Immortalization is a prerequisite for the clonal evolution and malignant transformation of normal mammalian cells in culture. In order to gain a mechanistic insight into the genetics of carcinogen-induced cellular immortality, a cell culture assay has been developed based on the use of freshly explanted Syrian hamster dermal (SHD) fibroblasts. The relative efficacies of a variety of chemical and physical carcinogens at immortalizing SHD cells (against a zero background of spontaneous immortalization) were compared. Ionizing radiation and nickel chloride appeared to be more effective as immortalizing agents than powerful point mutagens, suggesting (but not proving) that clastogenic damage may be more significant in the immortalization process than point mutation. Frequencies of induced immortality (10(-6)-10(-7)/treated cell) were arguably consistent with a direct mutational mechanism involving a single genetic target. However, detailed cytogenetic characterization of a panel of newly immortalized cell lines revealed no non-random chromosomal alterations in the cells at the level of G-banding. Furthermore, additional experiments with the SHD system have provided confirmatory evidence that immortalization can occur as an indirect consequence of carcinogen exposure following an induced high frequency change in the treated population, rather than through direct targeted mutagenesis. Previous somatic cell genetic studies have suggested the possibility that a target gene for immortalization exists on the human and Chinese hamster X chromosomes. Here we provide strong evidence that the normal SHD X chromosome displays powerful senescence-inducing properties when introduced, by microcell transfer, into newly immortalized SHD recipients. These results suggest that induction of the immortal phenotype in SHD cells by carcinogens results primarily from functional inactivation of a senescence gene which may be X-linked. One possible mechanism for senescence gene inactivation consistent with our observations is through a sub-microscopic interstitial genetic deletion. However, the considerable efficacy of nickel (a human carcinogen) as an immortalizing agent at nonmutagenic doses raises the alternative possibility that immortalization may occur through an epigenetic mechanism.

Despite complete surgical resection brain metastases are at significant risk of local recurrence without additional radiation therapy. Traditionally, the addition of postoperative whole brain radiotherapy (WBRT) has been considered the standard of care on the basis of randomized studies demonstrating its efficacy in reducing the risk of recurrence in the surgical bed as well as the incidence of new distant metastases. More recently, postoperative stereotactic radiosurgery (SRS) to the surgical bed has emerged as an effective and safe treatment option for resected brain metastases. Published randomized trials have demonstrated that postoperative SRS to the resection cavity provides superior local control compared to surgery alone, and significantly decreases the risk of neurocognitive decline compared to WBRT, without detrimental effects on survival. While studies support the use of postoperative SRS to the resection cavity as the standard of care after surgery, there are several issues that need to be investigated further with the aim of improving local control and reducing the risk of leptomeningeal disease and radiation necrosis, including the optimal dose prescription/fractionation, the timing of postoperative SRS treatment, and surgical cavity target delineation. We provide a clinical overview on current status and recent advances in resection cavity irradiation of brain metastases, focusing on relevant strategies that can improve local control and minimize the risk of radiation-induced toxicity.

In a series of related papers, we have recently presented the results of a thermodynamic approach to the conformational transitions of bulk chromatin induced in vitro by different structure-perturbing agents, such as the intercalating dye ethidium bromide or the ionic strength. In all these studies, we took advantage of the capability of differential scanning calorimetry to detect the changes in the melting behavior of the structural domains of chromatin (the linker and the core particle) associated with the order-disorder transitions. This technique also revealed that the higher-order structure undergoes a catastrophic decondensation process in the course of the transformation of rat hepatocytes as well as of cultured cells. Therefore, several questions arose concerning the biological function (if any) of the changes in the degree of condensation of bulk chromatin, as well as the mechanism of transition and the nature of the modulating agents. In this paper, we report a thermodynamic analysis of the reconstitution of H1-depleted calf thymus chromatin with the purpose of establishing (1) the binding mode of H1 and (2) the energetics and cooperativity of the transition from the unfolded to the condensed state. When H1 is progressively extracted from calf thymus nuclei by high-salt treatment, the endotherm at 107 degrees C, characteristic of the core particles interacting within condensed domains, converts into the thermal transition at 90 degrees C, resulting from the denaturation of noninteracting core particles. Binding of H1 fully restores the thermal profile of native chromatin. Analysis of H1 association shows that binding occurs at independent sites with KA = (3.67 +/- 0.60) x 10(4) M-1 and each site comprising 180 +/- 10 bp. The experimental dependence of the fraction of condensed chromatin on R, the moles of bound H1 per nucleosome mole, was compared with a simple thermodynamic model for the conformational change. This analysis yields a value of -5 kcal per nucleosome mole for the interaction free energy of nucleosomes within the ordered state. The process of condensation, is not, however, a highly cooperative (all-or-none) one, as expected from a consideration of the solenoidal model for the 30 nm fiber. Rather, nucleation of the helical state involves the face-to-face interaction between consecutive core particles, and the growth is largely determined by the mergence and rearrangement of neighboring clusters of helically arrayed nucleosomes.

The efficiency of magnetic resonance imaging (MRI) and that of transrectal ultrasound (TRUS) were compared in preoperative staging of 15 patients with rectal cancer and in postoperative follow-up of 12 patients. Thirteen of the 15 patients evaluated for preoperative staging were operated on. Preoperative staging and pathologic finding were identical in 11 patients (84.6 percent) examined by TRUS and in 10 patients (76.9 percent) examined by MRI. Recurrent cancer was detected in 3 of 12 patients in the follow-up group. MRI was able to diagnose correctly 10 of 12 patients (83.2 percent), one patient was misdiagnosed, and in one patient the MRI could not distinguish between fibrous tissue and recurrent cancer. TRUS diagnosed correctly only 5 of 12 patients (41.6 percent). One was falsely diagnosed, and, in 6 patients (50 percent), this examination could not differentiate between fibrous tissue and recurrent tumor. According to our results, both MRI and TRUS have a place in the preoperative staging of patients with rectal cancer. The main differences between the two methods were in the differential diagnoses of fibrous tissue and recurrent cancer. MRI being more specific in detection of recurrence.

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