An Evaluation of Atlas Selection Methods for Atlas-Based Automatic Segmentation in Radiotherapy Treatment Planning.

Atlas-based automatic segmentation is used in radiotherapy planning to accelerate the delineation of organs at risk (OARs). Atlas selection has been proposed as a way to improve the accuracy and execution time of segmentation, assuming that, the more similar the atlas is to the patient, the better the results will be. This paper presents an analysis of atlas selection methods in the context of radiotherapy treatment planning. For a range of commonly contoured OARs, a thorough comparison of a large class of typical atlas selection methods has been performed. For this evaluation, clinically contoured CT images of the head and neck ( N=316 ) and thorax ( N=280 ) were used. The state-of-the-art intensity and deformation similarity-based atlas selection methods were found to compare poorly to perfect atlas selection. Counter-intuitively, atlas selection methods based on a fixed set of representative atlases outperformed atlas selection methods based on the patient image. This study suggests that atlas-based segmentation with currently available selection methods compares poorly to the potential best performance, hampering the clinical utility of atlas-based segmentation. Effective atlas selection remains an open challenge in atlas-based segmentation for radiotherapy planning.

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 79.2Gy or lower when limited by either a mean lung dose (MLD) of 19Gy or a maximum spinal cord dose of 54Gy. 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 61.3Gy (range 44.9-80.4Gy) and 55.5Gy (43.9-76.8Gy), for CA and SA, respectively (p<0.001). Redesigning plans using the SA resulted in an almost identical PTV EUD of 55.1Gy (43.7-79.2Gy). For the subgroup (N=9) with MLD as dose-limiting factor a gain in PTV EUD of 2.7+/-1.8Gy (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.