Transition from a simple to a more advanced dose calculation algorithm for radiotherapy of non-small cell lung cancer (NSCLC): Implications for clinical implementation in an individualized dose-escalation protocol

Background and purpose: To investigate the clinical consequences of the transition from a simple convolution algorithm (CA) to a more advanced superposition dose calculation algorithm (SA) in an individualized isotoxic dose-escalation protocol for NSCLC patients. Material and methods: First, treatment plans designed according to ICRU50-criteria using the CA were recalculated using the SA, for 16 patients. Next, two additional plans were designed for each patient using only the SA: one with 95%-isodose coverage (ICRU50-criteria), the other allowing PTV coverage with 90%-isodose at the lung side. PTV dose was escalated to a maximum dose of 79.2 Gy or lower when limited by either a mean lung dose (MLD) of 19 Gy or a maximum spinal cord dose of 54 Gy. Equivalent uniform doses (EUD) in the PTV were compared. Results: Recalculation of the CA plans using the SA, showed PTV underdosage in the CA plans: the median PTV EUD was 61.3 Gy (range 44.9-80.4 Gy) and 55.5 Gy (43.9-76.8 Gy), for CA and SA, respectively (p < 0.001). Redesigning plans using the SA resulted in an almost identical PTV EUD of 55.1 Gy (43.7-79.2 Gy). For the subgroup (N = 9) with MLD as dose-limiting factor a gain in PTV EUD of 2.7 ± 1.8 Gy (p = 0.008) was achieved using the 90%-isodose coverage plan. Conclusions: Plans calculated using the CA caused large PTV underdosage. Plans designed using the SA often lead to lower maximum achievable tumour doses due to higher MLD values. Allowing somewhat relaxed PTV coverage criteria increased the PTV dose again for MLD restricted cases. Consequently, in clinics where isotoxic individual dose-escalation is applied, implementation of an SA should be accompanied by accepting limited PTV underdosage in patients with MLD as the dose-limiting factor