- Email: [email protected]
1090 poster An investigation of the dosimetric characteristics and clinical potential of the elekta beam modulator®
Posters
$457
with usual set-up controls on 200 patients treated last years. Inaccuracy values are inferior because there is smaller uncertainty in position, more accurate magnification and better image quality on CR instead of usual slab.
Further recruitment and analysis will evaluate the possible consequences of this in terms of predicted radiation dose to organs at risk.
Conclusions: CR digital images can be manipulated, analysed, fused and transmitted. CR can replace films for treatment characterization and verification, with many advantages: faster image elaboration, better image quality, quantitative analysis of patient position, electronical image transmission, efficient and economic manipulation of large amount of images. Future aim will be iMRT verification.
1089 poster Investigation of artefacts observed in helical CT images
1088 poster Distortion-corrected T2-weighted MRI: a comparison with CT for prostate contouring in radiotherapy planning
A.S.N. Jackson 1, S.A. Reinsberg 2, E.M. Moore 2, S.A. Sohaib ~, E.J. Adams 4, H.A. McNair~, D.P. Deamalej ~Royal Marsden Hospital and Institute of Cancer Research Academic Department of Radiotherapy and Oncology, Sutton, United Kingdom 2Royal Marsden Hospital and Institute of Cancer Research CRUK Clinical Magnetic Resonance Research Group, Sutton, United Kingdom 3Royal Marsden Hospital and Institute of Cancer Research Department of Radiology, Sutton, United Kingdom 4Royal Marsden Hospital and Institute of Cancer Research Joint Department of Physics, Sutton, United Kingdom 5Royal Marsden Hospital and Institute of Cancer Research Department of Radiotherapy, Sutton, United Kingdom Purpose: In this study T2-weighted distortion-corrected MRI images are compared with CT for prostate radiotherapy planning. Materials and Methods: The patient group consisted of men about to be treated with radical radiotherapy for prostate cancer and with no contra-indication to MRI scanning. Patients were scanned with CT (GE HiSpeed QX/i) and MRI (1.5T Philips Intera Gyroscan) using the same patient positioning and immobilisation technique. A novel flat couchtop has been designed for the MR system. Standard clinically available 3D T2-weighted MR images were corrected for both patient and system-based distortion, and images from both modalities transferred to a radiotherapy treatment planning workstation. The prostate and organs at risk were contoured on axial images firstly on CT, then on MRI. Image fusion was used to allow qualitative, as well as quantitative comparisons between image pairs. A total of fourteen patients will be imaged over the next 3 months, with results from the first four subjects described here. Results: The MRI scan took place a mean of 3 days after the CT (range 0-8 days). The mean volume of the prostate as defined with CT and MRI was 45cc and 39cc respectively (p=0.1). The MRI volume was never larger than the CT volume (mean difference 12%, range 1%-25%). The apex of the gland was visualised as lying more inferiorly on CT than MRI in three cases (range 2.5 - 7.5mm) and was the same in one case. There was no consistent effect of imaging on the visualisation of the antero-posterior location of the gland, which appeared to be more associated with the degree of rectal filling. Consequences to bladder and rectum dose volume histograms will be presented from the full cohort of patients. Conclusions: Standard clinical MR sequences obtained in radiotherapy treatment conditions have been processed to remove the effects of system and patient-based distortion. Early results are concordant with previous studies in suggesting that, as compared to MRI, CT overestimates the size of the prostate, and the inferior extent of the gland.
G.A. Cho, L.C. He//oway South West Sydney Cancer Service, Liverpool Cancer Therapy Centre, Sydney, Australia In current radiotherapy departments, acquisition of accurate CT patient anatomy information is of clinical importance. Registered artefacts in CT images used for radiotherapy treatment planning impact on the accuracy of dose calculations as well the Iocalisation of the treatment region and potentially the final clinical outcome. Limited data (both qualitative and quantitative) exist on the nature of the artefacts observed in CT images obtained using helical scanning protocols. For this reason images obtained with helical scanning have had limited use in our radiotherapy clinic due to concern about the impact of the possible artefact. A preliminary study has been performed to evaluate the difference between the two scanning methods in terms of possible artefacts introduced along the Z-axis. A well defined geometric object, a brachytherapy CT/MR gynaecological applicator with dummy sources of 10 mm separation, has been scanned using both spiral and sequential scanning protocols. A multislice CT scanner (Siemens Sensation 4) with slice collimation setting of 4 x 1 mm to produce CT data sets of an effective slice thickness of 1.0 mm for both scan protocols. A range of scan pitches between pitch factor of 0.5 to 1.0 were used for the helical scans. A 3D MPR image reconstruction algorithm, available in the scanning software was used to obtain a sagittal view of the source position of the reconstructed helical CT data. This image analysis tool was not available for the sequential scans. Both CT data sets were exported to a commercial 3-D radiotherapy Treatment Planning System (CMS FOCUS) to generate Digitally Reconstructed Radiographs (DRRs) which were compared with a plane film from a simulator. The helical CT images showed greater uncertainties on the DRR in terms of physical source position due to blurring of the dummy source objects. It is believed that the artefacts may be associated with broadening of the Slice Sensitivity Profile although apparent broadening has been observed only close to a pitch factor of 2. The same degree of degradation was not observed on sequential CT images. The resolution of DRR image quality was limited for both data sets. Higher helical pitch resulted in greater artefacts and further reduced image quality. 1090 poster An investigation of the dosimetric characteristics and clinical potential of the elekta beam modulator@
A. Glendinninq, I. Pate/, G. Shentall, M. Kirby Royal Preston Hospital, Radiotherapy Dept, Preston, United Kingdom Background and purpose: In recent years a number of mini-multileaf collimators (mMLCs) have been developed with leaf widths less than the standard l c m at isocentre. This finer spatial resolution makes them better suited to smaller target volumes. The Beam Modulator@ is such a device for Elekta linear accelerators which can be fully integrated into the normal dimensions of the radiation head. The: Beam Modulator@ has 40 leaves per leafbank, each projecting to a width of 0.4 cm at isocentre. The maximum fieldsize is 21 cm x 16 cm. There are no backup collimators, so fieldsizes in the non-MLC direction are defined by the
S458
sides of the leaves and increment in 0.4 cm steps. All leaves can interdigitate, so islands of radiation dose can be produced very easily. Those leaves not required for field shaping are 'closed' off to either side of the field beneath a set of fixed, outer diaphragms. This paper examines some of the dosimetric characteristics (for 6MV photons only) of the first production pilot Beam Modulator@, retrofitted to a standard Elekta SL15X linear accelerator. We also report on its clinical potential from a perspective of interfacing and planning with a commercial treatment planning system.
Materials and Methods: The following physical and dosimetric characteristics were investigated; (a) Leaf Bank Set-up (scaling, centring and alignment), (b) Leaf positioning (X-ray calibration, X-ray to light field coincidence, interleaf offsets and reproducibility), (c) Interleaf leakage and leaf transmission, (d) Leaf and shaped fields penumbra and (e) Percentage depth dose and relative output factors (ROPFs) for open fields. For (a), (c) and (d) X-Omat V film was used at isocentre and dmax (2 and 10 cm depth for the leakage and transmission measurements), analysed using Scanditronix-Wellhofer analysis software. All films were analysed in terms of absolute dose from a calibrated film scanner. (b) was examined using film, standard light field techniques and with a p-type diode in a plotting tank. Various head rotation and gantry angles were examined to test the stability of the system and the influence of gravity on interleaf leakage and leaf positioning reproducibility. (e) was measured using a standard small volume (0.06 cm 3) ion chamber in a water tank and miniphantom. The clinical potential was explored using Nucletron's PLATO ('Sunrise') treatment planning system. The physical design of the MLC was configured into the system in terms of leaf pitch, movement limits, etc.. Conformal and IMRT plans were simulated on both patient and Rando phantom based CT data. For IMRT, a sequencer was chosen allowing a combination of MLC interdigitation and fixed diaphragm modeling.
Results and discussion: Leafbank scaling and alignment were within 0.2 mm of nominal values; centring was 0.5 mm maximum from the isocentre. Leaf bank calibration was within 1 mm of nominal and the interleaf offsets were found to maintain their calibration throughout the test period. Leaf positioning reproducibility was found to be within 0.1 mm (1 std dev) with a range of < 0.5 mm. Maximum interleaf leakage was 1.7% and the average leaf transmission was < 1%. No significant differences were found with collimator or gantry angle variation. The leaf end penumbra (80-20% dose width) was between 4.5 and 5.0 mm across the full range of leaf travel, and between 2.7 and 3.5 mm for the leaf sides, across a full range of offset fields. The penumbra for shaped fields showed minimal stepping (compared to a standard 1 cm Elekta MLC) with an effective penumbra width of about 4.5 mm for a 450 edge. Percentage depth dose curves, inwater and mini-phantom ROPFs were all extremely similar to the previous machine data with asymmetric collimators. Differences of just over 1% were found only for fields > 16 cm eq. sq. Conformal and IMRT plans from PLATO could be transferred and delivered on the Beam Modulator@ very easily. Conformal plans showed improved conformation of isodoses around the target volume for all sites, when compared with standard l c m MLC plans. There was a significant improvement in penumbra and the DVHs for prostate and head and neck plans showed reduced OAR doses. Qualitative IMRT segment verification was easily performed using an EPID, with delivered segments comparing very well with the plan.
Posters
Conclusions: Overall the production pilot Beam Modulator@ showed very good dosimetric performance. The ease of data interfacing with a commercial TPS was impressive as was the potential clinical gains offered for conformal and IMRT plans. 1091 poster NTCP of the kidney: comparison of competing irradiation techniques of paraaortic lymph nodes M. Nevinny-Stickel, 7-. Seppi, K. Poljanc, B. Forthuber, P. Lukas Innsbruck Medical University, Austria, Dept. of RadiotherapyRadiooncology, Innsbruck, Austria
Introduction: Partial involvement of both kidneys is an unavoidable consequence for adequate dose delivery in paraaortic lymph node (PLN) irradiation. Depending on total dose ap/pa opposed fields or multi-field techniques are used. A comparison of four common irradiation techniques for the PLN areas on the basis of calculated NTCPs of co-irradiated kidneys will be presented in respect to an optimised treatment of potentially tumour affected PLN combined with tolerable kidney involvement. Materials and Methods: PTV delineation was performed in CT scans of 21 patients with a lateral safety margin of 3cm to the aorta and 2cm to the vena cava. Ventral and dorsal margins of the PTV were delineated 2cm ventral and dorsal of the vessels. PTV included > 97% of the potentially involved PLN. For each patient four hypothetical treatment plans were created: A) standard ap/pa opposed fields with lateral field margins along the tips of the transverse processes of the vertebral bodies B) individually planned ap/pa opposed fields with lateral field margins according to the PTV C) standard four-field box with lateral width as described for A, with dorsal borders at the center of the vertebral bodies and ventral margins 3cm in front of the vertebrae D) indivi-dually planned four-field box with lateral field margins according to the PTV.Calculation of radiation induced complication probability values for non uniform kidney irradiation were determined for model doses 50.4 Gy, 30.6 Gy and 19.8 Gy according to the Lyman-Wolbarst model (Statistics: U-Test by Mann-WhitneyWilcoxon). Results: Individually planned ap/pa opposed fields (B) are accompanied by the highest overall kidney NTCP rates (median 0.68, range 0.00 to 0.99). No statistically significant difference (P > 0.1) in the median overall kidney NTCP was observed among the other techniques (A: median 0.39, range 0.00 to 0.83, C: median 0.27, range 0.06 to 0.35 and D: 0.36, range 0.15 to 0.72). Conclusion: Regarding to expected overall moderate tissue complication probabilities in both kidneys with optimal inclusion (>97%) of potentially involved PLN the individually planned 4-box irradiation might be superior among the investigated techniques. 1092 poster Whole scalp irradiation using helical tomotherapy N. Orton, H. Jaradat, J. Welsh, W. Tom~
University of Wisconsin Hospital, Radiation Oncology, Madison, WI, U.S.A. Homogeneous irradiation of the scalp poses technical and dosimetric challenges due to the extensive, superficial curved treatment volume. Conventional treatments on a linear accelerator use multiple matched electron fields or a combination of electron and photon fields. Problems with this technique include dose inhomogeneity in the target due to
$457
with usual set-up controls on 200 patients treated last years. Inaccuracy values are inferior because there is smaller uncertainty in position, more accurate magnification and better image quality on CR instead of usual slab.
Further recruitment and analysis will evaluate the possible consequences of this in terms of predicted radiation dose to organs at risk.
Conclusions: CR digital images can be manipulated, analysed, fused and transmitted. CR can replace films for treatment characterization and verification, with many advantages: faster image elaboration, better image quality, quantitative analysis of patient position, electronical image transmission, efficient and economic manipulation of large amount of images. Future aim will be iMRT verification.
1089 poster Investigation of artefacts observed in helical CT images
1088 poster Distortion-corrected T2-weighted MRI: a comparison with CT for prostate contouring in radiotherapy planning
A.S.N. Jackson 1, S.A. Reinsberg 2, E.M. Moore 2, S.A. Sohaib ~, E.J. Adams 4, H.A. McNair~, D.P. Deamalej ~Royal Marsden Hospital and Institute of Cancer Research Academic Department of Radiotherapy and Oncology, Sutton, United Kingdom 2Royal Marsden Hospital and Institute of Cancer Research CRUK Clinical Magnetic Resonance Research Group, Sutton, United Kingdom 3Royal Marsden Hospital and Institute of Cancer Research Department of Radiology, Sutton, United Kingdom 4Royal Marsden Hospital and Institute of Cancer Research Joint Department of Physics, Sutton, United Kingdom 5Royal Marsden Hospital and Institute of Cancer Research Department of Radiotherapy, Sutton, United Kingdom Purpose: In this study T2-weighted distortion-corrected MRI images are compared with CT for prostate radiotherapy planning. Materials and Methods: The patient group consisted of men about to be treated with radical radiotherapy for prostate cancer and with no contra-indication to MRI scanning. Patients were scanned with CT (GE HiSpeed QX/i) and MRI (1.5T Philips Intera Gyroscan) using the same patient positioning and immobilisation technique. A novel flat couchtop has been designed for the MR system. Standard clinically available 3D T2-weighted MR images were corrected for both patient and system-based distortion, and images from both modalities transferred to a radiotherapy treatment planning workstation. The prostate and organs at risk were contoured on axial images firstly on CT, then on MRI. Image fusion was used to allow qualitative, as well as quantitative comparisons between image pairs. A total of fourteen patients will be imaged over the next 3 months, with results from the first four subjects described here. Results: The MRI scan took place a mean of 3 days after the CT (range 0-8 days). The mean volume of the prostate as defined with CT and MRI was 45cc and 39cc respectively (p=0.1). The MRI volume was never larger than the CT volume (mean difference 12%, range 1%-25%). The apex of the gland was visualised as lying more inferiorly on CT than MRI in three cases (range 2.5 - 7.5mm) and was the same in one case. There was no consistent effect of imaging on the visualisation of the antero-posterior location of the gland, which appeared to be more associated with the degree of rectal filling. Consequences to bladder and rectum dose volume histograms will be presented from the full cohort of patients. Conclusions: Standard clinical MR sequences obtained in radiotherapy treatment conditions have been processed to remove the effects of system and patient-based distortion. Early results are concordant with previous studies in suggesting that, as compared to MRI, CT overestimates the size of the prostate, and the inferior extent of the gland.
G.A. Cho, L.C. He//oway South West Sydney Cancer Service, Liverpool Cancer Therapy Centre, Sydney, Australia In current radiotherapy departments, acquisition of accurate CT patient anatomy information is of clinical importance. Registered artefacts in CT images used for radiotherapy treatment planning impact on the accuracy of dose calculations as well the Iocalisation of the treatment region and potentially the final clinical outcome. Limited data (both qualitative and quantitative) exist on the nature of the artefacts observed in CT images obtained using helical scanning protocols. For this reason images obtained with helical scanning have had limited use in our radiotherapy clinic due to concern about the impact of the possible artefact. A preliminary study has been performed to evaluate the difference between the two scanning methods in terms of possible artefacts introduced along the Z-axis. A well defined geometric object, a brachytherapy CT/MR gynaecological applicator with dummy sources of 10 mm separation, has been scanned using both spiral and sequential scanning protocols. A multislice CT scanner (Siemens Sensation 4) with slice collimation setting of 4 x 1 mm to produce CT data sets of an effective slice thickness of 1.0 mm for both scan protocols. A range of scan pitches between pitch factor of 0.5 to 1.0 were used for the helical scans. A 3D MPR image reconstruction algorithm, available in the scanning software was used to obtain a sagittal view of the source position of the reconstructed helical CT data. This image analysis tool was not available for the sequential scans. Both CT data sets were exported to a commercial 3-D radiotherapy Treatment Planning System (CMS FOCUS) to generate Digitally Reconstructed Radiographs (DRRs) which were compared with a plane film from a simulator. The helical CT images showed greater uncertainties on the DRR in terms of physical source position due to blurring of the dummy source objects. It is believed that the artefacts may be associated with broadening of the Slice Sensitivity Profile although apparent broadening has been observed only close to a pitch factor of 2. The same degree of degradation was not observed on sequential CT images. The resolution of DRR image quality was limited for both data sets. Higher helical pitch resulted in greater artefacts and further reduced image quality. 1090 poster An investigation of the dosimetric characteristics and clinical potential of the elekta beam modulator@
A. Glendinninq, I. Pate/, G. Shentall, M. Kirby Royal Preston Hospital, Radiotherapy Dept, Preston, United Kingdom Background and purpose: In recent years a number of mini-multileaf collimators (mMLCs) have been developed with leaf widths less than the standard l c m at isocentre. This finer spatial resolution makes them better suited to smaller target volumes. The Beam Modulator@ is such a device for Elekta linear accelerators which can be fully integrated into the normal dimensions of the radiation head. The: Beam Modulator@ has 40 leaves per leafbank, each projecting to a width of 0.4 cm at isocentre. The maximum fieldsize is 21 cm x 16 cm. There are no backup collimators, so fieldsizes in the non-MLC direction are defined by the
S458
sides of the leaves and increment in 0.4 cm steps. All leaves can interdigitate, so islands of radiation dose can be produced very easily. Those leaves not required for field shaping are 'closed' off to either side of the field beneath a set of fixed, outer diaphragms. This paper examines some of the dosimetric characteristics (for 6MV photons only) of the first production pilot Beam Modulator@, retrofitted to a standard Elekta SL15X linear accelerator. We also report on its clinical potential from a perspective of interfacing and planning with a commercial treatment planning system.
Materials and Methods: The following physical and dosimetric characteristics were investigated; (a) Leaf Bank Set-up (scaling, centring and alignment), (b) Leaf positioning (X-ray calibration, X-ray to light field coincidence, interleaf offsets and reproducibility), (c) Interleaf leakage and leaf transmission, (d) Leaf and shaped fields penumbra and (e) Percentage depth dose and relative output factors (ROPFs) for open fields. For (a), (c) and (d) X-Omat V film was used at isocentre and dmax (2 and 10 cm depth for the leakage and transmission measurements), analysed using Scanditronix-Wellhofer analysis software. All films were analysed in terms of absolute dose from a calibrated film scanner. (b) was examined using film, standard light field techniques and with a p-type diode in a plotting tank. Various head rotation and gantry angles were examined to test the stability of the system and the influence of gravity on interleaf leakage and leaf positioning reproducibility. (e) was measured using a standard small volume (0.06 cm 3) ion chamber in a water tank and miniphantom. The clinical potential was explored using Nucletron's PLATO ('Sunrise') treatment planning system. The physical design of the MLC was configured into the system in terms of leaf pitch, movement limits, etc.. Conformal and IMRT plans were simulated on both patient and Rando phantom based CT data. For IMRT, a sequencer was chosen allowing a combination of MLC interdigitation and fixed diaphragm modeling.
Results and discussion: Leafbank scaling and alignment were within 0.2 mm of nominal values; centring was 0.5 mm maximum from the isocentre. Leaf bank calibration was within 1 mm of nominal and the interleaf offsets were found to maintain their calibration throughout the test period. Leaf positioning reproducibility was found to be within 0.1 mm (1 std dev) with a range of < 0.5 mm. Maximum interleaf leakage was 1.7% and the average leaf transmission was < 1%. No significant differences were found with collimator or gantry angle variation. The leaf end penumbra (80-20% dose width) was between 4.5 and 5.0 mm across the full range of leaf travel, and between 2.7 and 3.5 mm for the leaf sides, across a full range of offset fields. The penumbra for shaped fields showed minimal stepping (compared to a standard 1 cm Elekta MLC) with an effective penumbra width of about 4.5 mm for a 450 edge. Percentage depth dose curves, inwater and mini-phantom ROPFs were all extremely similar to the previous machine data with asymmetric collimators. Differences of just over 1% were found only for fields > 16 cm eq. sq. Conformal and IMRT plans from PLATO could be transferred and delivered on the Beam Modulator@ very easily. Conformal plans showed improved conformation of isodoses around the target volume for all sites, when compared with standard l c m MLC plans. There was a significant improvement in penumbra and the DVHs for prostate and head and neck plans showed reduced OAR doses. Qualitative IMRT segment verification was easily performed using an EPID, with delivered segments comparing very well with the plan.
Posters
Conclusions: Overall the production pilot Beam Modulator@ showed very good dosimetric performance. The ease of data interfacing with a commercial TPS was impressive as was the potential clinical gains offered for conformal and IMRT plans. 1091 poster NTCP of the kidney: comparison of competing irradiation techniques of paraaortic lymph nodes M. Nevinny-Stickel, 7-. Seppi, K. Poljanc, B. Forthuber, P. Lukas Innsbruck Medical University, Austria, Dept. of RadiotherapyRadiooncology, Innsbruck, Austria
Introduction: Partial involvement of both kidneys is an unavoidable consequence for adequate dose delivery in paraaortic lymph node (PLN) irradiation. Depending on total dose ap/pa opposed fields or multi-field techniques are used. A comparison of four common irradiation techniques for the PLN areas on the basis of calculated NTCPs of co-irradiated kidneys will be presented in respect to an optimised treatment of potentially tumour affected PLN combined with tolerable kidney involvement. Materials and Methods: PTV delineation was performed in CT scans of 21 patients with a lateral safety margin of 3cm to the aorta and 2cm to the vena cava. Ventral and dorsal margins of the PTV were delineated 2cm ventral and dorsal of the vessels. PTV included > 97% of the potentially involved PLN. For each patient four hypothetical treatment plans were created: A) standard ap/pa opposed fields with lateral field margins along the tips of the transverse processes of the vertebral bodies B) individually planned ap/pa opposed fields with lateral field margins according to the PTV C) standard four-field box with lateral width as described for A, with dorsal borders at the center of the vertebral bodies and ventral margins 3cm in front of the vertebrae D) indivi-dually planned four-field box with lateral field margins according to the PTV.Calculation of radiation induced complication probability values for non uniform kidney irradiation were determined for model doses 50.4 Gy, 30.6 Gy and 19.8 Gy according to the Lyman-Wolbarst model (Statistics: U-Test by Mann-WhitneyWilcoxon). Results: Individually planned ap/pa opposed fields (B) are accompanied by the highest overall kidney NTCP rates (median 0.68, range 0.00 to 0.99). No statistically significant difference (P > 0.1) in the median overall kidney NTCP was observed among the other techniques (A: median 0.39, range 0.00 to 0.83, C: median 0.27, range 0.06 to 0.35 and D: 0.36, range 0.15 to 0.72). Conclusion: Regarding to expected overall moderate tissue complication probabilities in both kidneys with optimal inclusion (>97%) of potentially involved PLN the individually planned 4-box irradiation might be superior among the investigated techniques. 1092 poster Whole scalp irradiation using helical tomotherapy N. Orton, H. Jaradat, J. Welsh, W. Tom~
University of Wisconsin Hospital, Radiation Oncology, Madison, WI, U.S.A. Homogeneous irradiation of the scalp poses technical and dosimetric challenges due to the extensive, superficial curved treatment volume. Conventional treatments on a linear accelerator use multiple matched electron fields or a combination of electron and photon fields. Problems with this technique include dose inhomogeneity in the target due to