Dependence of radiochromic film response kinetics on fractionated doses.

Because the rate and magnitude of the post-exposure growth of the MD-55 radiochromic film (RCF) dosimeter response depends significantly on total dose, we have investigated the influence of fractionation and protracted dose delivery on optical density (OD) as a function of total dose and the exposure-to-densitometry time interval for a 633-nm scanning laser densitometer. Both measurements and models demonstrate that fractionation induces transient OD over responses, which can be as large as 20%, but rapidly dissipate within 24 h. However, the superposition model predicts 2-5% over responses that persist as long as 700 h.     

Direct analyses of in vivo colony survival after single and fractionated doses of radiation.

Several methods are described for analysing the results of in vivo colony assays using the statistical procedure called maximum-likelihood analysis. The methods differ in the way in which they take into account possible sources of variability in the data. The methods described here for analysing microcolony data are direct methods, in that they use the observed colony counts rather than transformed (e.g. Poisson-corrected) data. Each method can be used to estimate the average number of surviving cells per tissue structure (e.g. per jejunal crypt) in a single dose group, together with 95% confidence intervals, or to fit cell-survival models to data from a range of dose groups (e.g. to obtain estimates of D0 or of the linear-quadratic parameters alpha and beta). Experimental microcolony data from murine jejunum, colon, and hair follicles irradiated in anagen (proliferative) or telogen (resting) phase have been analysed. Estimates of D0 have been derived from single-dose data and estimates of alpha, beta, and the initial number of clonogenic cells per structure have been derived from fractionation data. For hair follicles, the half-time of repair of sublethal radiation injury has also been derived from fractionation data.     

The predictability of the radiotherapy response in epidermoid tumors of the head and neck. A review of the literature and mathematical models for choosing the fractionated doses

Literature data show that the term "squamous-cell head and neck cancer" includes a wide range of epidermoid cell subgroups, each of them with its own intrinsic radiosensitivity (Do values ranging from 107 to 184 in primary tumors, and 146-263 in recurrences; n values ranging from 1 to 5; and, if we consider linear-quadratic model alpha values from 0.273 to 0.490 and beta values from 0.029-0.045). Different sublethal and potential lethal repair times are also observed (4-6 hours and 12-24 hours, respectively), and structural tissue heterogeneity (hypoxic fraction oscillating 5%-30% of the neoplasm). Most important, different kinetic parameters are demonstrated, with Labelling Index ranging from 4% to 30%, phase-S time from 6 to 19 hours, and potential doubling time from 2 to 20 days. On the basis of Fowler's and Barendsen's mathematical models and knowing the potential doubling time and Labelling Index values (derived from bioptic specimens), as well as alpha/beta ratio for both tumor and normal tissue, we tried to identify the optimal fractionation (standard, accelerated, hyperfractionated) for slow/fast-growth tumors, also evaluating the relative acute and late side effects. Our analysis shows that: 1) tumors with aggressive biological behavior (Labelling Index greater than 15%, aneuploidy, potential doubling time less than 5 days) seem to respond to accelerated fractionation/hyperfractionation without split better than to standard regimens: 2) tumors with slow growth (Labelling Index less than 15%, potential doubling time greater than 5 days, euploidy) seem to respond not only to standard regimens, but also--and mainly--to hyperfractionation.     

Magnetic Resonance applications in dental implantology: technical notes and preliminary results

PURPOSE: To evaluate the possible use of Magnetic Resonance Imaging (MRI) in the field of dental implantology, for identifying the mandibular nerve, as proposed by several authors. MATERIALS AND METHODS: MRI was used for the study of the mandible in ten subjects (five healthy volunteers and five subjects awaiting dental implants). Imaging was performed on a 1.0-T MR scanner with a brain coil. T2 TSE, T1 spin-echo and T2 gradient-echo sequences were performed, both parallel and perpendicular to the horizontal portion of the mandible, with a thickness of 3 mm. RESULTS: In all the subjects MRI clearly identified the intraosseous course of the inferior alveolar neurovascular bundle within the mandibular canal. CONCLUSIONS: MRI appears useful for the depiction of the mandibular canal before dental implantation. Further studies are required to compare the accuracy of MRI and CT based on a statistically significant sample.     

Lens crystallin response to whole body irradiation with single and fractionated doses of gamma radiation

Abstract

Purpose: This study was aimed at determining the effect of whole body irradiation with single and fractionated low doses of γ-radiation on the structure of normal rats lens crystallin with the concept of correlating this with the changes in its molecular weight. Materials and methods: Three groups of Wistar albino rats were involved in this study; the first group is the control (n =?10) and the second group (n =?10) treated with a single dose of 4.0 Gy administered in one application from Cesium-137. The third group (n =?60) was treated for 8 weeks with fractionated dose rate of 0.5 Gy/week. Lens crystallin was analysed for protein content measurements and column chromatography. Results: Measurements of total soluble lens protein indicated pronounced decreases after exposure to single and fractionated doses of γ-radiation. The molecular weights and the protein content for the β_H (with high molecular weight), β_L (with low molecular weight) and γ-crystallin fractions revealed generalized alteration for all groups. Conclusion: Single and fractionated doses of γ-radiations induced lens crystallin changes that might lead to development of cataract due to induction of cross-linking and aggregation. β_H-crystallin was the most stable against radiation induced damage, and the molecular weight of γ-crystallin was the most sensitive to gamma radiation.     

The relative biological effectiveness of fractionated doses of fast neutrons (42 MeVd----Be) for normal tissues. III. Effects on lung function

The effect of single and fractionated doses of fast neutrons (42 MeVd----Be) on the early and late radiation responses of the pig lung have been assessed by the measurement of changes in lung function using a 133Xe washout technique. The results obtained for irradiation schedules with fast neutrons have been compared with those after photon irradiation. There was no statistically significant difference between the values for the relative biological effectiveness (RBE) for the early and late radiation response of the lung. The RBE of the neutron beam increased with decreasing size of dose/fraction with an upper limit value of 4.39 +/- 0.94 for infinitely small X-ray doses per fraction.