A practical methodology to improve the dosimetric accuracy of MR-based radiotherapy simulation for brain tumors.

PURPOSE: To investigate the dosimetric accuracy of synthetic computed tomography (sCT) images generated by a clinically-ready voxel-based MRI simulation package, and to develop a simple and feasible method to improve the accuracy. METHODS: 20 patients with brain tumor were selected to undergo CT and MRI simulation. sCT images were generated by a clinical MRI simulation package. The discrepancy between planning CT and sCT in CT number and body contour were evaluated. To resolve the discrepancies, an sCT specific CT-relative electron density (RED) calibration curve was used, and a layer of pseudo-skin was created on the sCT. The dosimetric impact of these discrepancies, and the improvement brought about by the modifications, were evaluated by a planning study. Volumetric modulated arc therapy (VMAT) treatment plans for each patient were created and optimized on the planning CT, which were then transferred to the original sCT and the modified-sCT for dose re-calculation. Dosimetric comparisons and gamma analysis between the calculated doses in different images were performed. RESULTS: The average gamma passing rate with 1%/1 mm criteria was only 70.8% for the comparison of dose distribution between planning CT and original sCT. The mean dose difference between the planning CT and the original sCT were -1.2% for PTV D95 and -1.7% for PTV Dmax, while the mean dose difference was within 0.7 Gy for all relevant OARs. After applying the modifications on the sCT, the average gamma passing rate was increased to 92.2%. Mean dose difference in PTV D95 and Dmax were reduced to -0.1% and -0.3% respectively. The mean dose difference was within 0.2 Gy for all OAR structures and no statistically significant difference were found. CONCLUSIONS: The modified-sCT demonstrated improved dosimetric agreement with the planning CT. These results indicated the overall dosimetric accuracy and practicality of this improved MR-based treatment planning method.

Systematic review with meta-analysis: the impact of Clostridium difficile infection on the short- and long-term risks of colectomy in inflammatory bowel disease.

BACKGROUND: Clostridium difficile infection (CDI) is associated with increased mortality in inflammatory bowel disease (IBD), but the risk of colectomy is variable and has not been adequately studied. AIM: To perform a systematic review and meta-analysis to assess the impact of CDI on colectomy risk in IBD. METHODS: Multiple databases were searched systematically for observational studies reporting colectomy risk in IBD, stratified by the presence of CDI, and the duration of follow-up (short term 3 months, and long term at least 1 year). Weighted summary estimates were calculated using generalised inverse variance with random-effects model. Study quality was assessed using the Newcastle-Ottawa scale. RESULTS: Twelve observational studies were identified and included 35 057 IBD patients with CDI, and 929 259 without CDI. CDI did not increase the short-term colectomy risk in IBD patients overall (10 studies) (OR: 1.35; 95% CI: 0.68-2.67), or in patients with ulcerative colitis (nine studies) (OR: 1.20; 95% CI: 0.39-3.76). In contrast, CDI was associated with higher long-term colectomy risk in patients with IBD overall (five studies) (OR: 2.23; 95% CI: 1.18-4.21), and in patients with ulcerative colitis (four studies) (OR: 2.96; 95% CI: 1.19-7.34). The results were stable in subgroups stratified by recruitment period, hospitalisation status and geographical location. All studies were at least of moderate quality. The results were limited in the ability to compare IBD severity and the type of anti-microbial therapy. CONCLUSION: Based on 12 observational studies with at least moderate quality, Clostridium difficile infection appears to increase colectomy risk in IBD in the long- but not short- term.

Effect of binocular cortical misalignment on ocular dominance and orientation selectivity.

We model a two-eye visual environment composed of natural images and study its effect on single cell synaptic modification. In particular, we study the effect of binocular cortical misalignment on receptive field formation after eye opening. We show that binocular misalignment affects principal component analysis (PCA) and Bienenstock, Cooper, and Munro (BCM) learning in different ways. For the BCM learning rule this misalignment is sufficient to produce varying degrees of ocular dominance, whereas for PCA learning binocular neurons emerge in every case.

Formation of receptive fields in realistic visual environments according to the Bienenstock, Cooper, and Munro (BCM) theory.

The Bienenstock, Cooper, and Munro (BCM) theory of synaptic plasticity has successfully reproduced the development of orientation selectivity and ocular dominance in kitten visual cortex in normal, as well as deprived, visual environments. To better compare the consequences of this theory with experiment, previous abstractions of the visual environment are replaced in this work by real visual images with retinal processing. The visual environment is represented by 24 gray-scale natural images that are shifted across retinal fields. In this environment, the BCM neuron develops receptive fields similar to the fields of simple cells found in kitten striate cortex. These fields display adjacent excitatory and inhibitory bands when tested with spot stimuli, orientation selectivity when tested with bar stimuli, and spatial-frequency selectivity when tested with sinusoidal gratings. In addition, their development in various deprived visual environments agrees with experimental results.