More optimal dose distributions for moving lung tumours: A planning study

Background and purposeTarget volumes for moving lung tumours encompass the full range of respiratory motion, increasing the risk of lung complications. Intensity modulated radiotherapy (IMRT) allows for more precise dose distributions. Distributions corresponding to the probability density function (PDF) of tumour motion may better spare lung yet deliver adequate target dose. The planning study purpose is to compare and evaluate different dose distributions on a moving lung tumour: (A) conformal RT (CRT) encompassing the full range of tumour motion, (B) CRT encompassing the modal tumour position only, and (C) an IMRT technique where the dose delivered corresponds to the tumour PDF.Materials and methodsA 5 cm diameter spherical target within a rectangular lung equivalent phantom was treated using a parallel-opposed pair technique with a 1.5 cm margin around the tumour (in the beam's eye view). Asymmetrical sinusoidal (superior–inferior) target movement (peak-trough=3 cm) was simulated for different dose distributions (prescription dose=60 Gy). Equivalent uniform dose (EUD) for the tumour and normal tissue complication probabilities (NTCPs) for radiation pneumonitis were evaluated.ResultsThe EUDs were 60.0, 48.5, and 57.9 Gy while the NTCPs were 5, 1, and 3% for cases A, B, and C, respectively (assuming survival fraction, SF2 Gy=0.5).ConclusionsSince these results rely on unvalidated radiobiologic models, they must be interpreted cautiously. However, more optimized dose distributions for moving lung targets appear feasible and can reduce lung complications with only a negligible impact on the expected EUD and, thus, deserve further study.