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OC-0233 RADIATION TECHNOLOGIST LED IMAGE GUIDED RADIOTHERAPY – IS IT SAFE AND ACHIEVABLE FOR LUNG SBRT?
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Conclusions: Scatter correction calculated by MC simulations are found to improve CBCT image quality compared to the current clinical practice where a constant background is subtracted. The MC simulation can be performed in a timeframe that allows for clinical implementations. In the clinic, a planning CT dataset could be used as the basis for determining the spatial scatter distribution for each projection using the MC code. This scatter distribution could then be subtracted from planar CBCT images before reconstruction, which leads to improvement in the image quality of routine CBCT scans. This improvement requires no additional data to be collected, and the image quality may improve sufficiently for the CBCT images to be used for dose calculations. OC-0231 MEGAVOLTAGE X-RAY SCATTER CORRECTION FOR SIMULTANEOUS CONE-BEAM CT IMAGING DURING VMAT DELIVERY C.J. Boylan1, T.E. Marchant1, J. Stratford2, J. Rodgers2, J. Malik3, A. Choudhury3, R.K. Shrimali3, C.G. Rowbottom1 1 The Christie NHS Foundation Trust, North Western Medical Physics, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, The Wade Radiotherapy Research Centre, Manchester, United Kingdom 3 The Christie NHS Foundation Trust, Department of Clinical Oncology, Manchester, United Kingdom Purpose/Objective: kV Cone Beam CT (CBCT) images can be acquired concurrently with the delivery of volumetric modulated arc therapy (VMAT). Such images are useful because they give an indication of patient anatomy during treatment rather than before or after, and also mean that fraction times may be reduced, improving patient throughput. However, the quality of these images is degraded by megavoltage (MV) scatter onto the imaging panel. Proposed scatter correction methods have involved reducing the number of kV projections, or interrupting the treatment arc. In this study, a novel correction technique has been developed which retains the number of projections and does not affect the treatment duration. The effectiveness of the technique is assessed with phantom and patient images, and results are compared to two other correction methods. Materials and Methods: Direct measurement of MV scatter was made during treatment delivery by allowing the imager to acquire without a kV beam. For phantom measurements, these scatter acquisitions were taken after the simultaneous CBCT. For the patients, they were taken on a non-imaging treatment fraction. The simultaneous CBCT projections were then corrected by subtracting the scatter projection at that gantry angle. CBCTs were taken during delivery of a VMAT prostate plan to an image quality phantom. Low contrast signal-to-noise ratios (SNR) were compared between a) the standard CBCT, b) the uncorrected simultaneous CBCT, c) the scatter-corrected CBCT, d) a simple correction using the measured mean scatter signal, and e) an analytical correction based on the plan parameters. Simultaneous CBCTs were also taken for three prostate VMAT patients. To assess clinical image quality, a scoring system for prostate CBCT was used. Two oncologists and two radiographers ranked the uncorrected and corrected CBCTs on a scale from 1 (good) to 5 (poor), with each point defined by the visibility of soft tissue boundaries. The CBCTs were anonymized and their order was randomized for each observer. Results: The image quality of the uncorrected simultaneous CBCTs was worse than the standard CBCTs, with a reduction in low contrast SNR (-78%) and in the mean image quality score (mean decrease of 1.4 points). Scatter correction improved the SNR within the phantom, compared to the uncorrected images. Method c) increased the SNR by 50%, method d) by 38% and method e) by 26%. For the patient images, a similar trend was observed. Method c) increased the mean image quality score by 0.67 points, method d) by 0.56 points and method e) by 0.50 points. Conclusions: Simultaneous CBCT images taken during VMAT are adversely affected by MV scatter, so it is important to attempt to recover the image quality. The method described here requires the acquisition of scatter-only images on the kV panel, which can be taken during treatment. Quantitative and qualitative assessments of image quality showed an improvement in the corrected CBCTs compared to the uncorrected.
ESTRO 31
OC-0232 CONE-BEAM CT - BASED RADIOTHERAPY PLANNING OF HEAD AND NECK CANCER U.V. Elstrøm1, S.K. Olsen1, B.A. Wysocka2, L.P. Muren1, J.B.B. Petersen3, C. Grau2 1 Aarhus University Hospital, Dept. of Oncology & Dept. of Medical Physics, Aarhus C, Denmark 2 Aarhus University Hospital, Dept. of Oncology, Aarhus C, Denmark 3 Aarhus University Hospital, Dept. of Medical Physics, Aarhus C, Denmark Purpose/Objective: The implications of anatomical changes during a course of radiotherapy (RT) for head and neck (H&N) cancer can potentially be reduced through image-based adaptive RT (ART) strategies. In-room imaging modalities like cone-beam CT (CBCT) provides images that potentially can be useful in ART. Developments in CBCT calibration and reconstruction have resulted in significantly improved CBCT image quality. The aim of this study was to directly compare treatment planning calculations based on optimised CBCT vs. conventional CT scans in a series of H&N cancer patients. Materials and Methods: The study included thirteen patients with locally advanced H&N cancer treated with intensity-modulated RT (IMRT) and daily CBCT for patient positioning. The patient selection criteria were that they had a conventional CT and a high-quality CBCT acquired on the same day, halfway through the treatment course, and full coverage of relevant targets and normal tissues by the CBCT fieldof-view. The CBCT projections were reconstructed twice, using both the on-board imager (OBI) standard method as well as a new full fan experimental (FFE) pre-clinical reconstruction algorithm with improved beam hardening and scatter correction. Stoichiometric calibration was used to determine the relation between CT-numbers and electron density ratios in the treatment planning system, separately for the two reconstructions. Delineated structures on CT were propagated to the CBCT scans using intensity-based deformable image registration. The original treated IMRT plan was re-calculated on both the CT and the two CBCTs, and key dose/volume-parameters for the plans were compared for high-dose and high-risk elective targets, parotid glands, mandible and spinal cord. Results: There was a significant reduction in volume from CT to CBCT for all volumes investigated for both reconstruction methods, although most pronounced with the standard clinical reconstruction, with the mean difference ranging from 2-13%. The largest difference was seen for the mandible due to image artefacts from teeth fillings. The mean doses to the two target volumes and the parotid glands were higher for CBCT (with at most 1.7%). There was also a significant difference between the reconstruction methods for the targets, with the mean dose in the FFE-reconstruction closer to CT. The maximum doses on the mandible and spinal cord decreased with at most 1.9% using CBCT-based dose calculation. Conclusions: The differences in dose/volume-parameters for both targets and critical normal tissues from CT- vs. CBCT-based planning were found to be within a clinically acceptable range. Improved CBCT image quality diminishes the discrepancies to CT. The results of this study open a potential for CBCT-based ART for patients with H&N cancer.
PROFFERED PAPERS: RTT 2: POSITION VERIFICATION OC-0233 RADIATION TECHNOLOGIST LED IMAGE GUIDED RADIOTHERAPY - IS IT SAFE AND ACHIEVABLE FOR LUNG SBRT? A. Needham1, H. Summers1, D.J.R. Sykes2, J. Lilley2, D.A.M Henry3, D.K.N Franks3 1 St. James's Institute of Oncology, Radiotherapy, Leeds - West Yorkshire, United Kingdom 2 St. James's Institute of Oncology, Medical Physics & Engineering, Leeds - West Yorkshire, United Kingdom 3 St. James's Institute of Oncology, Clinical Oncology, Leeds - West Yorkshire, United Kingdom Purpose/Objective: Implementation and maintenance of a SBRT programme is a resource intensive activity often requiring coordinated presence of key staff during treatment delivery. Three years into our programme, numbers treated daily, treatment session length and IGRT processes have fundamentally altered from initial service set-up, a result of improved use of technology and migration to 'Radiographer Technologist (RT) led' image guidance. Routine multidisciplinary (MD) attendance during IGRT has ended,
S92
except for pre-agreed cases or when issues were identified by treating RT's. However risks associated with extreme hypo-fractionation demand that objective decision making and conformance to protocols are maintained irrespective of changes to process or delegated responsibility. This paper investigates a number of performance metrics throughout our SBRT programme that are indicative of quality and safety of the delivered radiotherapy. Materials and Methods: The time interval between first CBCT and final post treatment CBCT was collected from patient cohorts representing 3 phases within our Lung SBRT programme corresponding to decisions to; 1) stop RT's entering the room following correction of patient position 2) reduce requirement for MD attendance at first fraction. Treatment session data was also analysed from cohorts, representative of early and current practice, identifying common aspects of technical decision making and quality of delivery. We measured the proportion of sessions where; multiple corrections were necessary; post treatment CBCT suggested tumour to be at, or outside accepted tolerance. Patient movement observed during delivery, requiring intervention. Proactive intervention was agreed for subsequent treatments. Results: Improvements in delivery efficiency were demonstrated between each phase, with median and mean treatment session times decreasing by approximately 4.5 minutes. (see figure 1) This can be attributed to the increased confidence and experience levels of RT's following the two changes in practice. Comparison of CBCT registration data (table 1) showed a decline in sessions requiring multiple corrections and of instances where tumour was considered 'out of tolerance' at treatment end. Fewer incidences of patient movement were observed although RT-led decisions to scan 'mid-treatment' on subsequent treatments had increased. Systematic and random set-up errors were also reduced.
ESTRO 31
Conclusions: During SBRT implementation, MD presence and joint decision making at all treatment sessions is necessary to build up confidence and expertise. However, once established, our experience shows that delegation of IGRT decision making to adequately trained RT's, who act as gatekeepers for intervention and MD discussion can reduce time for treatment delivery and improve patient experience, without a detrimental effect on quality and safety. OC-0234 IGRT DATA TO EVALUATE SET UP ACCURACY OF THE IMMOBILIZZATION SYSTEMS L. Balardi1, G. Mantello1, S. Costantini1, F. Fenu1, L. Vicenzi1, F. Cucciarelli1, M. Montisci1, R. Righetto2, M. Valenti2, M. Cardinali1 1 AOU Ospedali Riuniti, Radiotherapy, Ancona, Italy 2 AOU Ospedali Riuniti, Physics, Ancona, Italy Purpose/Objective: The aim of this analysis was to evaluate the shifts data recorded after each on line set up correction , according to our IGRT protocols, in order to calculate our immobilization systems accuracy and to define the set up margin to add per each device used. Materials and Methods: 311 immobilization devices were studied : 59 thermoplastic head only masks (47 ORFIT and 12 APU), 52 thorax poliuretan foams, 15 breast boards, 24 abdomen foams, 72 combifix, 25 pelvis and leg poliuretan foams, 18 bellyboards, 12 leg only poliuretan foams and 24 kneefix. Before each treatment section, after patient set up, according to Department policy a CBCT or two orthogonal kV images were acquired; a 3D/3D match (simulation CT - CBCT) or a 2D/2D match (DRR - kV) on bone structures were performed in order to evaluate the set up error. The shifts were applied to the couch to erase set up error and were recorded. The patient's systematic error (mean shifts value) and the patient's random error (standard deviation SD value) were calculated per each patient and per each direction. Then, the population systematic errors (Σ pop) and the random errors (σ pop) were found per each treated region, each immobilization system, and separately for Y(vrt) Z(lng), and X(lat), axes. Results: 8184 shifts (2D and 3D) were performed, on the bony structures, and recorded. The calculated Σpop and σ pop values, per each treated region, each immobilization system, each direction (Y, Z, X), are showed in the table. Σpop Treated region brain
Immobilization IGRT system (n) protocol Apu (12) 2D (kV) Orfit (47) 2D (kV) thorax Thorax foam (52) 3D (CBCT) breast board (15) 3D (CBCT) pelvis, combifix ( 72) 3D (CBCT) supine pelvic+leg foam 3D (CBCT) (25) pelvis, belly board (18) 3D (CBCT) prone leg foam (12) 3D (CBCT) spine Kneefix ( 24) 2D (kV) abdomen abdomen foam 3D (CBCT) (24)
σ pop
Y(vrt) Z(lng) X(lat) Y(vrt) Z(lng) X(lat) 0.11 0.11 0.19 0.13 0.19
0.13 0.12 0.30 0.10 0.36
0.18 0.13 0.28 0.24 0.25
0.17 0.14 0.26 0.05 0.55
0.21 0.18 0.36 0.07 0.38
0.17 0.18 0.39 0.21 0.32
0.16
0.18
0.24
0.31
0.28 0.28
0.71 0.11 0.04 0.34
0.49 0.27 0.22 0.40
0.23 0.29 0.39 0.32
0.36 0.48 0.32 0.40
0.45 0.39 0.69 0.38
0.34 0.39 0.51 0.34
As expected, due the characteristic of the treated region , the best accuracy was recorded when using thermoplastic head masks. While the analysis of the data showed an increased systematic and random error in the group of patients treated in pelvic region on the belly board and also a large random error when combifix or kneefix devices were used. This was due to the fact that our IGRT system cannot correct the frequent pelvic rotation (both in supine and prone positions) with couch rotation, so these errors are usually compensated with simple translations. The pelvis and leg poliuretan foams (used to treat the pelvis in supine position) guaranteed a better patient set up accuracy. Conclusions: Data available from IGRT applications allowed us to check the set up accuracy of immobilization systems in use in our Department and to calculate the set up margin to add to the CTV when IGRT protocols are not applied.
Conclusions: Scatter correction calculated by MC simulations are found to improve CBCT image quality compared to the current clinical practice where a constant background is subtracted. The MC simulation can be performed in a timeframe that allows for clinical implementations. In the clinic, a planning CT dataset could be used as the basis for determining the spatial scatter distribution for each projection using the MC code. This scatter distribution could then be subtracted from planar CBCT images before reconstruction, which leads to improvement in the image quality of routine CBCT scans. This improvement requires no additional data to be collected, and the image quality may improve sufficiently for the CBCT images to be used for dose calculations. OC-0231 MEGAVOLTAGE X-RAY SCATTER CORRECTION FOR SIMULTANEOUS CONE-BEAM CT IMAGING DURING VMAT DELIVERY C.J. Boylan1, T.E. Marchant1, J. Stratford2, J. Rodgers2, J. Malik3, A. Choudhury3, R.K. Shrimali3, C.G. Rowbottom1 1 The Christie NHS Foundation Trust, North Western Medical Physics, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, The Wade Radiotherapy Research Centre, Manchester, United Kingdom 3 The Christie NHS Foundation Trust, Department of Clinical Oncology, Manchester, United Kingdom Purpose/Objective: kV Cone Beam CT (CBCT) images can be acquired concurrently with the delivery of volumetric modulated arc therapy (VMAT). Such images are useful because they give an indication of patient anatomy during treatment rather than before or after, and also mean that fraction times may be reduced, improving patient throughput. However, the quality of these images is degraded by megavoltage (MV) scatter onto the imaging panel. Proposed scatter correction methods have involved reducing the number of kV projections, or interrupting the treatment arc. In this study, a novel correction technique has been developed which retains the number of projections and does not affect the treatment duration. The effectiveness of the technique is assessed with phantom and patient images, and results are compared to two other correction methods. Materials and Methods: Direct measurement of MV scatter was made during treatment delivery by allowing the imager to acquire without a kV beam. For phantom measurements, these scatter acquisitions were taken after the simultaneous CBCT. For the patients, they were taken on a non-imaging treatment fraction. The simultaneous CBCT projections were then corrected by subtracting the scatter projection at that gantry angle. CBCTs were taken during delivery of a VMAT prostate plan to an image quality phantom. Low contrast signal-to-noise ratios (SNR) were compared between a) the standard CBCT, b) the uncorrected simultaneous CBCT, c) the scatter-corrected CBCT, d) a simple correction using the measured mean scatter signal, and e) an analytical correction based on the plan parameters. Simultaneous CBCTs were also taken for three prostate VMAT patients. To assess clinical image quality, a scoring system for prostate CBCT was used. Two oncologists and two radiographers ranked the uncorrected and corrected CBCTs on a scale from 1 (good) to 5 (poor), with each point defined by the visibility of soft tissue boundaries. The CBCTs were anonymized and their order was randomized for each observer. Results: The image quality of the uncorrected simultaneous CBCTs was worse than the standard CBCTs, with a reduction in low contrast SNR (-78%) and in the mean image quality score (mean decrease of 1.4 points). Scatter correction improved the SNR within the phantom, compared to the uncorrected images. Method c) increased the SNR by 50%, method d) by 38% and method e) by 26%. For the patient images, a similar trend was observed. Method c) increased the mean image quality score by 0.67 points, method d) by 0.56 points and method e) by 0.50 points. Conclusions: Simultaneous CBCT images taken during VMAT are adversely affected by MV scatter, so it is important to attempt to recover the image quality. The method described here requires the acquisition of scatter-only images on the kV panel, which can be taken during treatment. Quantitative and qualitative assessments of image quality showed an improvement in the corrected CBCTs compared to the uncorrected.
ESTRO 31
OC-0232 CONE-BEAM CT - BASED RADIOTHERAPY PLANNING OF HEAD AND NECK CANCER U.V. Elstrøm1, S.K. Olsen1, B.A. Wysocka2, L.P. Muren1, J.B.B. Petersen3, C. Grau2 1 Aarhus University Hospital, Dept. of Oncology & Dept. of Medical Physics, Aarhus C, Denmark 2 Aarhus University Hospital, Dept. of Oncology, Aarhus C, Denmark 3 Aarhus University Hospital, Dept. of Medical Physics, Aarhus C, Denmark Purpose/Objective: The implications of anatomical changes during a course of radiotherapy (RT) for head and neck (H&N) cancer can potentially be reduced through image-based adaptive RT (ART) strategies. In-room imaging modalities like cone-beam CT (CBCT) provides images that potentially can be useful in ART. Developments in CBCT calibration and reconstruction have resulted in significantly improved CBCT image quality. The aim of this study was to directly compare treatment planning calculations based on optimised CBCT vs. conventional CT scans in a series of H&N cancer patients. Materials and Methods: The study included thirteen patients with locally advanced H&N cancer treated with intensity-modulated RT (IMRT) and daily CBCT for patient positioning. The patient selection criteria were that they had a conventional CT and a high-quality CBCT acquired on the same day, halfway through the treatment course, and full coverage of relevant targets and normal tissues by the CBCT fieldof-view. The CBCT projections were reconstructed twice, using both the on-board imager (OBI) standard method as well as a new full fan experimental (FFE) pre-clinical reconstruction algorithm with improved beam hardening and scatter correction. Stoichiometric calibration was used to determine the relation between CT-numbers and electron density ratios in the treatment planning system, separately for the two reconstructions. Delineated structures on CT were propagated to the CBCT scans using intensity-based deformable image registration. The original treated IMRT plan was re-calculated on both the CT and the two CBCTs, and key dose/volume-parameters for the plans were compared for high-dose and high-risk elective targets, parotid glands, mandible and spinal cord. Results: There was a significant reduction in volume from CT to CBCT for all volumes investigated for both reconstruction methods, although most pronounced with the standard clinical reconstruction, with the mean difference ranging from 2-13%. The largest difference was seen for the mandible due to image artefacts from teeth fillings. The mean doses to the two target volumes and the parotid glands were higher for CBCT (with at most 1.7%). There was also a significant difference between the reconstruction methods for the targets, with the mean dose in the FFE-reconstruction closer to CT. The maximum doses on the mandible and spinal cord decreased with at most 1.9% using CBCT-based dose calculation. Conclusions: The differences in dose/volume-parameters for both targets and critical normal tissues from CT- vs. CBCT-based planning were found to be within a clinically acceptable range. Improved CBCT image quality diminishes the discrepancies to CT. The results of this study open a potential for CBCT-based ART for patients with H&N cancer.
PROFFERED PAPERS: RTT 2: POSITION VERIFICATION OC-0233 RADIATION TECHNOLOGIST LED IMAGE GUIDED RADIOTHERAPY - IS IT SAFE AND ACHIEVABLE FOR LUNG SBRT? A. Needham1, H. Summers1, D.J.R. Sykes2, J. Lilley2, D.A.M Henry3, D.K.N Franks3 1 St. James's Institute of Oncology, Radiotherapy, Leeds - West Yorkshire, United Kingdom 2 St. James's Institute of Oncology, Medical Physics & Engineering, Leeds - West Yorkshire, United Kingdom 3 St. James's Institute of Oncology, Clinical Oncology, Leeds - West Yorkshire, United Kingdom Purpose/Objective: Implementation and maintenance of a SBRT programme is a resource intensive activity often requiring coordinated presence of key staff during treatment delivery. Three years into our programme, numbers treated daily, treatment session length and IGRT processes have fundamentally altered from initial service set-up, a result of improved use of technology and migration to 'Radiographer Technologist (RT) led' image guidance. Routine multidisciplinary (MD) attendance during IGRT has ended,
S92
except for pre-agreed cases or when issues were identified by treating RT's. However risks associated with extreme hypo-fractionation demand that objective decision making and conformance to protocols are maintained irrespective of changes to process or delegated responsibility. This paper investigates a number of performance metrics throughout our SBRT programme that are indicative of quality and safety of the delivered radiotherapy. Materials and Methods: The time interval between first CBCT and final post treatment CBCT was collected from patient cohorts representing 3 phases within our Lung SBRT programme corresponding to decisions to; 1) stop RT's entering the room following correction of patient position 2) reduce requirement for MD attendance at first fraction. Treatment session data was also analysed from cohorts, representative of early and current practice, identifying common aspects of technical decision making and quality of delivery. We measured the proportion of sessions where; multiple corrections were necessary; post treatment CBCT suggested tumour to be at, or outside accepted tolerance. Patient movement observed during delivery, requiring intervention. Proactive intervention was agreed for subsequent treatments. Results: Improvements in delivery efficiency were demonstrated between each phase, with median and mean treatment session times decreasing by approximately 4.5 minutes. (see figure 1) This can be attributed to the increased confidence and experience levels of RT's following the two changes in practice. Comparison of CBCT registration data (table 1) showed a decline in sessions requiring multiple corrections and of instances where tumour was considered 'out of tolerance' at treatment end. Fewer incidences of patient movement were observed although RT-led decisions to scan 'mid-treatment' on subsequent treatments had increased. Systematic and random set-up errors were also reduced.
ESTRO 31
Conclusions: During SBRT implementation, MD presence and joint decision making at all treatment sessions is necessary to build up confidence and expertise. However, once established, our experience shows that delegation of IGRT decision making to adequately trained RT's, who act as gatekeepers for intervention and MD discussion can reduce time for treatment delivery and improve patient experience, without a detrimental effect on quality and safety. OC-0234 IGRT DATA TO EVALUATE SET UP ACCURACY OF THE IMMOBILIZZATION SYSTEMS L. Balardi1, G. Mantello1, S. Costantini1, F. Fenu1, L. Vicenzi1, F. Cucciarelli1, M. Montisci1, R. Righetto2, M. Valenti2, M. Cardinali1 1 AOU Ospedali Riuniti, Radiotherapy, Ancona, Italy 2 AOU Ospedali Riuniti, Physics, Ancona, Italy Purpose/Objective: The aim of this analysis was to evaluate the shifts data recorded after each on line set up correction , according to our IGRT protocols, in order to calculate our immobilization systems accuracy and to define the set up margin to add per each device used. Materials and Methods: 311 immobilization devices were studied : 59 thermoplastic head only masks (47 ORFIT and 12 APU), 52 thorax poliuretan foams, 15 breast boards, 24 abdomen foams, 72 combifix, 25 pelvis and leg poliuretan foams, 18 bellyboards, 12 leg only poliuretan foams and 24 kneefix. Before each treatment section, after patient set up, according to Department policy a CBCT or two orthogonal kV images were acquired; a 3D/3D match (simulation CT - CBCT) or a 2D/2D match (DRR - kV) on bone structures were performed in order to evaluate the set up error. The shifts were applied to the couch to erase set up error and were recorded. The patient's systematic error (mean shifts value) and the patient's random error (standard deviation SD value) were calculated per each patient and per each direction. Then, the population systematic errors (Σ pop) and the random errors (σ pop) were found per each treated region, each immobilization system, and separately for Y(vrt) Z(lng), and X(lat), axes. Results: 8184 shifts (2D and 3D) were performed, on the bony structures, and recorded. The calculated Σpop and σ pop values, per each treated region, each immobilization system, each direction (Y, Z, X), are showed in the table. Σpop Treated region brain
Immobilization IGRT system (n) protocol Apu (12) 2D (kV) Orfit (47) 2D (kV) thorax Thorax foam (52) 3D (CBCT) breast board (15) 3D (CBCT) pelvis, combifix ( 72) 3D (CBCT) supine pelvic+leg foam 3D (CBCT) (25) pelvis, belly board (18) 3D (CBCT) prone leg foam (12) 3D (CBCT) spine Kneefix ( 24) 2D (kV) abdomen abdomen foam 3D (CBCT) (24)
σ pop
Y(vrt) Z(lng) X(lat) Y(vrt) Z(lng) X(lat) 0.11 0.11 0.19 0.13 0.19
0.13 0.12 0.30 0.10 0.36
0.18 0.13 0.28 0.24 0.25
0.17 0.14 0.26 0.05 0.55
0.21 0.18 0.36 0.07 0.38
0.17 0.18 0.39 0.21 0.32
0.16
0.18
0.24
0.31
0.28 0.28
0.71 0.11 0.04 0.34
0.49 0.27 0.22 0.40
0.23 0.29 0.39 0.32
0.36 0.48 0.32 0.40
0.45 0.39 0.69 0.38
0.34 0.39 0.51 0.34
As expected, due the characteristic of the treated region , the best accuracy was recorded when using thermoplastic head masks. While the analysis of the data showed an increased systematic and random error in the group of patients treated in pelvic region on the belly board and also a large random error when combifix or kneefix devices were used. This was due to the fact that our IGRT system cannot correct the frequent pelvic rotation (both in supine and prone positions) with couch rotation, so these errors are usually compensated with simple translations. The pelvis and leg poliuretan foams (used to treat the pelvis in supine position) guaranteed a better patient set up accuracy. Conclusions: Data available from IGRT applications allowed us to check the set up accuracy of immobilization systems in use in our Department and to calculate the set up margin to add to the CTV when IGRT protocols are not applied.