title: Online 3D EPID-based dose verification: Proof of concept.
abstract: OBJECTIVE
  Delivery errors during radiotherapy may lead to medical harm and reduced life expectancy for patients. Such serious incidents can be avoided by performing dose verification online, i.e., while the patient is being irradiated, creating the possibility of halting the linac in case of a large overdosage or underdosage. The offline EPID-based 3D in vivo dosimetry system clinically employed at our institute is in principle suited for online treatment verification, provided the system is able to complete 3D dose reconstruction and verification within 420 ms, the present acquisition time of a single EPID frame. It is the aim of this study to show that our EPID-based dosimetry system can be made fast enough to achieve online 3D in vivo dose verification.
  
  
  METHODS
  The current dose verification system was sped up in two ways. First, a new software package was developed to perform all computations that are not dependent on portal image acquisition separately, thus removing the need for doing these calculations in real time. Second, the 3D dose reconstruction algorithm was sped up via a new, multithreaded implementation. Dose verification was implemented by comparing planned with reconstructed 3D dose distributions delivered to two regions in a patient: the target volume and the nontarget volume receiving at least 10 cGy. In both volumes, the mean dose is compared, while in the nontarget volume, the near-maximum dose (D2) is compared as well. The real-time dosimetry system was tested by irradiating an anthropomorphic phantom with three VMAT plans: a 6 MV head-and-neck treatment plan, a 10 MV rectum treatment plan, and a 10 MV prostate treatment plan. In all plans, two types of serious delivery errors were introduced. The functionality of automatically halting the linac was also implemented and tested.
  
  
  RESULTS
  The precomputation time per treatment was ∼180 s/treatment arc, depending on gantry angle resolution. The complete processing of a single portal frame, including dose verification, took 266 ± 11 ms on a dual octocore Intel Xeon E5-2630 CPU running at 2.40 GHz. The introduced delivery errors were detected after 5-10 s irradiation time.
  
  
  CONCLUSIONS
  A prototype online 3D dose verification tool using portal imaging has been developed and successfully tested for two different kinds of gross delivery errors. Thus, online 3D dose verification has been technologically achieved.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Hanno
authors.initials: H
authors.lastname: Spreeuw
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Roel
authors.initials: R
authors.lastname: Rozendaal
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Igor
authors.initials: I
authors.lastname: Olaciregui-Ruiz
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Patrick
authors.initials: P
authors.lastname: González
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Anton
authors.initials: A
authors.lastname: Mans
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam 1066 CX, The Netherlands.
authors.forename: Ben
authors.initials: B
authors.lastname: Mijnheer
authors.affiliation: The University of Manchester, Manchester Academic Health Science Centre, The Christie NHS Foundation Trust, Manchester M20 4BX, United Kingdom.
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
mesh_terms: D001331:Automation; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods*; D008297:Male; D019300:Medical Errors / Q000517:prevention & control; D008953:Models, Anatomic; D058958:Organs at Risk; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012004:Rectal Neoplasms / Q000532:radiotherapy; D012984:Software; D013997:Time Factors