Nonrigid Image Registration for Head and Neck Cancer Radiotherapy Treatment Planning With PET/CT

Abstract

Purpose: Head and neck radiotherapy planning with positron emission tomography/computed tomography (PET/CT) requires the images to be reliably registered with treatment planning CT. Acquiring PET/CT in treatment position is problematic, and in practice for some patients it may be beneficial to use diagnostic PET/CT for radiotherapy planning. Therefore, the aim of this study was first to quantify the image registration accuracy of PET/CT to radiotherapy CT and, second, to assess whether PET/CT acquired in diagnostic position can be registered to planning CT.Methods and Materials: Positron emission tomography/CT acquired in diagnostic and treatment position for five patients with head and neck cancer was registered to radiotherapy planning CT using both rigid and nonrigid image registration. The root mean squared error for each method was calculated from a set of anatomic landmarks marked by four independent observers.Results: Nonrigid and rigid registration errors for treatment position PET/CT to planning CT were 2.77 ± 0.80 mm and 4.96 ± 2.38 mm, respectively, p = 0.001. Applying the nonrigid registration to diagnostic position PET/CT produced a more accurate match to the planning CT than rigid registration of treatment position PET/CT (3.20 ± 1.22 mm and 4.96 ± 2.38 mm, respectively, p = 0.012).Conclusions: Nonrigid registration provides a more accurate registration of head and neck PET/CT to treatment planning CT than rigid registration. In addition, nonrigid registration of PET/CT acquired with patients in a standardized, diagnostic position can provide images registered to planning CT with greater accuracy than a rigid registration of PET/CT images acquired in treatment position. This may allow greater flexibility in the timing of PET/CT for head and neck cancer patients due to undergo radiotherapy. Purpose: Head and neck radiotherapy planning with positron emission tomography/computed tomography (PET/CT) requires the images to be reliably registered with treatment planning CT. Acquiring PET/CT in treatment position is problematic, and in practice for some patients it may be beneficial to use diagnostic PET/CT for radiotherapy planning. Therefore, the aim of this study was first to quantify the image registration accuracy of PET/CT to radiotherapy CT and, second, to assess whether PET/CT acquired in diagnostic position can be registered to planning CT. Methods and Materials: Positron emission tomography/CT acquired in diagnostic and treatment position for five patients with head and neck cancer was registered to radiotherapy planning CT using both rigid and nonrigid image registration. The root mean squared error for each method was calculated from a set of anatomic landmarks marked by four independent observers. Results: Nonrigid and rigid registration errors for treatment position PET/CT to planning CT were 2.77 ± 0.80 mm and 4.96 ± 2.38 mm, respectively, p = 0.001. Applying the nonrigid registration to diagnostic position PET/CT produced a more accurate match to the planning CT than rigid registration of treatment position PET/CT (3.20 ± 1.22 mm and 4.96 ± 2.38 mm, respectively, p = 0.012). Conclusions: Nonrigid registration provides a more accurate registration of head and neck PET/CT to treatment planning CT than rigid registration. In addition, nonrigid registration of PET/CT acquired with patients in a standardized, diagnostic position can provide images registered to planning CT with greater accuracy than a rigid registration of PET/CT images acquired in treatment position. This may allow greater flexibility in the timing of PET/CT for head and neck cancer patients due to undergo radiotherapy. Improved availability of functional imaging such as positron emission tomography (PET) and the limitations of morphological computed tomography (CT) and magnetic resonance imaging (MRI) have generated increased interest in the role of PET in the management of head and neck cancer. In particular, the additional metabolic information about the primary tumor and lymph nodes may be useful when planning radiotherapy treatment (1Wang D. Schultz C.J. Jursinic P.A. et al.Initial experience of FDG-PET/CT guided IMRT of head-and-neck carcinoma.Int J Radiat Oncol Biol Phys. 2006; 65: 143-151Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar, 2Paulino A.C. Koshy M. Howell R. et al.Comparison of CT- and FDG-PET-defined gross tumor volume in intensity-modulated radiotherapy for head-and-neck cancer.Int J Radiat Oncol Biol Phys. 2005; 61: 1385-1392Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar, 3Grosu A.L. Piert M. Weber W.A. et al.Positron emission tomography for radiation treatment planning.Strahlenther Onkol. 2005; 181: 483-499Crossref PubMed Scopus (177) Google Scholar). Ideally patients would have virtual simulation performed on a dedicated PET/CT scanner for treatment planning. The main advantage is that both PET and CT images are acquired sequentially during a single imaging session, which minimizes patient movement and provides inherently registered images, unless there is obvious patient movement. Although a small number of centers may have this facility (4Heron D.E. Andrade R.S. Flickinger J. et al.Hybrid PET-CT simulation for radiation treatment planning in head-and-neck cancers: A brief technical report.Int J Radiat Oncol Biol Phys. 2004; 60: 1419-1424Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar, 5Ciernik I.F. Dizendorf E. Baumert B.G. et al.Radiation treatment planning with an integrated positron emission and computer tomography (PET/CT): A feasibility study.Int J Radiat Oncol Biol Phys. 2003; 57: 853-863Abstract Full Text Full Text PDF PubMed Scopus (473) Google Scholar), in practice it is likely that separate PET/CT and treatment planning CT will remain more common for the majority of patients for whom PET imaging is an option (2Paulino A.C. Koshy M. Howell R. et al.Comparison of CT- and FDG-PET-defined gross tumor volume in intensity-modulated radiotherapy for head-and-neck cancer.Int J Radiat Oncol Biol Phys. 2005; 61: 1385-1392Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar, 6Lavely W.C. Scarfone C. Cevikalp H. et al.Phantom validation of coregistration of PET and CT for image-guided radiotherapy.Med Phys. 2004; 31: 1083-1092Crossref PubMed Scopus (51) Google Scholar). Even when a PET/CT system is used to acquire the treatment-planning CT, image registration may still be required (1Wang D. Schultz C.J. Jursinic P.A. et al.Initial experience of FDG-PET/CT guided IMRT of head-and-neck carcinoma.Int J Radiat Oncol Biol Phys. 2006; 65: 143-151Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar). A number of methods have been applied to the problem of head and neck PET to planning CT image registration, including the use of fiducial markers (1Wang D. Schultz C.J. Jursinic P.A. et al.Initial experience of FDG-PET/CT guided IMRT of head-and-neck carcinoma.Int J Radiat Oncol Biol Phys. 2006; 65: 143-151Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar, 6Lavely W.C. Scarfone C. Cevikalp H. et al.Phantom validation of coregistration of PET and CT for image-guided radiotherapy.Med Phys. 2004; 31: 1083-1092Crossref PubMed Scopus (51) Google Scholar) and registration via the transmission PET (7Scarfone C. Lavely W.C. Cmelak A.J. et al.Prospective feasibility trial of radiotherapy target definition for head and neck cancer using 3-dimensional PET and CT imaging.J Nucl Med. 2004; 45: 543-552PubMed Google Scholar, 8Klabbers B.M. de Munck J.C. Slotman B.J. et al.Matching PET and CT scans of the head and neck area: Development of method and validation.Med Phys. 2002; 29: 2230-2238Crossref PubMed Scopus (32) Google Scholar). Most applications have involved either manual–interactive registration (2Paulino A.C. Koshy M. Howell R. et al.Comparison of CT- and FDG-PET-defined gross tumor volume in intensity-modulated radiotherapy for head-and-neck cancer.Int J Radiat Oncol Biol Phys. 2005; 61: 1385-1392Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar, 9Daisne J.F. Sibomana M. Bol A. et al.Evaluation of a multimodality image (CT, MRI and PET) coregistration procedure on phantom and head and neck cancer patients: Accuracy, reproducibility and consistency.Radiother Oncol. 2003; 69: 237-245Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar) or automatic rigid registration (8Klabbers B.M. de Munck J.C. Slotman B.J. et al.Matching PET and CT scans of the head and neck area: Development of method and validation.Med Phys. 2002; 29: 2230-2238Crossref PubMed Scopus (32) Google Scholar), whereas Schwartz et al. applied a nonrigid algorithm (10Schwartz D.L. Ford E. Rajendran J. et al.FDG-PET/CT imaging for preradiotherapy staging of head-and-neck squamous cell carcinoma.Int J Radiat Oncol Biol Phys. 2005; 61: 129-136Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar, 11Schwartz D.L. Ford E.C. Rajendran J. et al.FDG-PET/CT-guided intensity modulated head and neck radiotherapy: A pilot investigation.Head Neck. 2005; 27: 478-487Crossref PubMed Scopus (112) Google Scholar). However, when a PET/CT system is used, the availability of the CT acquired during the same imaging session as the PET provides the opportunity to improve the registration accuracy of PET to radiotherapy planning CT. In this article, automatic methods, including intensity-based nonrigid algorithms, are used to first register the attenuation correction CT to treatment planning CT. Then, because PET is assumed to be “hardware” registered to the attenuation correction CT, the derived CT to CT image transformation is applied directly to the PET to provide the necessary PET to planning CT registration. For the functional data to be used in head and neck treatment planning, it is necessary to estimate the accuracy of the PET/CT to treatment planning CT image registration and to assess the impact of nonrigid deformations (8Klabbers B.M. de Munck J.C. Slotman B.J. et al.Matching PET and CT scans of the head and neck area: Development of method and validation.Med Phys. 2002; 29: 2230-2238Crossref PubMed Scopus (32) Google Scholar, 9Daisne J.F. Sibomana M. Bol A. et al.Evaluation of a multimodality image (CT, MRI and PET) coregistration procedure on phantom and head and neck cancer patients: Accuracy, reproducibility and consistency.Radiother Oncol. 2003; 69: 237-245Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar). Therefore, the first original contribution of our study was to quantify the accuracy of both rigid and nonrigid image registration of CT acquired during a PET imaging session to CT acquired for radiotherapy treatment planning. When PET is used for radiotherapy target volume delineation, Goerres et al. (12Goerres G.W. von Schulthess G.K. Steinert H.C. Why most PET of lung and head-and-neck cancer will be PET/CT.J Nucl Med. 2004; 45: 66S-71SPubMed Google Scholar) proposed that patients should be positioned in the exact treatment position. However, this requires custom-made immobilization to be available early enough for the PET/CT to be acquired before the start of patient treatment. Because of the significant costs and logistical problems involved, it is unlikely that many patients would undergo both a staging and dedicated treatment planning PET/CT as part of their routine patient management. Furthermore, significant differences can occur between a staging and treatment planning PET (13Stewart D. Eakin R. McAleese J. et al.The use of PET-CT for treatment planning after induction chemotherapy in non–small cell lung cancer may lead to geographical and marginal miss.Radiother Oncol. 2006; 81 ([abstract]): S398Google Scholar). Therefore, a decision must be made as to the most useful timing of PET/CT imaging in the patient pathway (Table 1). When PET/CT is used early in the diagnostic and staging phase of patient management, nodal involvement can be established early, but treatment planning with PET/CT is complicated by the lack of custom immobilization. In contrast, if PET is used after a decision has been made to proceed with radiotherapy, the potential for assessing nodal involvement remains, but the data are available much later in the patient management with the further disadvantage that a flatbed and custom immobilization may be required.Table 1Comparison of diagnostic/staging PET and treatment planning PETDiagnostic/ staging PETTreatment planning PETIdentification of nodesYesYesTiming during patient managementEarlyLateTime to assess treatment optionsYesLimitedCustom immobilizationNoYesFlat bedNoYesAbbreviation: PET = positron emission tomography. Open table in a new tab Abbreviation: PET = positron emission tomography. In practice, it would be useful to obtain the PET/CT earlier as part of the diagnostic and staging workup for the patients but also to be able to use the information for radiotherapy planning. This would require an accurate and reliable method of image registration that can accommodate the nonrigid deformations that can occur in the head and neck region. Therefore, the second original contribution of this study was to investigate the feasibility of using a standard method of immobilization in conjunction with nonrigid registration to achieve the desired PET/CT to planning CT image fusion. Five patients with head and neck cancer underwent PET/CT in addition to conventional X-ray CT for radical radiotherapy treatment planning. All patients gave written informed consent to participate, and the study was approved by the Local Research Ethics Committee. Positron emission tomography 18-F fluorodeoxyglucose (FDG) imaging was conducted on a Millenium VG Hawkeye dual headed gamma camera capable of coincidence detection (GE Healthcare, Chalfont St. Giles, Bucks, UK), with a 40 cm field of view (FOV) extending from the top of the skull to the upper thorax. Following PET acquisition and with the patient in the same position, CT for attenuation correction and image registration was acquired using an integrated, low-dose Hawkeye CT system. The Hawkeye CT was acquired over 10 minutes and consisted of 256 × 256 pixels, with pixel size 2 mm and slice thickness 10 mm. To address the question of whether PET/CT acquired in diagnostic position can subsequently be registered to treatment planning CT, patients underwent repeated PET/CT in two setup positions. First, patients were imaged with their custom-made immobilization shell attached to a modified base plate that was placed on a flat bed. The upper half of the shell was adapted for improved patient comfort around the mouth and eyes while maintaining suitable immobilization of the neck during the 40-min imaging procedure. Second, after an interval of a few minutes, each patient underwent a repeated PET/CT with a Repovac vacuum cushion (Sinmed Radiotherapy Products, Reeuwijk, The Netherlands) and standard-sized headrest on a conventional diagnostic curved bed. Radiotherapy planning CT was performed with patients on a flat bed wearing a full custom-made immobilization shell. Images were acquired at 512 × 512 pixels with pixel size determined by the field of view (FOV). The first two subjects were imaged on a PQS CT (Philips Medical Systems, Eindhoven, The Netherlands) (slice thickness 3 mm, FOV 48 cm), and the other 3 patients were scanned on a CT (slice thickness mm, two patients at FOV patient at FOV Hawkeye CT was registered to treatment planning CT using both rigid and nonrigid The nonrigid method of images using image and simulation 2005; 29: PubMed Scopus Google Scholar) provides a and a to that the transformation between the two images is and The nonrigid is on a of The between is a The of the between is by The of the at the of the are by a the of is the or image is and the after a with a has been applied to the is the is The first is a of of intensity between the two images, and the second is a that the by the to be is a second The is by the is that the of of intensity differences can be to differences in intensity between the registered However, and of images using image and simulation 2005; 29: PubMed Scopus Google Scholar) have a intensity correction can be the that for such differences and the of the registration algorithm to The of J can be in of the of a set of Although the set is the is and an can be In practice, images were first to the immobilization when The planning CT was to 256 × 256 pixels for nonrigid registration. The Hawkeye CT was and to match the of the planning CT. All was performed using and image were to × × was performed in 1). First, images were to a position The exact of this impact on the automatic registration Second, images were and using a rigid the of which was used as the for nonrigid registration the nonrigid registration of images using image and simulation 2005; 29: PubMed Scopus Google Scholar) was with a which the in the images and the as the The rigid and nonrigid procedure an of in using a with that CT in was used by four to set five anatomic landmarks of their on both and registered CT. The between the marked on the two image was calculated as the root mean squared error D. C. et setup error using intensity-based image registration J Radiat Oncol Biol Phys. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). and two at CT images, anatomic landmarks on the study was with with the on both were which the of the of the of the and the top of the error and reproducibility was by landmarks on two head and neck CT with a registered data set at a rigid registration The Hawkeye CT is acquired at a to the planning CT. Therefore, the of CT on error was by using four CT with registered data with a registration marked anatomic landmarks with registered at the same as the CT Second, the was repeated for the same registered that been to the of Hawkeye CT. For four registration accuracy was for the and to assess the impact of on To the accuracy of the rigid and nonrigid registration methods, both methods were applied to register Hawkeye CT to a treatment planning CT. All data are as mean ± standard and was conducted with the Five patients written informed consent and the All patients head and neck cancer. All patients the PET/CT imaging the of the procedure. significant movement were on images acquired with either the custom-made or standard immobilization For the four data error calculated from anatomic landmarks was ± mm. the of landmarks with the registered data to the of the Hawkeye CT increased the error to ± 1.22 mm = reproducibility was ± mm for the data and ± for the For patients set in treatment position (Table the error for rigid registration was 4.96 ± 2.38 mm, with 2.77 ± 0.80 mm for nonrigid registration = For patients set in diagnostic position, the error for rigid registration was ± mm while for nonrigid registration the error was ± 1.22 mm of registration accuracy ± ± = ± ± Open table in a new tab Applying the nonrigid registration to diagnostic position PET/CT produced a more accurate match to the planning CT than rigid registration of treatment position PET/CT (3.20 ± 1.22 mm and 4.96 ± 2.38 mm, respectively, p = 0.012). have rigid registration in with patient immobilization. For et al. E. et and of computer computed tomography and positron emission tomography with head and neck J Full Text PDF PubMed Scopus Google Scholar) a mean mm error of anatomic landmarks when assessing rigid registration accuracy of PET to CT acquired with patients Lavely et al. W.C. Scarfone C. Cevikalp H. et al.Phantom validation of coregistration of PET and CT for image-guided radiotherapy.Med Phys. 2004; 31: 1083-1092Crossref PubMed Scopus (51) Google Scholar) performed phantom validation of a information rigid algorithm for head and neck cancer and applied the method to a single patient PET/CT with mean error of mm. et al. J.F. Sibomana M. Bol A. et al.Evaluation of a multimodality image (CT, MRI and PET) coregistration procedure on phantom and head and neck cancer patients: Accuracy, reproducibility and consistency.Radiother Oncol. 2003; 69: 237-245Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar) rigid registration of four patients been and registration errors mm. The impact of immobilization was by et al. (8Klabbers B.M. de Munck J.C. Slotman B.J. et al.Matching PET and CT scans of the head and neck area: Development of method and validation.Med Phys. 2002; 29: 2230-2238Crossref PubMed Scopus (32) Google Scholar), five patients and two patients were scanned a to assess the impact of nonrigid rigid registration errors were the with a mean of mm with immobilization. To for such nonrigid registration has been used by Schwartz et al. (10Schwartz D.L. Ford E. Rajendran J. et al.FDG-PET/CT imaging for preradiotherapy staging of head-and-neck squamous cell carcinoma.Int J Radiat Oncol Biol Phys. 2005; 61: 129-136Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar, 11Schwartz D.L. Ford E.C. Rajendran J. et al.FDG-PET/CT-guided intensity modulated head and neck radiotherapy: A pilot investigation.Head Neck. 2005; 27: 478-487Crossref PubMed Scopus (112) Google Scholar), applied the algorithm to register head and neck PET to CT that been conducted with radiotherapy immobilization. However, the registration accuracy was in this the first aim of our study was to quantify the accuracy of a nonrigid image registration algorithm for CT acquired during a PET imaging session to CT acquired for head and neck radiotherapy treatment planning. first established accuracy with a set of CT anatomic landmarks were used to assess the PET/CT to planning CT registration The in the that for the set of patients the nonrigid algorithm the registration error with rigid registration for PET/CT acquired in treatment position. In addition to the registration the second original of our is the of head and neck PET/CT acquired in both diagnostic and treatment position on the same Because the attenuation correction CT is acquired during the same imaging session as intensity-based nonrigid registration can be used to register this CT to CT. The calculated image transformation can be applied to the PET to provide the PET to planning CT image registration. This procedure has to investigate whether nonrigid registration can for the lack of radiotherapy immobilization. The in the that for PET/CT acquired in diagnostic position, the nonrigid registration provides registration accuracy the mm proposed by et al. (2Paulino A.C. Koshy M. Howell R. et al.Comparison of CT- and FDG-PET-defined gross tumor volume in intensity-modulated radiotherapy for head-and-neck cancer.Int J Radiat Oncol Biol Phys. 2005; 61: 1385-1392Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar) and is than rigid registration of treatment position PET/CT to planning CT. Although this study that custom-made immobilization is required for accurate PET/CT to planning CT image registration, must be to acquire the diagnostic images with an of immobilization that the neck is at an to treatment position. In our diagnostic position images were acquired with a standard-sized headrest and a vacuum cushion head The advantage of using this of immobilization in conjunction with nonrigid image registration is that PET/CT can be used in treatment planning custom-made immobilization has been used during the acquisition of the functional should also be that the of the image transformation derived from the CT to CT registration on the PET/CT However, because the PET is acquired over many minutes, additional must be made to the of this and, an estimate of PET to attenuation correction CT should be et the use of to in the head and neck in PET/CT Nucl Med. 2005; Google Scholar). For the system used in this the PET to CT registration is to be mm in the of registration accuracy is a problem D. H. et image registration in the using 2003; PubMed Scopus Google Scholar). in a on the role of nonrigid registration in treatment planning R. et 3-dimensional registration of PET and CT from separate or Nucl Med. 2005; Google Scholar), in this study anatomic landmarks are used to provide a of registration of the limitations of using anatomic landmarks over a error is that the landmarks provide the error at and of in that the is made in conjunction with a of image registration the method provides a method of registration CT the Hawkeye CT system used in this study is an integrated, low-dose system that provides images with than PET/CT The impact of CT is on error and image registration accuracy is also if the planning CT and PET/CT image is This is because the registration algorithm is an intensity-based method that when the two of images have been acquired at the same Therefore, it is likely that using PET/CT acquired at the same as planning CT would further improve both registration accuracy and accuracy of registration In this CT has been used for the treatment planning CT as is used at our for this M. R. et accuracy in head and and J 2006; PubMed Scopus Google Scholar). The use of planning CT have an impact on the accuracy of the image registration as would be used for the attenuation correction CT acquired for may be to this problem by the on the CT et al.The of on intensity-modulated radiation for head and neck J Oncol. 2005; PubMed Scopus Google Scholar). For patients with head and neck this study provides that a nonrigid algorithm can provide a more accurate registration of PET/CT to radiotherapy planning CT than a rigid In addition, nonrigid registration of PET/CT acquired with the patient in a standardized, diagnostic position can provide images registered to planning CT with greater accuracy than a rigid registration of PET/CT images acquired in treatment position. For suitable this may a staging than a treatment position to be used in radiotherapy treatment planning. to the image registration of PET/CT to treatment planning CT can be with improved PET/CT image and by further of the nonrigid registration