Dose heterogeneity in the target volume and intensity-modulated radiotherapy to escalate the dose in the treatment of non-small-cell lung cancer

Purpose: To quantify the dose escalation achievable in the treatment of non-small-cell lung cancer (NSCLC) by allowing dose heterogeneity in the target volume or using intensity-modulated radiotherapy (IMRT), or both. Methods and Materials: Computed tomography data and contours of 10 NSCLC patients with limited movements of the tumor and representing a broad spectrum of clinical cases were selected for this study. Four irradiation techniques were compared: two conformal (CRT) and two IMRT techniques, either prescribing a homogeneous dose in the planning target volume (PTV) (CRThom and IMRThom) or allowing dose heterogeneity (CRTinhom and IMRTinhom). The dose heterogeneity was allowed only toward high doses, i.e., the minimum dose in the target for CRTinhom and IMRTinhom could not be lower than for the corresponding homogeneous plan. The dose in the PTV was escalated (fraction size of 2.25 Gy) until either an organ at risk reached the maximum allowed dose or the mean PTV dose reached a maximum level set at 101.25 Gy. Results: When small and convex tumors were irradiated, CRThom could achieve the maximum dose of 101.25 Gy, whereas for bigger and/or concave PTVs the dose level achievable with CRThom was significantly lower, in 1 case even below 60 Gy. The CRTinhom allowed on average a 6% dose escalation with respect to CRThom. The IMRThom achieved in all except 1 case a mean PTV dose of at least 75 Gy. The gain in mean PTV dose of IMRThom with respect to CRThom ranged from 7.7 to 14.8 Gy and the IMRThom plans were always more conformal than the corresponding CRThom plans. The IMRTinhom provided an additional advantage over IMRThom of at least 5 Gy. For all CRT plans the achievable dose was determined by the lung dose threshold, whereas for more than half of the IMRT plans the esophagus was the dose-limiting organ. The IMRT plans were deliverable with 10-12 segments per beam and did not produce an increase of lung volume irradiated at low doses (< 20 Gy). Conclusions: The dose in NSCLC treatments can be escalated by loosening the constraints on maximum dose in the target volume or using IMRT, or both. For large and concave tumors, an average dose escalation of 6% and 17% was possible when dose heterogeneity and IMRT were applied alone. When they were combined, the average dose increase was as high as 35%. Intensity-modulated RT delivered in a static mode can produce homogeneous dose distributions in the target and does not lead to an increase of lung volume receiving (very) low doses, even down to 5 Gy. (c) 2005 Elsevier Inc.