ESTRO 2022 - Abstract Book

S601

Abstract book

ESTRO 2022

PD-0667 Introduction of IMPT for NSCLC patients: the learning curve in practice.

R. Wijsman 1 , O. Chouvalova 1 , 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

Purpose or Objective In October 2019, we started to treat NSCLC patients with intensity modulated proton radiotherapy (IMPT). We planned to initiate a dose escalation RCT, but we felt that this should only start beyond the steepest part of the learning curve. Therefore, the objective of this study was to explore our learning curve in terms of treatment planning and delivery. Materials and Methods The model-based approach was used to select patients for IMPT. Model-based selection (IMPT compared to VMAT) was based on 2 NTCP-models (Grade ≥ 2 acute esophageal toxicity (AET) and Grade ≥ 2 radiation pneumonitis (RP)) and one prediction model for 2-year mortality (2yM). All patients received (chemo)radiation (25x2.4 Gy) for NSCLC. Robust IMPT plans were created using typically three beams and a 3D robustly optimized planning technique (5 times layered rescanning). Initially, maximum target motion allowed for IMPT was 10 mm, from March 2020 on this was extended to 15 mm. 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 visualized on the repeated CT. The following characteristics were evaluated: Δ Dose (mean doses for heart, lungs and esophagus) and Δ NTCP (for AET, RP and 2yM), all based on the nominal treatment plan. Treatment delivery times at the gantry and the number of plan adaptations were evaluated also. To evaluate the learning curve, the cohort was split in two: Cohort A ranged from October 2019 to March 31 st 2020 (n=24), Cohort B ranged from April 2020 to September 2021 (n=90). Results For patient and treatment characteristics, see Table 1. After radiotherapy plan comparison (IMPT vs VMAT), the median Δ Dose (mean) for heart (3.1 Gy vs 4.2 Gy, p=0.087), lung (2.9 Gy vs 3.4 Gy, p=0.13) and esophagus (2.4 Gy vs 2.8 Gy, p=0.48) was larger in Cohort B compared to Cohort A. Median ∆ NTCP increased in Cohort B compared to Cohort A: from 4.9% to 6.0% (2yM, p=0.26), 3.2% to 5.7% (RP, p=0.06) and 3.3% to 4.7% (AET, p=0.49), respectively (figure 1). Treatment delivery times decreased from an average of 34 minutes (Cohort A) to 25 minutes (Cohort B). Eleven (46%) of the treatment plans in Cohort A needed adaptation, compared to 20 (22%) in Cohort B. Distribution of the timing of these plan adaptations were equally balanced over the radiotherapy course and were similar for both cohorts. Comparing the first half of Cohort B with the second half of Cohort B: Δ Dose, ∆ NTCP and treatment delivery parameters were similar for both groups.