title: Evaluation of deformable image registration between external beam radiotherapy and HDR brachytherapy for cervical cancer with a 3D-printed deformable pelvis phantom.
abstract: OBJECTIVE
  In this study, we developed a 3D-printed deformable pelvis phantom for evaluating spatial DIR accuracy. We then evaluated the spatial DIR accuracies of various DIR settings for cervical cancer.
  
  
  METHODS
  A deformable female pelvis phantom was created based on patient CT data using 3D printing. To create the deformable uterus phantom, we first 3D printed both a model of uterus and a model of the internal cavities of the vagina and uterus. We then made a mold using the 3D printed uterus phantom. Finally, urethane was poured into the mold with the model of the internal cavities in place, creating the deformable uterus phantom with a cavity into which an applicator could be inserted. To create the deformable bladder phantom, we first 3D printed models of the bladder and of the same bladder scaled down by 2 mm. We then made a mold using the larger bladder model. Finally, silicone was poured into the mold with the smaller bladder model in place to create the deformable bladder phantom with a wall thickness of 2 mm. To emulate the anatomical bladder, water was poured into the created bladder. We acquired phantom image without applicator for EBRT. Then, we inserted the applicator into the phantom to simulate BT. In this situation, we scanned the phantom again to obtain the phantom image for BT. We performed DIR using the two phantom images in two cases: Case A, with full bladder (170 ml) in both EBRT and BT images; and Case B with full bladder in the BT image and half-full bladder (100 ml) in the EBRT image. DIR was evaluated using Dice similarity coefficients (DSCs) and 31 landmarks for the uterus and 25 landmarks for the bladder. A hybrid intensity and structure DIR algorithm implemented in RayStation with four DIR settings was evaluated.
  
  
  RESULTS
  On visual inspection, reasonable agreement in shape of the uterus between the phantom and patient CT images was observed for both EBRT and BT, although some regional disagreements in shape of the bladder and rectum were apparent. The created phantom could reproduce the actual patient's uterus deformation by the applicator. For both Case A and B, large variation was seen in landmark error among the four DIR parameters. In addition, although DSCs were comparable, moderate differences in landmark error existed between the two different DIR parameters selected from the four DIR parameters (i.e., DSC = 0.96, landmark error = 13.2 ± 5.7 mm vs. DSC = 0.97, landmark error = 9.7 ± 4.0 mm). This result suggests that landmark error evaluation might thus be more effective than DSC for evaluating DIR accuracy.
  
  
  CONCLUSIONS
  Our developed phantom enabled the evaluation of spatial DIR accuracy for the female pelvic region for the first time. Although the DSCs are high, the spatial errors can still be significant and our developed phantom facilitates their quantification. Our results showed that optimization is needed to identify suitable DIR settings. For determining suitable DIR settings, our method of evaluating spatial DIR accuracy using the 3D-printed phantom may prove helpful.
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Kadoya
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Yuya
authors.initials: Y
authors.lastname: Miyasaka
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Yujiro
authors.initials: Y
authors.lastname: Nakajima
authors.affiliation: Department of Bioengineering, Graduate School of Engineering Science, Osaka University, Osaka, 565-0871, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Kuroda
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Kengo
authors.initials: K
authors.lastname: Ito
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Mizuki
authors.initials: M
authors.lastname: Chiba
authors.affiliation: Department of Radiology, Tohoku University Hospital, Sendai, 980-8574, Japan.
authors.forename: Kiyokazu
authors.initials: K
authors.lastname: Sato
authors.affiliation: Department of Therapeutic Radiology, Tohoku University School of Medicine, Miyagi, 980-8574, Japan.
authors.forename: Suguru
authors.initials: S
authors.lastname: Dobashi
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Takaya
authors.initials: T
authors.lastname: Yamamoto
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Noriyoshi
authors.initials: N
authors.lastname: Takahashi
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Kubozono
authors.affiliation: Department of Therapeutic Radiology, Tohoku University School of Medicine, Miyagi, 980-8574, Japan.
authors.forename: Ken
authors.initials: K
authors.lastname: Takeda
authors.affiliation: Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, 980-8574, Japan.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Jingu
mesh_terms: D001918:Brachytherapy; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation*; D010388:Pelvis / Q000000981:diagnostic imaging*; D019047:Phantoms, Imaging; D066330:Printing, Three-Dimensional; D011879:Radiotherapy Dosage; D014057:Tomography, X-Ray Computed; D002583:Uterine Cervical Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*
keywords: 3D printer; cervical cancer; deformable image registration; dose accumulation; radiotherapy