title: Using a reduced spot size for intensity-modulated proton therapy potentially improves salivary gland-sparing in oropharyngeal cancer.
abstract: OBJECTIVE
  To investigate whether intensity-modulated proton therapy with a reduced spot size (rsIMPT) could further reduce the parotid and submandibular gland dose compared with previously calculated IMPT plans with a larger spot size. In addition, it was investigated whether the obtained dose reductions would theoretically translate into a reduction of normal tissue complication probabilities (NTCPs).
  
  
  METHODS
  Ten patients with N0 oropharyngeal cancer were included in a comparative treatment planning study. Both IMPT plans delivered simultaneously 70 Gy to the boost planning target volume (PTV) and 54 Gy to the elective nodal PTV. IMPT and rsIMPT used identical three-field beam arrangements. In the IMPT plans, the parotid and submandibular salivary glands were spared as much as possible. rsIMPT plans used identical dose-volume objectives for the parotid glands as those used by the IMPT plans, whereas the objectives for the submandibular glands were tightened further. NTCPs were calculated for salivary dysfunction and xerostomia.
  
  
  RESULTS
  Target coverage was similar for both IMPT techniques, whereas rsIMPT clearly improved target conformity. The mean doses in the parotid glands and submandibular glands were significantly lower for three-field rsIMPT (14.7 Gy and 46.9 Gy, respectively) than for three-field IMPT (16.8 Gy and 54.6 Gy, respectively). Hence, rsIMPT significantly reduced the NTCP of patient-rated xerostomia and parotid and contralateral submandibular salivary flow dysfunction (27%, 17%, and 43% respectively) compared with IMPT (39%, 20%, and 79%, respectively). In addition, mean dose values in the sublingual glands, the soft palate and oral cavity were also decreased. Obtained dose and NTCP reductions varied per patient.
  
  
  CONCLUSIONS
  rsIMPT improved sparing of the salivary glands and reduced NTCP for xerostomia and parotid and submandibular salivary dysfunction, while maintaining similar target coverage results. It is expected that rsIMPT improves quality of life during and after radiotherapy treatment.
authors.affiliation: Department of Radiation Oncology, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands. t.a.van.de.water@rt.umcg.nl
authors.forename: Tara A
authors.initials: TA
authors.lastname: van de Water
authors.forename: Antony J
authors.initials: AJ
authors.lastname: Lomax
authors.forename: Hendrik P
authors.initials: HP
authors.lastname: Bijl
authors.forename: Cornelis
authors.initials: C
authors.lastname: Schilstra
authors.forename: Eugen B
authors.initials: EB
authors.lastname: Hug
authors.forename: Johannes A
authors.initials: JA
authors.lastname: Langendijk
mesh_terms: D002294:Carcinoma, Squamous Cell / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D009055:Mouth / Q000528:radiation effects; D059351:Organ Sparing Treatments / Q000379:methods*; D058958:Organs at Risk / Q000528:radiation effects; D009959:Oropharyngeal Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D010160:Palate, Soft / Q000528:radiation effects; D010306:Parotid Gland / Q000528:radiation effects; D011336:Probability; D061766:Proton Therapy; D011832:Radiation Injuries / Q000517:prevention & control*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012469:Salivary Glands / Q000528:radiation effects*; D013361:Sublingual Gland / Q000528:radiation effects; D013363:Submandibular Gland / Q000528:radiation effects; D014987:Xerostomia / Q000517:prevention & control; D011522:Protons
chemical_list: D011522:Protons