceived as costly with regards to equipment and time. The purpose of this study was to evaluate different IG protocol frequencies, using cone-beam CT (CBCT), and assess their impact on geometric accuracy with respect to daily IG. Materials/Methods: Daily CBCT datasets (4,237) for 100 patients were retrospectively registered to planning scans using automatic bone registration. Subsequently, four hypothetical IG protocols were simulated by sampling this daily positional data: (1) no IG; (2) IG only on the first 5 fractions with a mean systematic shift calculated and applied subsequently; (3) weekly IG; and, (4) alternate day IG. P Frequency of setup error occurrence and magnitude of setup error using systematic ( ) and random (s) deviations were determined. Statistical differences in mean setup error were also calculated between the hypothetical protocols and compared to daily IG, using a t P test from a mixed effects (regression) model. Lastly, setup margins were calculated (2.5 + 0.7s) for each protocol. Results: No imaging resulted in 71% and 35% of all fractions incurring geometric errors above ± 3 mmP and ± 5 mm, respectively; with daily IG, these proportions were reduced systematic error ( ) occurred as image freP to 27% and 6%.POverall, reductions inP quency increased from no IG to daily IG: = 1.5–0.9 mm, = 2.2–0.7 mm and = 1.6–1.0 mm in the medio-lateral (ML), P cranio-caudal (CC) and Panterior-posterior (AP) directions, respectively. First 5 days IG ( = 1.8–2.6 mm) though, proved to be worse than no IG ( = 1.5–2.2 mm). Modest fluctuations in random error (s) occurred between hypothetical protocols, but a substantial decrease was seen from no IG to daily IG: s = 2.9–1.7 mm, s = 3.7–2.0 mm and s = 2.5–1.7 mm in the ML, CC, and AP directions, respectively. Statistically significant differences in mean setup error between daily IG and first 5 fractions IG were present in the CC (p \ 0.0001) and AP (p \ 0.0001) planes but not the ML (p = 0.17) plane. Population-based setup margins were 6, 8, 6 mm (no IG), 7, 9, 6 mm (first 5 days IG), 5, 7, 5 mm (weekly IG), 4, 5, 5 mm (alternate IG) and 3, 3, 4 mm (daily IG) in the ML, CC, and AP directions, respectively. Conclusions: No IG and first 5 fractions IG yield unfavorable levels of geometric uncertainty in comparison to weekly, alternate, and daily IG. Weekly and alternate IG can be considered when PTV margins of 5–7 mm are used. Overall, daily CBCT substantially reduces setup error compared to all other IG protocols and allows PTV setup margin reduction to go as low as 3–4 mm, leading to greater potential of reductions in lung complications without compromising target coverage. Author Disclosure: J.A. Higgins, None; A. Bezjak, None; A. Hope, None; T. Panzarella, None; W. Li, None; T. Craig, None; A. Brade, None; A. Sun, None; J. Cho, None; J. Bissonnette, None.
209
Incorporation of Prior Volumetric Image Information into Cone-beam CT (CBCT) Reconstruction: A Novel Strategy of Imaging Dose Reduction for Daily Patient Setup and Adaptive Radiation Therapy
J. Wang, L. Xing Stanford University School of Medicine, Stanford, CA Purpose/Objective(s): Patient imaging dose is a major limiting factor for frequent use of volumetric CBCT imaging in clinical patient setup and future adaptive therapy applications. The purpose of this work is to develop an effective dose reduction strategy by effectively utilizing the image information from previously acquired simulation CT/CBCT during the on-treatment CBCT reconstruction process. Materials/Methods: The on-treatment CBCT is acquired with a protocol of an order of magnitude lower mAs as compared with current practice. The proposed algorithm extracts information from the previously acquired simulation CT or CBCT through
S98
Volume 75, Number 3, Supplement, 2009
image quality is sufficient, the in-treatment image can be used for re-calculation the delivered dose distribution. However, increased scatter from the MV beam and nonuniformly spaced projections may degrade Hounsfield unit (HU) accuracy of the image. This study assesses the quality of CBCT acquired during VMAT, and potential improvements using a postprocessing correction algorithm. Materials/Methods: The CBCT projection images were acquired simultaneously with delivery of a VMAT prostate treatment (8 MV, 46 control points, 517 MU per fraction) to the Rando phantom pelvis using Elekta Synergy. A standard CBCT pelvis image was also acquired using the same imaging settings (120 kV, 40 mA, 16 ms per frame). The CBCT image quality was compared to a high-quality reference CT scan using regions of interest placed throughout the phantom to measure noise, nonuniformity (NU), contrast to noise ratio (CNR), and HU accuracy (DHUmax). A postprocessing algorithm to reduce low frequency shading artifacts (Marchant et al., Physics in Medicine and Biology 2008;53:5719) was applied to the CBCT images and its effect on image quality evaluated. Results: The CBCT image acquired during VMAT delivery (CBCT-V) was reconstructed from 750 projections, while the standard CBCT (CBCT-S) had 650 projections. The CBCT-V had the highest noise level (s = 28), followed by CBCT-S (s = 21) and reference CT (s = 9). Shading correction had little effect on noise levels. Uniformity was much poorer in CBCT-S (NU = 18%) and CBCT-V (NU = 13%) than the reference CT (NU = 1%). Shading correction greatly improved uniformity of both CBCTs so that it was similar to the reference CT. The CNR was poorest for CBCT-V (CNR = 14), compared to CBCT-S (CNR = 23) and reference CT (CNR = 50). Shading correction had little effect on CNR. The HU accuracy was poor for both CBCT-S (DHUmax = 22%) and CBCT-V (DHUmax = 14%). After shading correction, both CBCT images had much improved HU accuracy (DHUmax = 1%). Streak artifacts were more prominent in CBCT-V than CBCT-S, probably due to uneven spacing of projection angles. Conclusions: The CBCT images acquired during VMAT delivery have lower quality than standard CBCT. The drop in image quality was characterized by increased noise, decreased CNR, and increased streak artifacts. Uniformity and HU accuracy were unexpectedly improved slightly in the presence of additional MV scatter, although were still much worse than reference CT. A shading correction algorithm for CBCT improved both uniformity and HU accuracy, making the images more suitable for accurate assessment of delivered dose. Author Disclosure: T. Marchant, None; C. Rowbottom, None; C. Moore, None.
208
Impact of Different Image-guided Protocols on Geometric Accuracy of Radiotherapy for Locally-advanced Lung Cancer
J. A. Higgins1,2, A. Bezjak1,2, A. Hope1,2, T. Panzarella3, W. Li1, T. Craig1,2, A. Brade1,2, A. Sun1,2, J. Cho1,2, J. Bissonnette1,2 Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada, 2University of Toronto, Toronto, ON, Canada, 3Department of Biostatistics, Toronto, ON, Canada
1
Purpose/Objective(s): Daily volumetric image-guidance (IG) improves geometric accuracy for lung cancer radiotherapy but can be perceived as costly with regards to equipment and time. The purpose of this study was to evaluate different IG protocol frequencies, using cone-beam CT (CBCT), and assess their impact on geometric accuracy with respect to daily IG. Materials/Methods: Daily CBCT datasets (4,237) for 100 patients were retrospectively registered to planning scans using automatic bone registration. Subsequently, four hypothetical IG protocols were simulated by sampling this daily positional data: (1) no IG; (2) IG only on the first 5 fractions with a mean systematic shift calculated and applied subsequently; (3) weekly IG; and, (4) alternate day IG. P Frequency of setup error occurrence and magnitude of setup error using systematic ( ) and random (s) deviations were determined. Statistical differences in mean setup error were also calculated between the hypothetical protocols and compared to daily IG, using a t P test from a mixed effects (regression) model. Lastly, setup margins were calculated (2.5 + 0.7s) for each protocol. Results: No imaging resulted in 71% and 35% of all fractions incurring geometric errors above ± 3 mmP and ± 5 mm, respectively; with daily IG, these proportions were reduced systematic error ( ) occurred as image freP to 27% and 6%.POverall, reductions inP quency increased from no IG to daily IG: = 1.5–0.9 mm, = 2.2–0.7 mm and = 1.6–1.0 mm in the medio-lateral (ML), P cranio-caudal (CC) and Panterior-posterior (AP) directions, respectively. First 5 days IG ( = 1.8–2.6 mm) though, proved to be worse than no IG ( = 1.5–2.2 mm). Modest fluctuations in random error (s) occurred between hypothetical protocols, but a substantial decrease was seen from no IG to daily IG: s = 2.9–1.7 mm, s = 3.7–2.0 mm and s = 2.5–1.7 mm in the ML, CC, and AP directions, respectively. Statistically significant differences in mean setup error between daily IG and first 5 fractions IG were present in the CC (p \ 0.0001) and AP (p \ 0.0001) planes but not the ML (p = 0.17) plane. Population-based setup margins were 6, 8, 6 mm (no IG), 7, 9, 6 mm (first 5 days IG), 5, 7, 5 mm (weekly IG), 4, 5, 5 mm (alternate IG) and 3, 3, 4 mm (daily IG) in the ML, CC, and AP directions, respectively. Conclusions: No IG and first 5 fractions IG yield unfavorable levels of geometric uncertainty in comparison to weekly, alternate, and daily IG. Weekly and alternate IG can be considered when PTV margins of 5–7 mm are used. Overall, daily CBCT substantially reduces setup error compared to all other IG protocols and allows PTV setup margin reduction to go as low as 3–4 mm, leading to greater potential of reductions in lung complications without compromising target coverage. Author Disclosure: J.A. Higgins, None; A. Bezjak, None; A. Hope, None; T. Panzarella, None; W. Li, None; T. Craig, None; A. Brade, None; A. Sun, None; J. Cho, None; J. Bissonnette, None.
209
Incorporation of Prior Volumetric Image Information into Cone-beam CT (CBCT) Reconstruction: A Novel Strategy of Imaging Dose Reduction for Daily Patient Setup and Adaptive Radiation Therapy
J. Wang, L. Xing Stanford University School of Medicine, Stanford, CA Purpose/Objective(s): Patient imaging dose is a major limiting factor for frequent use of volumetric CBCT imaging in clinical patient setup and future adaptive therapy applications. The purpose of this work is to develop an effective dose reduction strategy by effectively utilizing the image information from previously acquired simulation CT/CBCT during the on-treatment CBCT reconstruction process. Materials/Methods: The on-treatment CBCT is acquired with a protocol of an order of magnitude lower mAs as compared with current practice. The proposed algorithm extracts information from the previously acquired simulation CT or CBCT through