ESTRO 2021 Abstract Book

S122

ESTRO 2021

Left-Superior

Quarter

N/A

N/A

DVH

Right-Inferior

Quarter

N/A

V%_2Gy (HR=1.007/1%, p=0.02)

DVH

Left-Inferior Quarter DVH N/A

N/A

High

resolution

V%_55Gy (HR = 1.04/1%, p<0.001) 19/64 segments

V%_2Gy (HR = 1.01/1%, p=0.01) 17/64 segments

subdivision DVH

Note:

this

result

approximates the complete solution which is a linear combination of V%_1Gy – V%_5Gy indices in the predictive heart region.

Conclusion Heart irradiation is associated with reduced OS in LA-NSCLC patients and in patients treated with lung SBRT. A comparison of the two patient cohorts suggests that different etiologies may be responsible for reduced OS in each. OC-0193 Introduction of IMPT for (N)SCLC patients: NTCP model based selection of patients and first results R. Wijsman 1 , O. Chouvalova 2 , P. Klinker 1 , A. Niezink 1 , A. Van der Leest 1 , F. Ubbels 1 , M. Woltman - Van Iersel 1 , S. Both 1 , E. Korevaar 1 , J. Langendijk 1 1 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands; 2 University Medical Center Groningen, Radiation oncology, Groningen, The Netherlands Purpose or Objective In October 2019, we started to treat (N)SCLC patients with intensity modulated proton radiotherapy (IMPT) after model-based selection (MBS). The objective was to evaluate MBS for these patients and to report the Patients with (N)SCLC referred for (chemo)radiation (typically 25 x 2.4 Gy, Table 1) with curative intent were selected for IMPT according to the national indication protocol containing guidelines for MBS of these patients. MBS is based on a difference in NTCP (ΔNTCP) for Grade≥2 radiation pneumonitis (RP), Grade≥2 acute esophageal toxicity (AET) and/or 2-year mortality (2yM) based on a plan comparison (VMAT vs. IMPT). The ΔNTCP thresholds are 10% for RP and AET and 2% for 2yM. The dosimetric parameters in these NTCP models are mean lung dose (MLD), mean esophagus dose (MED) and mean heart dose (MHD), respectively. Maximum target motion allowed for IMPT delivery was 15 mm. Robust IMPT plans were created using three beams and a 3D robustly optimized planning technique (including 5 times layered rescanning). Online position verification consisted of daily (consecutive) body surface scanning, 2D kV imaging and cone-beam CT imaging. Weekly repeated (4D) CTs were acquired; the treatment plan was adapted in case of inadequate target coverage. Concurrent chemotherapy for NSCLC typically consisted of weekly courses of low-dose cisplatinum/docetaxel. Toxicity data were prospectively collected from our standard follow-up program. Results Seventy-seven patients (43%) of the 193 patients referred for radiotherapy with curative intent met the predefined ΔNTCP thresholds and were eligible for IMPT (average maximum target motion 8.4 mm, range 3-19 mm). The majority of patients qualified for IMPT based on an improvement in 2yM (n=51, 66%). The remaining 26 patients qualified not only based on improvement of 2yM but also based on ΔNTCP for RP and/or AET (Table 1). Compared with VMAT, IMPT resulted in significantly lower MHD (3.8 Gy vs. 9.2 Gy; p<0.001), MLD (9.2 Gy vs. 12.5 Gy; p<0.001) and MED (16.5 Gy vs. 19.5 Gy; p<0.001). This resulted in an average ΔNTCP with IMPT for 2yM (42% vs. 48%), RP (14.1% vs. 20.2%) and AET (36.2% vs. 42%, Fig. 1). The observed Grade 2 and 3 AET rates first results on toxicity. Materials and Methods