PO-1165 Dosimetric parameters in hypofractionated radiotherapy in NSCLC cohort during SARS-COV-2 pandemia

Abstract

Purpose or Objective: Moderate hypofractionation has been more popular in the last years, however no standard dosimetric parameters has been stablished. The majority of the dose-volume-constraints (DVCs) published refer to conventional 2 Gy/ fraction. In this study, we analyse different dosimetric parameters in patients (pts) with non-small cell lung cancer (NSCLC) treated with concomitant radiochemotherapy (RCT) or radiotherapy (RT) alone with radical intention and hypofractionated scheme in pandemic era. Due to the lack of consensus on this aspect, we analyse the relationship between tolerance to treatment and dosimetric parameters to aid its use in the clinic. Materials and Methods: Retrospective and multicentric study of 49 pts with locally advanced NSCLC treated from November 2019 to December 2020. During SARS-CoV-2 pandemic, hypofractionated schedules have allowed to decrease the duration of thoracic radiotherapy. The hypofractionated scheme used was 20 fractions of 2.75 Gy/daily (total dose 55 Gy, BED10=70 Gy). 3-dimensional conformal radiation therapy (3DCRT), intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) were allowed. We report gross tumour volume (GTV), planning tumour volume (PTV) and D98% and D95% PTV. Organ-at-risk (OARs) examined included lungs- GTV (V20<35% and mean-dose<20Gy), heart (V25<10%) and oesophagus (V50<40% and mean-dose<34 Gy). To evaluate toxicity during the treatment, we use CTCAE v.5 scale. Results: Mean GTV was 85.6 cc (3.3cc-581.3cc) and mean PTV was 268.8 cc (13cc-1047.4cc). Mean D98% PTV was 53 Gy and mean D95% PTV was 53.8 Gy. Most pts had G1-G2 cardiac toxicity like pericarditis, oesophagitis and pneumonitis. Only two pts (4%) had G3 oesophagitis and G3 pneumonitis (2%). No grade 4-5 toxicity was reported. In the analysis of DVCs, lungs-GTV V20<35% and lungs-GTV mean-dose < 20 Gy were associated with more pneumonitis regardless of grade (p 0.018 and p 0.027). In terms of oesophagitis, V50<40%, was associated with more oesophagitis regardless of grade (p 0.037). Mean dose < 34 Gy in oesophagus and heart DVCs were no associated with more toxicity in our study. There were not differences between T stage (<T3 vs ≥ T3) and N stage (<N2 vs ≥ N2) and mean PTV (p 0.55 and p 0.178). There were not differences in terms of cancer-specific survival and PTV (p 0.195). Conclusion: In moderately hypofractionation, it is important to consider that dosimetric parameters cannot be the same as in standard fractionation (2 Gy/fraction). Due to changing radiotherapy technique, DVCs may need to be adjusted based on different dose distribution. According to our results, hypofractionated radiotherapy in NSCLC is well-tolerated with low rates of grade 3-4- 5 toxicity but lungs-GTV V20<35%, lungs-GTV mean-dose < 20 Gy and oesophagus V50<40% were associated with more toxicity regardless of grade. Because of that, we consider it worth investigating the relationship between dosimetric parameters and toxicity in order to reach a consensus in daily clinical practice.