In Reply to Yamazaki et al.

To the Editor: On behalf of our co-authors, we appreciate the kind words and interest in our recent study. The planning target volumes (PTVs) were defined as a 3-mm expansion of the clinical target volumes (CTVs). Strictly speaking, the PTV concept commonly used in photon therapy cannot be directly used for proton therapy because the range uncertainties are beam direction specific ( 1 Phan J. Sio T.T. Nguyen T.P. et al. Reirradiation of head and neck cancers with proton therapy: outcomes and analyses. Int J Radiat Oncol Bio Phys. 2016; 96: 30-41 Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar , 2 Yamazaki H. Nakamura S. Suzuki G. Yoshida K. In Regard to Phan et al. Int J Radiat Oncol Biol Phys. 2017; 97: 868 Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar , 3 Liu W. Zhang X. Li Y. et al. Robust optimization of intensity modulated proton therapy. Med Phys. 2012; 39: 1079-1091 Crossref PubMed Scopus (243) Google Scholar ). However, beam-specific PTVs ( 4 Park P.C. Zhu X.R. Lee A.K. et al. A beam-specific planning target volume (PTV) design for proton therapy to account for setup and range uncertainties. Int J Radiat Oncol Biol Phys. 2012; 82: e329-e336 Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar ) are not available in our clinical treatment planning system. We nevertheless used the PTV concept in spot scanning proton therapy to manage delivery uncertainties for plans with non–parallel-opposed beam arrangements ( 5 Lomax A.J. Intensity modulated proton therapy and its sensitivity to treatment uncertainties 1: The potential effects of calculational uncertainties. Phys Med Biol. 2008; 53: 1027-1042 Crossref PubMed Scopus (275) Google Scholar ). The PTVs created by 3-mm expansion of CTVs were also justified by the fact that the depths of distal edge of the target volumes for head and neck patients were typically 10 cm or less; 3-mm margin would correspond to 3% range uncertainty.