In Reply to Oskan

We would kindly thank Mr. Oskan for his critical comments 1 Oskan F In regard to Moustakis et al. Int J Radiat Oncol Biol Phys. 2022; 114: 372-373 Abstract Full Text Full Text PDF Scopus (1) Google Scholar on our international multicenter work, “Planning benchmark study for SBRT of liver metastases,” which has been completed with the participation of 35 experienced liver stereotactic body radiation therapy (SBRT) centers and received an independently verified Radiotherapy Treatment plannINg study Guidelines (RATING) score of 98% (201 out of 205 points). 2 Hansen CR Crijns W Hussein M et al. Radiotherapy Treatment plannINg study Guidelines (RATING): A framework for setting up and reporting on scientific treatment planning studies. Radiother Oncol. 2020; 153: 67-78 Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar In his letter, he implies that we are advocating a change of general SBRT practice from the established planning target volume (PTV) surrounding isodose to “utopic” gross tumor volume (GTV) D50% prescription concepts. The careful reader will perceive that this is not the case. However, it is important to consider the striking physical and clinical evidence supporting the use of GTV D50% for plan evaluation as a predictor for efficacy and especially a means for harmonization of dose prescription and reporting in future clinical trials. (1)The main problem of published studies on SBRT was often the nonstandardized dose reporting with statistical analyses of the prescription doses alone. With that, the minimal and maximal doses within the PTV stayed unknown, making dose-response evaluations almost impossible. (2)One also has to keep in mind that the PTV is not defined by any anatomic concept like the GTV. In fact, the GTV is the most robustly defined volume in SBRT whereas the PTV is dependent on irradiation platforms, motion management techniques, and tissue heterogeneities. 3 Guckenberger M Baus WW Blanck O et al. Definition and quality requirements for stereotactic radiotherapy: Consensus statement from the DEGRO/DGMP working group stereotactic radiotherapy and radiosurgery. Strahlenther Onkol. 2020; 196: 417-420 Crossref PubMed Scopus (53) Google Scholar , 4 Schmitt D Blanck O Gauer T et al. Technological quality requirements for stereotactic radiotherapy: Expert review group consensus from the DGMP working group for physics and technology in stereotactic radiotherapy. Strahlenther Onkol. 2020; 196: 421-443 Crossref PubMed Scopus (47) Google Scholar , 5 Seuntjens J Lartigau EF Cora S et al. ICRU report 91. Prescribing, recording, and reporting of stereotactic treatments with small photon beams. J ICRU. 2014; 14: 1-160 PubMed Google Scholar , 6 Wilke L Andratschke N Blanck O et al. ICRU report 91 on prescribing, recording, and reporting of stereotactic treatments with small photon beams: Statement from the DEGRO/DGMP working group stereotactic radiotherapy and radiosurgery. Strahlenther Onkol. 2019; 195: 193-198 Crossref PubMed Scopus (68) Google Scholar In essence, the PTV surrounding isodose could be described as “theoretical” as it does not reflect the actual delivered dose in vivo, whereas the GTV D50% is comparatively more robust against dose calculation and delivery inaccuracies as has been demonstrated for countless techniques. 4 Schmitt D Blanck O Gauer T et al. Technological quality requirements for stereotactic radiotherapy: Expert review group consensus from the DGMP working group for physics and technology in stereotactic radiotherapy. Strahlenther Onkol. 2020; 196: 421-443 Crossref PubMed Scopus (47) Google Scholar (3)That said, we need to consider the available clinical evidence that supports the physical concepts of robustness. Arguably, the largest data come from our German Expert Group on liver SBRT, where we pooled 623 metastases for joined analysis. 7 Klement RJ Guckenberger M Alheid H et al. Stereotactic body radiotherapy for oligo-metastatic liver disease - Influence of pre-treatment chemotherapy and histology on local tumor control. Radiother Oncol. 2017; 123: 227-233 Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar ,8 Andratschke N Alheid H Allgäuer M et al. The SBRT database initiative of the German Society for Radiation Oncology (DEGRO): Patterns of care and outcome analysis of stereotactic body radiotherapy (SBRT) for liver oligometastases in 474 patients with 623 metastases. BMC Cancer. 2018; 18: 283 Crossref PubMed Scopus (80) Google Scholar We only found a weak dose-response-relationship between PTV surrounding isodose and local control, and hence used isocenter maximum dose, which is a close surrogate to GTV D50%, as primary dose parameter for modeling. In a later work on 1500 lung SBRT cases we could demonstrate a similar dose-response-relationship between GTV D50% and local control. 9 Klement RJ Sonke JJ Allgäuer M et al. Correlating dose variables with local tumor control in stereotactic body radiation therapy for early-stage non-small cell lung cancer: A modeling study on 1500 individual treatments. Int J Radiat Oncol Biol Phys. 2020; 107: 579-586 Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar