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What is the Setup Correction Threshold for Image Guided Head and Neck Radiotherapy?
I. J. Radiation Oncology d Biology d Physics
S586
2901
Volume 75, Number 3, Supplement, 2009
Improved Fixation and Position Verification for Salivary Gland Sparing Head and Neck IMRT
E. van der Wal, S. van der Meer, A. C. Houweling, C. H. J. Terhaard, C. P. J. Raaijmakers University Medical Center Utrecht, Utrecht, Netherlands Purpose/Objective(s): Parotid gland sparing IMRT in head and neck radiotherapy is feasible and reduces xerostomia. Sparing of the contralateral submandibular gland is now investigated as a further improvement for the treatment of oropharyngeal tumors. To ensure optimal tumor coverage and gland sparing, adequate fixation and position verification becomes more and more important. In this study we investigate the use of a patient specific thermoplast headrest for extra immobilization in combination with kV cone-beam CT for position verification. Further, our off-line position correction protocol is evaluated and optimized to further reduce systematic setup errors. Materials/Methods: Twenty head and neck cancer patients were immobilized with either standard (std, Posifix supine) or individual headrests (ind, both Civco, Kalona USA) in combination with an individual five-point thermoplastic mask. Cone-beam CTs (XVI, Elekta Stockholm Sweden) were made before and after the first five fractions and weekly thereafter. Position correction was done according to a NAL protocol. Positioning data were gathered and analyzed for random and systematic errors for several setup correction protocols, and for time trends. Setup data for standard and individual headrests were tested non-parametrically for differences in variance using a Wilcoxon rank-sum test (p\0.05). Margins were calculated according to van Herk. Finally, setup data for the patients with a standard headrest were compared to historical EPI positioning data. Results: Using an individual headrest, intrafraction motion was reduced significantly in the lateral direction (std: 0.97 mm, ind: 0.46 mm). Variance in the uncorrected interfraction setup was reduced both in the lateral (std: 1.13 mm, ind: 0.64 mm) and vertical direction (std: 1.42 mm, ind: 0.86 mm). Our current NAL protocol effectively halved the systematic interfraction setup error. Minimal isotropic margins for a standard and individual headrest would be 3.2 mm en 2.8 mm, respectively. No population averaged time trends were observed. Further, considerable non-rigid local deformations within the head-and-neck area are observed. A CBCT setup procedure did not result in markedly decreased random setup errors when compared to an EPI setup. Conclusions: An individual headrest reduces setup errors significantly in the LR and AP directions. Intrafraction motion in the LR direction is reduced. Further research needs to be conducted to determine local margins for the target and the elective lymph nodes, depending on their location, and the impact of these deformations on the dose to the salivary glands. Concluding, from this study we see that individual head support in combination with CBCT improves reproducibility of patient setup and fixation during treatment. Author Disclosure: E. van der Wal, None; S. van der Meer, None; A.C. Houweling, None; C.H.J. Terhaard, None; C.P.J. Raaijmakers, None.
2902
What is the Setup Correction Threshold for Image Guided Head and Neck Radiotherapy?
J. L. Wloch, M. Matuszak, Q. Wu, D. Yan William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): Despite the use of advanced immobilization devices in head and neck (HN) radiotherapy, setup corrections are not straightforward. A rigid body correction based on registration of the bony anatomy near the GTV alone may induce additional errors in other regions of the treated volume. The purpose of this work is to present an analysis of setup errors and to suggest correction thresholds for cone beam CT (CBCT) guided HN-IMRT treatment. Materials/Methods: Total of 1500 Fx of CBCT images was acquired from 57 patients undergoing HN IMRT. Under an IRB approved study, CBCTs were retrospectively registered to treatment planning CTs using 4 masking techniques. Separate bony affine registrations were performed using imaging regions that represented only the head (top of CBCT to bottom of C1), neck (C2 to C6), or shoulder (top of C7 to bottom of CBCT) regions. The displacements were compared to those acquired using our clinical baseline mask of the GTV area (GTV + spine behind GTV) to determine the residual errors that would be seen with zero threshold corrections. Displacement data of all 4 registration areas and residual displacements (DRL, DSI, DAP) with respect to the baseline correction were analyzed. Correlations on rotations about the 3 axes (qRL, qSI, qAP) to DRL, DSI, and DAP were also analyzed. Results: The means of the average displacements for the 57 patients for the baseline registration in the RL, SI, and AP directions were -0.01, -0.01, and -0.03 cm, respectively. The systematic errors were 0.12, 0.17, and 0.12 cm. The RMS of the standard deviations (RMSsi) was 0.18, 0.20, and 0.20 cm, in the RL, SI, and AP directions. Analysis of RMSsi in the head, neck, and shoulder regions showed the largest variability in the shoulder region (RMSsi = 0.31, 0.26, 0.32 cm in the RL, SI, and AP directions). After using the baseline registration for position correction, residual displacements were larger than the original displacement in 19 - 36 % of fractions, depending on the region and direction. The largest residual RMSsI values were 0.18 cm in the AP direction for the head region, and 0.20, 0.21, and 0.27 cm in the RL, SI, and AP directions for the shoulder region. qRL, qSI, qAP were most correlated with DAPhead, DRLshoulder and both DRLhead and DRLshoulder, respectively. Conclusions: Making positional corrections based on registration of the GTV + spine area of a HN CBCT to the reference CT can induce errors in the head, neck, and shoulder regions. In an analysis of 1500 Fx, 19-35 % of Fx has larger post-correction displacements than pre-correction displacements, depending on the region and direction. Correction thresholds of 0.2, 0.2, and 0.3 cm in the RL, SI, and AP directions are suggested to minimize residual displacement errors. AP rotational errors may be minimized by adding a setup mark in the head and/or shoulder regions. Author Disclosure: J.L. Wloch, None; M. Matuszak, None; Q. Wu, None; D. Yan, None.
2903
Characterization of Potential Radiobiological Benefit from Design of Patient-specific Margins for Prostate Conformal Treatment using a Real Time Tracking System
P. Rassiah1, B. Wang1, M. Szegedi1,2, H. Zhao1, Y. Huang1, J. D. Tward1, D. C. Shrieve1, B. J. Salter1 1
University of Utah, Salt Lake City, UT, 2University of Texas HSC, San Antonio, TX
Purpose/Objective(s): To investigate the potential radiobiological benefits of designing patient-specific margins for prostate external beam radiation treatment based on an observation of prostate motion during the initial few treatments using the Calypso 4D Localization System.
S586
2901
Volume 75, Number 3, Supplement, 2009
Improved Fixation and Position Verification for Salivary Gland Sparing Head and Neck IMRT
E. van der Wal, S. van der Meer, A. C. Houweling, C. H. J. Terhaard, C. P. J. Raaijmakers University Medical Center Utrecht, Utrecht, Netherlands Purpose/Objective(s): Parotid gland sparing IMRT in head and neck radiotherapy is feasible and reduces xerostomia. Sparing of the contralateral submandibular gland is now investigated as a further improvement for the treatment of oropharyngeal tumors. To ensure optimal tumor coverage and gland sparing, adequate fixation and position verification becomes more and more important. In this study we investigate the use of a patient specific thermoplast headrest for extra immobilization in combination with kV cone-beam CT for position verification. Further, our off-line position correction protocol is evaluated and optimized to further reduce systematic setup errors. Materials/Methods: Twenty head and neck cancer patients were immobilized with either standard (std, Posifix supine) or individual headrests (ind, both Civco, Kalona USA) in combination with an individual five-point thermoplastic mask. Cone-beam CTs (XVI, Elekta Stockholm Sweden) were made before and after the first five fractions and weekly thereafter. Position correction was done according to a NAL protocol. Positioning data were gathered and analyzed for random and systematic errors for several setup correction protocols, and for time trends. Setup data for standard and individual headrests were tested non-parametrically for differences in variance using a Wilcoxon rank-sum test (p\0.05). Margins were calculated according to van Herk. Finally, setup data for the patients with a standard headrest were compared to historical EPI positioning data. Results: Using an individual headrest, intrafraction motion was reduced significantly in the lateral direction (std: 0.97 mm, ind: 0.46 mm). Variance in the uncorrected interfraction setup was reduced both in the lateral (std: 1.13 mm, ind: 0.64 mm) and vertical direction (std: 1.42 mm, ind: 0.86 mm). Our current NAL protocol effectively halved the systematic interfraction setup error. Minimal isotropic margins for a standard and individual headrest would be 3.2 mm en 2.8 mm, respectively. No population averaged time trends were observed. Further, considerable non-rigid local deformations within the head-and-neck area are observed. A CBCT setup procedure did not result in markedly decreased random setup errors when compared to an EPI setup. Conclusions: An individual headrest reduces setup errors significantly in the LR and AP directions. Intrafraction motion in the LR direction is reduced. Further research needs to be conducted to determine local margins for the target and the elective lymph nodes, depending on their location, and the impact of these deformations on the dose to the salivary glands. Concluding, from this study we see that individual head support in combination with CBCT improves reproducibility of patient setup and fixation during treatment. Author Disclosure: E. van der Wal, None; S. van der Meer, None; A.C. Houweling, None; C.H.J. Terhaard, None; C.P.J. Raaijmakers, None.
2902
What is the Setup Correction Threshold for Image Guided Head and Neck Radiotherapy?
J. L. Wloch, M. Matuszak, Q. Wu, D. Yan William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): Despite the use of advanced immobilization devices in head and neck (HN) radiotherapy, setup corrections are not straightforward. A rigid body correction based on registration of the bony anatomy near the GTV alone may induce additional errors in other regions of the treated volume. The purpose of this work is to present an analysis of setup errors and to suggest correction thresholds for cone beam CT (CBCT) guided HN-IMRT treatment. Materials/Methods: Total of 1500 Fx of CBCT images was acquired from 57 patients undergoing HN IMRT. Under an IRB approved study, CBCTs were retrospectively registered to treatment planning CTs using 4 masking techniques. Separate bony affine registrations were performed using imaging regions that represented only the head (top of CBCT to bottom of C1), neck (C2 to C6), or shoulder (top of C7 to bottom of CBCT) regions. The displacements were compared to those acquired using our clinical baseline mask of the GTV area (GTV + spine behind GTV) to determine the residual errors that would be seen with zero threshold corrections. Displacement data of all 4 registration areas and residual displacements (DRL, DSI, DAP) with respect to the baseline correction were analyzed. Correlations on rotations about the 3 axes (qRL, qSI, qAP) to DRL, DSI, and DAP were also analyzed. Results: The means of the average displacements for the 57 patients for the baseline registration in the RL, SI, and AP directions were -0.01, -0.01, and -0.03 cm, respectively. The systematic errors were 0.12, 0.17, and 0.12 cm. The RMS of the standard deviations (RMSsi) was 0.18, 0.20, and 0.20 cm, in the RL, SI, and AP directions. Analysis of RMSsi in the head, neck, and shoulder regions showed the largest variability in the shoulder region (RMSsi = 0.31, 0.26, 0.32 cm in the RL, SI, and AP directions). After using the baseline registration for position correction, residual displacements were larger than the original displacement in 19 - 36 % of fractions, depending on the region and direction. The largest residual RMSsI values were 0.18 cm in the AP direction for the head region, and 0.20, 0.21, and 0.27 cm in the RL, SI, and AP directions for the shoulder region. qRL, qSI, qAP were most correlated with DAPhead, DRLshoulder and both DRLhead and DRLshoulder, respectively. Conclusions: Making positional corrections based on registration of the GTV + spine area of a HN CBCT to the reference CT can induce errors in the head, neck, and shoulder regions. In an analysis of 1500 Fx, 19-35 % of Fx has larger post-correction displacements than pre-correction displacements, depending on the region and direction. Correction thresholds of 0.2, 0.2, and 0.3 cm in the RL, SI, and AP directions are suggested to minimize residual displacement errors. AP rotational errors may be minimized by adding a setup mark in the head and/or shoulder regions. Author Disclosure: J.L. Wloch, None; M. Matuszak, None; Q. Wu, None; D. Yan, None.
2903
Characterization of Potential Radiobiological Benefit from Design of Patient-specific Margins for Prostate Conformal Treatment using a Real Time Tracking System
P. Rassiah1, B. Wang1, M. Szegedi1,2, H. Zhao1, Y. Huang1, J. D. Tward1, D. C. Shrieve1, B. J. Salter1 1
University of Utah, Salt Lake City, UT, 2University of Texas HSC, San Antonio, TX
Purpose/Objective(s): To investigate the potential radiobiological benefits of designing patient-specific margins for prostate external beam radiation treatment based on an observation of prostate motion during the initial few treatments using the Calypso 4D Localization System.