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PO-0864: Accuracy of fiducial based correction of target motion in prostate SBRT treatments
S470 ESTRO 36 _______________________________________________________________________________________________
S.H. Jeon1, S.Y. Park1, J.H. Kim1, J.I. Kim1, J.M. Park1 1 Seoul National University Hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To suggest an optimal planning target volume (PTV) margin in stereotactic body radiotherapy (SBRT) of the spine. Material and Methods From December 2014 to July 2016, 40 patients received 42 fractions of SBRT for spinal tumors to thoracic or lumbosacral spines using a volumetric modulated arc therapy technique and patient immobilization. Before treatment, kilovoltage cone-beam CT (CBCT) images were obtained for a 4 degrees of freedom (DoF) correction of patients alignment (translation + yaw). After corrections were made, additional CBCT was acquired just before treatment delivery (pretreatment CBCT). Immediately following SBRT, CBCT was acquired again (posttreatment CBCT). Residual setup errors for pretreatment CBCT was determined by a 6 DoF manual matching. Intrafraction motions were calculated as differences in errors between pretreatment and posttreatment CBCT. Three clinical target volumes (CTVs) were generated by translating and rotating original CTV by residual setup errors alone (CTV_R), intrafraction motions alone (CTV_I), and residual setup errors and intrafraction motions combined (CTV_R+I), respectively. Adding various uniform margins to original CTV generated PTVs. The impact of PTV margins on CTV coverage was evaluated. A provisional criterion of adequate CTV coverage was that PTV encompasses at least 97% of CTV. Results Time interval between pre-treatment and post-treatment CBCTs was 6.8±2.5 min (mean±2SD). The 2SD values of lateral, vertial, longitudinal translations and pitch, roll, and yaw were 0.7mm, 0.8mm, 1.1mm, 1.7°, 1.1°, and 1.6°for residual setup errors and 1.0mm, 0.9mm, 0.9mm, 1.1°, 0.8°, and 1.1°for intrafraction motions, respectively. Without margins, PTV showed adequate coverage for CTV_R, CTV_I, and CTV_R+I in 48% (20/42), 71% (30/42), and 48% (20/42) of fractions, respectively. With 1-mm uniform margins, PTV was adequate for 95% (40/42), 98% (41/42), and 100% (42/42) of fractions, respectively. 2-mm uniform margin was adequate in all fractions for all three CTVs. Conclusion With appropriate immobilizations and 4DoF corrections, a uniform 1-mm PTV margin may ensure an adequate CTV coverage in most treatment sessions of spine SBRT. Combined with a shortened treatment time, the small extent of intrafraction motions may obviate the need of treatment interruption for additional intra-session image guidance. Despite perfect 6 DoF patient alignment, 1-mm PTV margin is still needed to address intrafraction motions. PO-0864 Accuracy of fiducial based correction of target motion in prostate SBRT treatments T. Viren1, M. Korhonen2, J. Seppälä1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Applied Physics, Kuopio, Finland Purpose or Objective Robotic stereotactic body radiotherapy (SBRT) incorporating a fiducial based motion tracing system has enabled almost real-time correction of intra-fraction motion of a prostate during SBRT treatments of prostate cancer. However, the effect of number and positioning of the fiducials and the amount of prostate movements on the accuracy of the treatment has not been reported. The aim of the present study was to investigate the accuracy of the fiducial based correction of target motion in
prostate SBRT treatments and to evaluate the effect of fiducial number and positioning to the accuracy of the fiducial tracking. Material and Methods CT image was acquired from custom-made phantom incorporating different fiducial configurations (Fig 1). Subsequently, typical prostate SBRT treatment plan (5x7.25Gy) was calculated in the phantom using treatment planning software (Ray Tracing algorithm, Multiplan, Accuray, USA). To measure the dose distribution within the phantom calibrated Gafchormic films (4 x 4 inch, Gafchromic EBT3, RPD Inc., USA) were placed inside the phantom. A prostate treatment was irradiated in three different phantom positions: no movement, typical clinical prostate movements, and maximum movements allowed by the automatic fiducial tracing system (Fig 1).The phantom movements were conducted using Robochouch (Accuray, USA).To mimic the suboptimal positioning of the fiducials the measurements were repeated with four different seed configurations (optimal, typical clinical case, clinical case with three fiducials, clinical case with two fiducially). Measurements were conducted in coronal and sagittal planes. Finally, the films were scanned (Perfection V700, Epson, USA) 72 hours after the irradiation and the measured and calculated dose distributions were compared using gamma-analysis (5%/2mm threshold).
Figure 1. A) Custom made phantom used to measure prostate SBRT treatment plans. B) The directions of the prostate movements and rotations. C) Typical clinical and maximum intra-fraction prostate movements used in the present study Results The accuracy of the automatic correction of intra-fraction motion of the target was clinically acceptable when three or four seed configuration was used in the motion tracking (Table 1). No significant changes in gamma pass rates were detected when the amount of phantom movement was increased. Clinically unacceptable gamma pass rates were detected only when two fiducials where used in tracking. Table 1. Gamma pass rates of measured and calculated treatment plan comparisons for different fiducial configurations and phantom movements.
Conclusion Automatic correction of the target movement was reasonably accurate for clinical use when three or four
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fiducials were used. Optimal positioning of the fiducials did not improve the accuracy of the treatment when compared to the accuracy achieved with typical clinical fiducial positions or with three fiducials. Usage of only two fiducials in the target tracking resulted clinically unacceptable accuracy. PO-0865 Commissioning and clinical implementation of intra-fractional 4D-CBCT imaging for lung SBRT R. Sims1 1 ARO - Auckland Radiation Oncology, Radiotherapy Physics, Auckland, New Zealand Purpose or Objective Geometric verification of the tumour for free-breathing lung SBRT patients is challenging due to limitations of CBCT imaging at the treatment unit. This can be overcome by using novel acquisition and reconstruction tools to produce a 4D-CBCT dataset that can be acquired both before (inter-fraction) and during (intra-fraction) beam delivery. The commissioning and clinical experience of such a system for lung SBRT will be presented. Material and Methods An anthropomorphic phantom was used to investigate system efficacy for identifying changes in reconstructed motion with different acquisition settings for a variety of clinical situations. The sensitivity of the system to detect changes to programmed motion was investigated and compared to baseline 4DCT imaging with changes to image quality and kV absorbed dose being quantified using additional phantoms. The use of the system during MV treatment for VMAT deliveries was investigated and compared to baseline 4D-CBCT imaging with overall system performance being assessed in terms of image quality and image registration accuracy at the treatment console. Results For inter-fraction imaging, the system successfully identifies changes in amplitude motion to within ±2mm and is sensitive to image distortion/artefacts with different/irregular respiratory cycles and number of image projections. The absorbed dose for standard scan settings is 23.0 ± 1.6mGy with registration accuracy of ±0.4mm and ±0.3degrees. When used intra-fraction there is a reduction in image quality owing to the dependence on VMAT delivery and MV scatter. This can be seen in Figure 1 as a function of VMAT arc length, with the quicker arcs resulting in poorer image quality (for a given BPM of the phantom). Measuring this in terms of contrast-to-noise ratio (between the tumour and surrounding lung tissue) demonstrates that as the arc length and breathing rate increases, the contrast-to-noise ratio approaches that of the inter-fraction 4D-CBCT (see Figure 2). The automatic 4D matching algorithm was found to be influenced by image noise, causing a reduction in the measured amplitude of tumour motion, however despite this the accuracy of automatic registration was excellent varying by ±0.9mm (2SD) for compared to inter-fraction imaging baselines.
Conclusion Intra-fractional 4D-CBCT imaging has been implemented successfully and is now mandated for all lung SBRT patients at our clinic. The system has also been implemented for 3D spinal SBRT imaging although limitations of the MV scatter correction algorithm have resulted in our centre limiting the MU/Arc for VMAT delivery for these cases. Future studies will investigate different acquisition methods for existing conventionallyfractionated treatments to improve the workflow and improve image quality. Poster: Physics track: Inter-fraction management (excl. adaptive radiotherapy)
motion
PO-0866 Visibility, image artifacts and proton dose perturbation of fiducial markers V.C. Hamming1, C.L. Brouwer1, M.J. Van Goethem1, R.I. Jolck2, C. Van Leijsen1, A.C.M. Van den Bergh1 1 UMCG University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands 2 NANOVI radiotherapy, DTU scion, Lyngby, Denmark Purpose or Objective Fiducial markers (FMs) are necessary for an accurate photon and proton radiation treatment for prostatecancer. However, conventional FMs may cause problems with dose calculations and perturbations in proton therapy. Therefore, specific proton-treatment FMs are available having smaller dimensions and different material compositions. The goal of this research was to survey the visibility, CT artifacts and proton dose perturbations of available FMs to choose the optimal FM for proton therapy.
S.H. Jeon1, S.Y. Park1, J.H. Kim1, J.I. Kim1, J.M. Park1 1 Seoul National University Hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To suggest an optimal planning target volume (PTV) margin in stereotactic body radiotherapy (SBRT) of the spine. Material and Methods From December 2014 to July 2016, 40 patients received 42 fractions of SBRT for spinal tumors to thoracic or lumbosacral spines using a volumetric modulated arc therapy technique and patient immobilization. Before treatment, kilovoltage cone-beam CT (CBCT) images were obtained for a 4 degrees of freedom (DoF) correction of patients alignment (translation + yaw). After corrections were made, additional CBCT was acquired just before treatment delivery (pretreatment CBCT). Immediately following SBRT, CBCT was acquired again (posttreatment CBCT). Residual setup errors for pretreatment CBCT was determined by a 6 DoF manual matching. Intrafraction motions were calculated as differences in errors between pretreatment and posttreatment CBCT. Three clinical target volumes (CTVs) were generated by translating and rotating original CTV by residual setup errors alone (CTV_R), intrafraction motions alone (CTV_I), and residual setup errors and intrafraction motions combined (CTV_R+I), respectively. Adding various uniform margins to original CTV generated PTVs. The impact of PTV margins on CTV coverage was evaluated. A provisional criterion of adequate CTV coverage was that PTV encompasses at least 97% of CTV. Results Time interval between pre-treatment and post-treatment CBCTs was 6.8±2.5 min (mean±2SD). The 2SD values of lateral, vertial, longitudinal translations and pitch, roll, and yaw were 0.7mm, 0.8mm, 1.1mm, 1.7°, 1.1°, and 1.6°for residual setup errors and 1.0mm, 0.9mm, 0.9mm, 1.1°, 0.8°, and 1.1°for intrafraction motions, respectively. Without margins, PTV showed adequate coverage for CTV_R, CTV_I, and CTV_R+I in 48% (20/42), 71% (30/42), and 48% (20/42) of fractions, respectively. With 1-mm uniform margins, PTV was adequate for 95% (40/42), 98% (41/42), and 100% (42/42) of fractions, respectively. 2-mm uniform margin was adequate in all fractions for all three CTVs. Conclusion With appropriate immobilizations and 4DoF corrections, a uniform 1-mm PTV margin may ensure an adequate CTV coverage in most treatment sessions of spine SBRT. Combined with a shortened treatment time, the small extent of intrafraction motions may obviate the need of treatment interruption for additional intra-session image guidance. Despite perfect 6 DoF patient alignment, 1-mm PTV margin is still needed to address intrafraction motions. PO-0864 Accuracy of fiducial based correction of target motion in prostate SBRT treatments T. Viren1, M. Korhonen2, J. Seppälä1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Applied Physics, Kuopio, Finland Purpose or Objective Robotic stereotactic body radiotherapy (SBRT) incorporating a fiducial based motion tracing system has enabled almost real-time correction of intra-fraction motion of a prostate during SBRT treatments of prostate cancer. However, the effect of number and positioning of the fiducials and the amount of prostate movements on the accuracy of the treatment has not been reported. The aim of the present study was to investigate the accuracy of the fiducial based correction of target motion in
prostate SBRT treatments and to evaluate the effect of fiducial number and positioning to the accuracy of the fiducial tracking. Material and Methods CT image was acquired from custom-made phantom incorporating different fiducial configurations (Fig 1). Subsequently, typical prostate SBRT treatment plan (5x7.25Gy) was calculated in the phantom using treatment planning software (Ray Tracing algorithm, Multiplan, Accuray, USA). To measure the dose distribution within the phantom calibrated Gafchormic films (4 x 4 inch, Gafchromic EBT3, RPD Inc., USA) were placed inside the phantom. A prostate treatment was irradiated in three different phantom positions: no movement, typical clinical prostate movements, and maximum movements allowed by the automatic fiducial tracing system (Fig 1).The phantom movements were conducted using Robochouch (Accuray, USA).To mimic the suboptimal positioning of the fiducials the measurements were repeated with four different seed configurations (optimal, typical clinical case, clinical case with three fiducials, clinical case with two fiducially). Measurements were conducted in coronal and sagittal planes. Finally, the films were scanned (Perfection V700, Epson, USA) 72 hours after the irradiation and the measured and calculated dose distributions were compared using gamma-analysis (5%/2mm threshold).
Figure 1. A) Custom made phantom used to measure prostate SBRT treatment plans. B) The directions of the prostate movements and rotations. C) Typical clinical and maximum intra-fraction prostate movements used in the present study Results The accuracy of the automatic correction of intra-fraction motion of the target was clinically acceptable when three or four seed configuration was used in the motion tracking (Table 1). No significant changes in gamma pass rates were detected when the amount of phantom movement was increased. Clinically unacceptable gamma pass rates were detected only when two fiducials where used in tracking. Table 1. Gamma pass rates of measured and calculated treatment plan comparisons for different fiducial configurations and phantom movements.
Conclusion Automatic correction of the target movement was reasonably accurate for clinical use when three or four
S471 ESTRO 36 _______________________________________________________________________________________________
fiducials were used. Optimal positioning of the fiducials did not improve the accuracy of the treatment when compared to the accuracy achieved with typical clinical fiducial positions or with three fiducials. Usage of only two fiducials in the target tracking resulted clinically unacceptable accuracy. PO-0865 Commissioning and clinical implementation of intra-fractional 4D-CBCT imaging for lung SBRT R. Sims1 1 ARO - Auckland Radiation Oncology, Radiotherapy Physics, Auckland, New Zealand Purpose or Objective Geometric verification of the tumour for free-breathing lung SBRT patients is challenging due to limitations of CBCT imaging at the treatment unit. This can be overcome by using novel acquisition and reconstruction tools to produce a 4D-CBCT dataset that can be acquired both before (inter-fraction) and during (intra-fraction) beam delivery. The commissioning and clinical experience of such a system for lung SBRT will be presented. Material and Methods An anthropomorphic phantom was used to investigate system efficacy for identifying changes in reconstructed motion with different acquisition settings for a variety of clinical situations. The sensitivity of the system to detect changes to programmed motion was investigated and compared to baseline 4DCT imaging with changes to image quality and kV absorbed dose being quantified using additional phantoms. The use of the system during MV treatment for VMAT deliveries was investigated and compared to baseline 4D-CBCT imaging with overall system performance being assessed in terms of image quality and image registration accuracy at the treatment console. Results For inter-fraction imaging, the system successfully identifies changes in amplitude motion to within ±2mm and is sensitive to image distortion/artefacts with different/irregular respiratory cycles and number of image projections. The absorbed dose for standard scan settings is 23.0 ± 1.6mGy with registration accuracy of ±0.4mm and ±0.3degrees. When used intra-fraction there is a reduction in image quality owing to the dependence on VMAT delivery and MV scatter. This can be seen in Figure 1 as a function of VMAT arc length, with the quicker arcs resulting in poorer image quality (for a given BPM of the phantom). Measuring this in terms of contrast-to-noise ratio (between the tumour and surrounding lung tissue) demonstrates that as the arc length and breathing rate increases, the contrast-to-noise ratio approaches that of the inter-fraction 4D-CBCT (see Figure 2). The automatic 4D matching algorithm was found to be influenced by image noise, causing a reduction in the measured amplitude of tumour motion, however despite this the accuracy of automatic registration was excellent varying by ±0.9mm (2SD) for compared to inter-fraction imaging baselines.
Conclusion Intra-fractional 4D-CBCT imaging has been implemented successfully and is now mandated for all lung SBRT patients at our clinic. The system has also been implemented for 3D spinal SBRT imaging although limitations of the MV scatter correction algorithm have resulted in our centre limiting the MU/Arc for VMAT delivery for these cases. Future studies will investigate different acquisition methods for existing conventionallyfractionated treatments to improve the workflow and improve image quality. Poster: Physics track: Inter-fraction management (excl. adaptive radiotherapy)
motion
PO-0866 Visibility, image artifacts and proton dose perturbation of fiducial markers V.C. Hamming1, C.L. Brouwer1, M.J. Van Goethem1, R.I. Jolck2, C. Van Leijsen1, A.C.M. Van den Bergh1 1 UMCG University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands 2 NANOVI radiotherapy, DTU scion, Lyngby, Denmark Purpose or Objective Fiducial markers (FMs) are necessary for an accurate photon and proton radiation treatment for prostatecancer. However, conventional FMs may cause problems with dose calculations and perturbations in proton therapy. Therefore, specific proton-treatment FMs are available having smaller dimensions and different material compositions. The goal of this research was to survey the visibility, CT artifacts and proton dose perturbations of available FMs to choose the optimal FM for proton therapy.