title: Impact of Spot Size and Beam-Shaping Devices on the Treatment Plan Quality for Pencil Beam Scanning Proton Therapy.
abstract: OBJECTIVE
  This study aimed to assess the clinical impact of spot size and the addition of apertures and range compensators on the treatment quality of pencil beam scanning (PBS) proton therapy and to define when PBS could improve on passive scattering proton therapy (PSPT).
  
  
  METHODS
  The patient cohort included 14 pediatric patients treated with PSPT. Six PBS plans were created and optimized for each patient using 3 spot sizes (∼12-, 5.4-, and 2.5-mm median sigma at isocenter for 90- to 230-MeV range) and adding apertures and compensators to plans with the 2 larger spots. Conformity and homogeneity indices, dose-volume histogram parameters, equivalent uniform dose (EUD), normal tissue complication probability (NTCP), and integral dose were quantified and compared with the respective PSPT plans.
  
  
  RESULTS
  The results clearly indicated that PBS with the largest spots does not necessarily offer a dosimetric or clinical advantage over PSPT. With comparable target coverage, the mean dose (Dmean) to healthy organs was on average 6.3% larger than PSPT when using this spot size. However, adding apertures to plans with large spots improved the treatment quality by decreasing the average Dmean and EUD by up to 8.6% and 3.2% of the prescribed dose, respectively. Decreasing the spot size further improved all plans, lowering the average Dmean and EUD by up to 11.6% and 10.9% compared with PSPT, respectively, and eliminated the need for beam-shaping devices. The NTCP decreased with spot size and addition of apertures, with maximum reduction of 5.4% relative to PSPT.
  
  
  CONCLUSIONS
  The added benefit of using PBS strongly depends on the delivery configurations. Facilities limited to large spot sizes (>∼8 mm median sigma at isocenter) are recommended to use apertures to reduce treatment-related toxicities, at least for complex and/or small tumors.
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts. Electronic address: mmoteabbed@partners.org.
authors.forename: Maryam
authors.initials: M
authors.lastname: Moteabbed
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Torunn I
authors.initials: TI
authors.lastname: Yock
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Depauw
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Madden
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Hanne M
authors.initials: HM
authors.lastname: Kooy
authors.affiliation: Radiation Oncology Department, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Harald
authors.initials: H
authors.lastname: Paganetti
mesh_terms: D016543:Central Nervous System Neoplasms / Q000532:radiotherapy; D002648:Child; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy*; D016011:Normal Distribution; D059351:Organ Sparing Treatments / Q000295:instrumentation / Q000379:methods; D058958:Organs at Risk / Q000528:radiation effects*; D010386:Pelvic Neoplasms / Q000532:radiotherapy; D061766:Proton Therapy / Q000295:instrumentation / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D013899:Thoracic Neoplasms / Q000532:radiotherapy