title: Effect of inhomogeneity in a patient's body on the accuracy of the pencil beam algorithm in comparison to Monte Carlo.
abstract: The pencil beam algorithm (PBA) is reasonably accurate and fast. It is, therefore, the primary method used in routine clinical treatment planning for proton radiotherapy; still, it needs to be validated for use in highly inhomogeneous regions. In our investigation of the effect of patient inhomogeneity, PBA was compared with Monte Carlo (MC). A software framework was developed for the MC simulation of radiotherapy based on Geant4. Anatomical sites selected for the comparison were the head/neck, liver, lung and pelvis region. The dose distributions calculated by the two methods in selected examples were compared, as well as a dose volume histogram (DVH) derived from the dose distributions. The comparison of the off-center ratio (OCR) at the iso-center showed good agreement between the PBA and MC, while discrepancies were seen around the distal fall-off regions. While MC showed a fine structure on the OCR in the distal fall-off region, the PBA showed smoother distribution. The fine structures in MC calculation appeared downstream of very low-density regions. Comparison of DVHs showed that most of the target volumes were similarly covered, while some OARs located around the distal region received a higher dose when calculated by MC than the PBA.
authors.affiliation: Hyogo Ion Beam Medical Center, Shingu-cho, Tatsuno, Hyogo, Japan. t.yamashita@ham-progress.jp
authors.forename: T
authors.initials: T
authors.lastname: Yamashita
authors.forename: T
authors.initials: T
authors.lastname: Akagi
authors.forename: T
authors.initials: T
authors.lastname: Aso
authors.forename: A
authors.initials: A
authors.lastname: Kimura
authors.forename: T
authors.initials: T
authors.lastname: Sasaki
mesh_terms: D000465:Algorithms; D006801:Humans; D009010:Monte Carlo Method; D058958:Organs at Risk / Q000528:radiation effects; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*