Evaluation and optimization of the parameters used in multiple-atlas-based segmentation of prostate cancers in radiation therapy.

OBJECTIVE: To evaluate and optimize the parameters used in multiple-atlas-based segmentation of prostate cancers in radiation therapy. METHODS: A retrospective study was conducted, and the accuracy of the multiple-atlas-based segmentation was tested on 30 patients. The effect of library size (LS), number of atlases used for contour averaging and the contour averaging strategy were also studied. The autogenerated contours were compared with the manually drawn contours. Dice similarity coefficient (DSC) and Hausdorff distance were used to evaluate the segmentation agreement. RESULTS: Mixed results were found between simultaneous truth and performance level estimation (STAPLE) and majority vote (MV) strategies. Multiple-atlas approaches were relatively insensitive to LS. A LS of ten was adequate, and further increase in the LS only showed insignificant gain. Multiple atlas performed better than single atlas for most of the time. Using more atlases did not guarantee better performance, with five atlases performing better than ten atlases. With our recommended setting, the median DSC for the bladder, rectum, prostate, seminal vesicle and femurs was 0.90, 0.77, 0.84, 0.56 and 0.95, respectively. CONCLUSION: Our study shows that multiple-atlas-based strategies have better accuracy than single-atlas approach. STAPLE is preferred, and a LS of ten is adequate for prostate cases. Using five atlases for contour averaging is recommended. The contouring accuracy of seminal vesicle still needs improvement, and manual editing is still required for the other structures. ADVANCES IN KNOWLEDGE: This article provides a better understanding of the influence of the parameters used in multiple-atlas-based segmentation of prostate cancers.

A new approach to computing normal tissue complication probability of an intensity-modulated radiotherapy treatment with stereotactic radiotherapy boost of nasopharyngeal carcinoma: a case study.

We attempted to develop a method to compute the normal tissue complication probability (NTCP) of various critical organs from combined intensity-modulated radiotherapy (IMRT) and stereotactic radiotherapy (SRT) boost treatment of nasopharyngeal carcinoma (NPC), with the aid of a nonlinear image registration method. The SRT's planning computed tomography (CT) of a NPC patient treated with IMRT was warped to the IMRT's planning CT using a nonlinear image registration. Because CT and dose were inherently in-register, the entire dose distribution could be deformed using the same deformation field derived from the two CT sets. Using the biologically effective dose concept and the linear-quadratic model, physical doses of IMRT and SRT were converted to a 2 Gy-per-fraction equivalent dose to facilitate dose summation. The variation of organs' maximum doses of the combined treatments between traditional maximum dose sum and the proposed method was 1.5 Gy ± 1.7 Gy. After the correction of the effect of fractionation and dose heterogeneity within each organ, NTCP of each organ of interest was computed for the combined treatments. Based on the results of this case study, it is believed that dose registration could be a method for the NTCP computation of various critical organs when different fractionation schemes of radiation therapy treatment are instituted.