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Variability in Liver Shape and Impact on GTV Position During Liver Stereotactic Radiotherapy With Abdominal Compression
I. J. Radiation Oncology d Biology d Physics
S186
1088
Volume 69, Number 3, Supplement, 2007
Variability in Liver Shape and Impact on GTV Position During Liver Stereotactic Radiotherapy With Abdominal Compression
C. L. Eccles1, K. K. Brock1, J. Moseley2, D. Moseley1, D. Jaffray1, L. A. Dawson1 1
Princess Margaret Hospital, University of Toronto, Radiation Medicine Program, Toronto, ON, Canada, 2Radiation Medicine Program, Princess Margaret Hospital, University Health Network, Toronto, ON, Canada Background: For patients receiving liver stereotactic body radiotherapy (SBRT), abdominal compression can reduce organ motion and daily image guidance using liver-lo-liver matching can reduce setup error. Purpose/Objective(s): To measure residual variability in liver shape in patients treated with SBRT using abdominal compression, and its impact on GTV position following best fit rigid liver-to-liver registration of kV cone beam CTs (CBCTs) to planning CTs. Materials/Methods: Six patients with liver cancer treated with abdominal compression on a 6-fraction image-guided SBRT study were investigated. kVCBCTs were acquired in the treatment position and reconstructed offline, generating exhale respiratory sorted CBCTs. Manual rigid liver-to-liver registrations were performed from liver CBCT reconstructions to planning CTs (exhale). Each CBCT liver was contoured, and exported for evaluation of spatial differences from CBCT to planning CT using in-house developed finite element model based deformable registration (MOFEUS). The impact of liver deformations on the position of 10 GTVs in the right caudate (2), right post. (2), right inf. (1), right ant. (1) and mid right (4) liver lobes was estimated. As GTVs were not visible on CBCT, a centre of mass (COM) displacement was predicted based on liver deformations to estimate changes in GTV position due to liver deformation. Results: For 23 CBCTs (1–6/patient), regional liver deformations were measured. The average absolute residual deformation of 95% liver volume was 3.9, 4.9 and 3.4 mm in the RL, AP and SI directions (Table 1). One patient (6) had large deformations under the compression plate, with up to .10 mm change in 39% of the liver volume. For all patients deformations were greatest in the left lobe, and appeared to be related to variations in stomach filling and compression plate positioning. In 13/23 CBCTs (56%), the left lobe deformed by 6.4 mm (range 3.2–11.5) under the compression plate in the sup-posterior (11) or anterior (2) direction. 14/23 (74%) of all livers had \5.1 mm deformation in all directions. For all GTVs, excluding those in patient 6, the average absolute predicted GTV displacements were 0.5, 0.7 and 0.7 mm in RL, AP and SI directions. The changes in GTV in the outlier with the largest liver deformations (pt 6) were 4.9, 6.7 and 1.7 mm in the RL, AP and SI directions. Conclusions: Liver deformations using abdominal compression were small in most patients (\5.1 mm in 74% of CBCTs). However, one patient had substantial deformation under the compression plate resulting in clinically significant changes in GTV position. Methods to ensure reproducibility of abdominal compression are warranted. Table 1: Mean of Abs. 95% Volume Displacement for Liver, and ML, AP, and CC Components
Patient
ML mm
AP mm
CC mm
VM mm
1 2 3 4 5 6 All Repro.
3.3 2.6 2.4 3.0 1.5 10.5 3.9 1.9
5.1 3.1 2.9 3.0 2.1 12.9 4.9 2.6
2.4 3.7 2.9 3.5 2.1 5.6 3.4 1.7
5.7 5.4 4.0 5.0 3.1 15.6 6.5 3.4
Author Disclosure: C.L. Eccles, None; K.K. Brock, Varian Medical Systems, B. Research Grant; J. Moseley, None; D. Moseley, None; D. Jaffray, Elekta, B. Research Grant; L.A. Dawson, Elekta, B. Research Grant.
1089
Dosimetric Comparison of Various Online Adaptive Prostate Cancer Treatment Techniques
D. Z. Schulze, T. Zhang, J. Liang, D. Yan William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): The purpose of this study was to evaluate the benefits of different planning and online adaptive treatment techniques for prostate cancer. Materials/Methods: In this study, 3D conformal radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) treatment plans were generated for 9 prostate cancer patients, prescribing 91 Gy to the PTV. The CTV included prostate and seminal vesicles, organs-at-risk (OARs) included bladder and rectum, CTV-to-PTV margins of 0 cm and 1 cm were used, and IMRT plans used either OAR volumes or walls as inverse-planning objectives. Treatment methods included 3D-CRT and IMRT aligned to bony landmarks using 1 cm margins. Three online adaptive treatment methods used 0 cm margins: daily re-optimized 3D-CRT by aperture change, IMRT with rigid body target registration, and daily re-optimized IMRT. Using contours from daily CTs, doses were accumulated over 10 treatment fractions with finite element-based dose mapping techniques. Organ motion relative to the planning CT was evaluated using the Dice Similarity Coefficient (DSC). Results: Dose to normal tissue from several planning and treatment methods was evaluated. The reference plan utilized IMRT with OAR volume constraints, a 1 cm motion margin, and daily alignment to bony landmarks. Table 1 shows the planned dose, for which the IMRT zero-margin, OAR-wall plan achieved the greatest OAR reduction. Among treatment methods, close-to-zero slopes in accumulated CTV EUD vs. DSC (Fig. 1) indicates that these treatments are robust to interfraction motion by maintaining target dose. IMRT re-optimization with OAR walls showed the greatest OAR dose reduction compared to the accumulated reference plan, 14.5% with rectum and 25% with bladder, while remaining within 2.3% of the target prescription.
S186
1088
Volume 69, Number 3, Supplement, 2007
Variability in Liver Shape and Impact on GTV Position During Liver Stereotactic Radiotherapy With Abdominal Compression
C. L. Eccles1, K. K. Brock1, J. Moseley2, D. Moseley1, D. Jaffray1, L. A. Dawson1 1
Princess Margaret Hospital, University of Toronto, Radiation Medicine Program, Toronto, ON, Canada, 2Radiation Medicine Program, Princess Margaret Hospital, University Health Network, Toronto, ON, Canada Background: For patients receiving liver stereotactic body radiotherapy (SBRT), abdominal compression can reduce organ motion and daily image guidance using liver-lo-liver matching can reduce setup error. Purpose/Objective(s): To measure residual variability in liver shape in patients treated with SBRT using abdominal compression, and its impact on GTV position following best fit rigid liver-to-liver registration of kV cone beam CTs (CBCTs) to planning CTs. Materials/Methods: Six patients with liver cancer treated with abdominal compression on a 6-fraction image-guided SBRT study were investigated. kVCBCTs were acquired in the treatment position and reconstructed offline, generating exhale respiratory sorted CBCTs. Manual rigid liver-to-liver registrations were performed from liver CBCT reconstructions to planning CTs (exhale). Each CBCT liver was contoured, and exported for evaluation of spatial differences from CBCT to planning CT using in-house developed finite element model based deformable registration (MOFEUS). The impact of liver deformations on the position of 10 GTVs in the right caudate (2), right post. (2), right inf. (1), right ant. (1) and mid right (4) liver lobes was estimated. As GTVs were not visible on CBCT, a centre of mass (COM) displacement was predicted based on liver deformations to estimate changes in GTV position due to liver deformation. Results: For 23 CBCTs (1–6/patient), regional liver deformations were measured. The average absolute residual deformation of 95% liver volume was 3.9, 4.9 and 3.4 mm in the RL, AP and SI directions (Table 1). One patient (6) had large deformations under the compression plate, with up to .10 mm change in 39% of the liver volume. For all patients deformations were greatest in the left lobe, and appeared to be related to variations in stomach filling and compression plate positioning. In 13/23 CBCTs (56%), the left lobe deformed by 6.4 mm (range 3.2–11.5) under the compression plate in the sup-posterior (11) or anterior (2) direction. 14/23 (74%) of all livers had \5.1 mm deformation in all directions. For all GTVs, excluding those in patient 6, the average absolute predicted GTV displacements were 0.5, 0.7 and 0.7 mm in RL, AP and SI directions. The changes in GTV in the outlier with the largest liver deformations (pt 6) were 4.9, 6.7 and 1.7 mm in the RL, AP and SI directions. Conclusions: Liver deformations using abdominal compression were small in most patients (\5.1 mm in 74% of CBCTs). However, one patient had substantial deformation under the compression plate resulting in clinically significant changes in GTV position. Methods to ensure reproducibility of abdominal compression are warranted. Table 1: Mean of Abs. 95% Volume Displacement for Liver, and ML, AP, and CC Components
Patient
ML mm
AP mm
CC mm
VM mm
1 2 3 4 5 6 All Repro.
3.3 2.6 2.4 3.0 1.5 10.5 3.9 1.9
5.1 3.1 2.9 3.0 2.1 12.9 4.9 2.6
2.4 3.7 2.9 3.5 2.1 5.6 3.4 1.7
5.7 5.4 4.0 5.0 3.1 15.6 6.5 3.4
Author Disclosure: C.L. Eccles, None; K.K. Brock, Varian Medical Systems, B. Research Grant; J. Moseley, None; D. Moseley, None; D. Jaffray, Elekta, B. Research Grant; L.A. Dawson, Elekta, B. Research Grant.
1089
Dosimetric Comparison of Various Online Adaptive Prostate Cancer Treatment Techniques
D. Z. Schulze, T. Zhang, J. Liang, D. Yan William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): The purpose of this study was to evaluate the benefits of different planning and online adaptive treatment techniques for prostate cancer. Materials/Methods: In this study, 3D conformal radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) treatment plans were generated for 9 prostate cancer patients, prescribing 91 Gy to the PTV. The CTV included prostate and seminal vesicles, organs-at-risk (OARs) included bladder and rectum, CTV-to-PTV margins of 0 cm and 1 cm were used, and IMRT plans used either OAR volumes or walls as inverse-planning objectives. Treatment methods included 3D-CRT and IMRT aligned to bony landmarks using 1 cm margins. Three online adaptive treatment methods used 0 cm margins: daily re-optimized 3D-CRT by aperture change, IMRT with rigid body target registration, and daily re-optimized IMRT. Using contours from daily CTs, doses were accumulated over 10 treatment fractions with finite element-based dose mapping techniques. Organ motion relative to the planning CT was evaluated using the Dice Similarity Coefficient (DSC). Results: Dose to normal tissue from several planning and treatment methods was evaluated. The reference plan utilized IMRT with OAR volume constraints, a 1 cm motion margin, and daily alignment to bony landmarks. Table 1 shows the planned dose, for which the IMRT zero-margin, OAR-wall plan achieved the greatest OAR reduction. Among treatment methods, close-to-zero slopes in accumulated CTV EUD vs. DSC (Fig. 1) indicates that these treatments are robust to interfraction motion by maintaining target dose. IMRT re-optimization with OAR walls showed the greatest OAR dose reduction compared to the accumulated reference plan, 14.5% with rectum and 25% with bladder, while remaining within 2.3% of the target prescription.