Dosimetric impact of gastrointestinal air column in radiation treatment of pancreatic cancer

OBJECTIVE: Dosimetric evaluation of air column in gastrointestinal (GI) structures in intensity modulated radiation therapy (IMRT) of pancreatic cancer. METHODS: Nine sequential patients were retrospectively chosen for dosimetric analysis of air column in the GI apparatus in pancreatic cancer using cone beam CT (CBCT). The four-dimensional CT (4DCT) was used for target and organs at risk (OARs) and non-coplanar IMRT was used for treatment. Once a week, these patients underwent CBCT for air filling, isocentre verification and dose calculations retrospectively. RESULTS: Abdominal air column variation was as great as ±80% between weekly CBCT and 4DCT. Even with such a large air column in the treatment path for pancreatic cancer, changes in anteroposterior dimension were minimal (2.8%). Using IMRT, variations in air column did not correlate dosimetrically with large changes in target volume. An average dosimetric deviation of mere -3.3% and a maximum of -5.5% was observed. CONCLUSION: CBCT revealed large air column in GI structures; however, its impact is minimal for target coverage. Because of the inherent advantage of segmentation in IMRT, where only a small fraction of a given beam passes through the air column, this technique might have an advantage over 3DCRT in treating upper GI malignancies where the daily air column can have significant impact. Advances in knowledge: Radiation treatment of pancreatic cancer has significant challenges due to positioning, imaging of soft tissues and variability of air column in bowels. The dosimetric impact of variable air column is retrospectively studied using CBCT. Even though, the volume of air column changes by ± 80%, its dosimetric impact in IMRT is minimum

Self-organizing strategies for a column-store database

Column-store database systems open new vistas for improved maintenance through self-organization. Individual columns are the focal point, which simplify balancing conflicting requirements. This work presents two workload-driven self-organizing techniques in a column-store, i.e. adaptive segmentation and adaptive replication. Adaptive segmentation splits a column into non-overlapping segments based on the actual query load. Likewise, adaptive replication creates segment replicas. The strategies can support different application requirements by trading off the reorganization overhead for storage cost. Both techniques can significantly improve system performance as demonstrated in an evaluation of different scenarios

Analysis of the Zebrafish Segmentation Clock

In vertebrates, the repeating vertebrae in the vertebral column are the clearest indicators of the segmented body plan. The embryonic precursors of the vertebral column and skeletal musculature are bilaterally symmetric blocks of tissue flanking the notochord called somites. Somites are generated sequentially and periodically from an unsegmented tissue called the presomitic mesoderm (PSM) by a process called somitogenesis. Underlying the periodicity of somitogenesis are transcriptional oscillations of cyclic genes in the cells of the PSM. On a tissue level, these oscillations manifest as travelling waves, departing from the posterior and arresting in the anterior. The position of arrest prefigures the position of the new somite boundary. The molecular network that comprises the cyclic genes and their regulation in the PSM is termed the segmentation clock. Retinoic acid (RA) has been previously proposed to be a differentiation signal that acts to arrest the oscillations at the anterior of the PSM. This thesis shows the zebrafish RA catabolism mutant giraffe has an altered cyclic gene wave pattern, an observation that suggests that rather than stop their oscillations, cells tune their frequencies in response to RA signalling, introducing a novel function for RA in the zebrafish segmentation clock. In amniotes, the segmentation clock instructs the metamery of the vertebral column, but in zebrafish, the relationship is not established. This thesis demonstrates that the segmentation clock is not required in zebrafish for the development of a periodic vertebral column by using a novel segmentation clock mutant, thereby supporting a role for the notochord in the development of vertebral column metamery. Therefore, two periodic patterning processes establish zebrafish body pattern – one segments the somites and musculature, and the second segments the vertebral column. This thesis advances the understanding of the mechanisms of body pattern establishment by way of these novel insights