Biomedical subjects
C C Ling
Publications and source records attributed to C C Ling.
Moderate hyperthermia and low dose rate irradiation.
Moderate hyperthermia (4 h at 40 degrees C) enhances V-79 cell radiosensitivity at low irradiation dose rates with a maximum thermal enhancement ratio (TER) of 1.38. In comparison, the TER measured at acute dose rate is 1.13. Heat treatments given before and during irradiation are equally effective, and more so than postirradiation hyperthermia. Hyperthermia-induced inhibition of sublethal damage repair is a probable cause of the observed effect.
Using measured dose distribution data of the Fletcher-Suit-Delclos colpostat in brachytherapy treatment planning.
A program to use the measured dose distribution of a shielded ovoid has been written and interfaced with a commercial system. The processing of the data and the method of reconstructing isodose curves in any desired plane is described. The effect of the shielding on calculated dose distributions is evaluated.
Dose distributions of model 6702 I-125 seeds in water.
Dose distributions in water of Model 6702 I-125 seeds have been measured to a distance of 9 cm from the seed center as a function of angle about the seed axis. A silicon diode and electrometer were used in the measurement. Two empirical models have been fitted to the data. The data have also been compared with Monte Carlo calculations.
CT-assisted assessment of bladder and rectum dose in gynecological implants.
Eight patients who received gynecological implants with Fletcher-Suit type applicators were involved in this study. An orthogonal pair of films and computed tomographic scans were obtained for each patient. In the CT study, judicious use of contrast materials and selective window and level settings permitted clear delineation of the bladder and the rectum boundaries relative to the implanted applicators. In comparison to reference organ doses derived from the orthogonal film pair method, the maximum organ doses estimated from the CT-assisted evaluation were considerably higher, by approximately twofold on the average. The differences between the values estimated from the two methods vary from patient to patient, being highly dependent on the individual anatomy and the geometry of the implanted sources. These preliminary results point to the inaccuracy of the conventional method of estimating organ doses. CT-assisted evaluation may be necessary to accurately calculate organ doses in gynecological applications.
Irradiation of bilateral orbital lymphoma: a non-coplanar technique with case reports.
Bilateral synchronous involvement of the orbits by lymphoma is an uncommon event. Therefore, the irradiation techniques for this disease are not well refined. Depending on the technique used, one has to decide between underdosing the anterior segment of the orbit and the frequently involved lacrimal glands or to tolerate shield related in homogeneities in the range of 40 to 50% as well as higher doses to the midline structures, e.g. the optic chiasma. To avoid the above shortcomings we developed a technique by which half the prescribed dose is delivered by pairs of beams angulated in the transverse plane and the other half by pairs of beams angulated in the sagittal plane. Film dosimetry in a phantom confirms that the entire orbit, including its anterior portion, is well covered, the lens is protected, and the dose to the mid-sagittal structures is not increased. The dose inhomogeneity is in the range of 20% which represents a substantial improvement over alternative methods. Two irradiated patients with bilateral orbital lymphoma are presented with local control and no ophthalmic or other side effects as of last follow-up.
The effect of hypoxic cell sensitizers at different irradiation dose rates.
The effect of 0.1-5 mM misonidazole and SR 2508 on hypoxic V79 cellular survival at acute (498 cGy/min) and low (890 and 933 cGy/h) irradiation dose rates was measured and compared. The experiments were designed to delineate the oxygen mimetic phenomenon and the preincubation effect of these chemicals at these dose rates. Linear regression analysis of the survival data in terms of the linear quadratic model yielded values of alpha and beta. In the absence of drug, the linear coefficient was independent of dose rate, whereas the quadratic term was greatly reduced at low dose rate. At all dose rates, the preincubation effect affected primarily the alpha term, with little influence on beta. In contrast, the oxygen mimetic phenomenon predominantly affected the beta term. Overall, the radiosensitizing ability of these drugs was higher at low dose rate than at acute dose rate.
Transfection of a human gene for the repair of X-ray- and EMS-induced DNA damage.
EM9 cells are a line of Chinese hamster ovary cells that are sensitive to killing by ethylmethanesulfonate (EMS) and X ray, since they are unable to repair the DNA damage inflicted by these agents. Through DNA-mediated gene transfer, human DNA and a selectable marker gene, pSV2neo, were transfected into EM9 cells. Resistant clones of transfected cells were selected for by growth in EMS and G418 (an antibiotic lethal to mammalian cells not containing the transfected neo gene). One primary clone (APEX1) and one secondary clone (TEMS2) were shown to contain both marker and human DNA sequences by Southern blot. In cell survival studies, APEX1 was shown to be as resistant to EMS and X ray as the parental cell type AA8 (CHO cells). TEMS2 cells were found to be partially resistant to EMS and X ray, displaying an intermediate phenotype more sensitive than AA8 cells but more resistant than EM9 cells. Alkaline elution was used to assess the DNA strand-break rejoining ability of these cells at 23 degrees C. APEX1 cells showed DNA repair capacity equal to that of AA8 cells; 75% of the strand breaks were repaired with a rejoining T 1/2 of 3 min. TEMS2 showed similar levels of repair but a T 1/2 for repair of 9 min. EM9 cells repaired only 25% of the breaks and showed a T 1/2 for repair of 16 min. The DNA repair data are consistent with the survival data in that the more resistant cell lines showed a greater capacity for DNA repair. The data support the conclusion that APEX1 and TEMS2 cells contain a human DNA repair gene.
Detection of S-phase overreplication following chronic hypoxia using a monoclonal anti-BrdUrd.
We have examined cytokinetic perturbations induced in Chinese hamster V-79 cells in vitro during and following exposure to chronic hypoxia employing simultaneous flow cytometric measurement of incorporated BrdUrd and DNA content. These data indicate hypoxia inhibited G1 progression into S-phase, but did not significantly delay G2M division and progression into G1. Also, upon reaeration after 20 hr in hypoxia, cells originally in G1 exhibited significant kinetic delay. BrdUrd pulse/chase and pulse/fix data indicated DNA replication was reduced, but not completely inhibited during hypoxia. Also, between 6 and 20 hr of chronic hypoxia and following reaeration, a subset of the original S-phase cells overreplicated their DNA, such that these cells had greater than 2C DNA content. This subpopulation was estimated on the average to comprise approximately 20% of the total population (30% of the treated S-phase subpopulation) by 24 hr following reaeration after 20 hr hypoxia. These results are discussed in light of the similarities between overreplication and gene amplification observed under certain conditions with other agents, which like chronic hypoxia, are used to transiently disrupt DNA synthesis.
The variation of OER with dose rate.
The oxygen enhancement ratio (OER) has been measured as a function of dose rate from 276 Gy/hr to 0.89 Gy/hr for V-79 cells irradiated at 23 degrees C or 37 degrees C. As dose rate is decreased, the OER initially increases, from a value of 3.0, to a maximum value of 3.7 to 4.0, at a dose rate between 20 and 60 Gy/hr. The OER subsequently decreases with further dose rate reduction to a minimum value of 2.4 at the lowest dose rate. Similar experiments conducted with cells in nutrient deprived conditions exhibited a monotonic decrease in OER from 3.0 to 1.7 with dose rate reduction. These experimental findings can be understood in terms of the sublethal damage repair capability of cells under different pO2 and nutrient conditions.
Air-generated electron contamination of 4 and 10 MV photon beams: a comparison of theory and experiment.
The contribution to the surface dose of contaminant electrons produced in air is studied for the photon beams of the Clinac 4 and Clinac 18 by measuring the change in per cent surface ionisation when a portion of the air path is replaced by helium. Measurements are made both with and without a magnetic field to sweep away electrons produced in the jaws. The measured changes are compared with the predictions of the theory of Nilsson and Brahme and also with the theory of Howarth et al, modified for the photon spectrum.
Orbital tumor irradiation using non-coplanar beams.
Irradiation of the orbit is often associated with substantial dose inhomogeneity resulting from the insertion of lens blocks. We postulated that such dose inhomogeneity, which often exceeds 40%, could be halved by the use of two pairs of wedged beams, one angled in the sagittal plane and the other in the commonly employed transverse plane. The sagittal plane is obtained by turning the treatment couch 90 degrees. All beams carried a central-axis lens block and were angled 30 degrees relative to the vertical. Verification of dose distribution was obtained by film dosimetry in a head phantom for central- and off-axis planes. These measurements indicate that significant improvement in dose homogeneity over alternate methods can be achieved with this technique.
Oxygen radiosensitisation at low dose rate.
Oxygen radiosensitisation has been studied at dose rates of 600, 3.37 and 0.89 Gy/h at pO2 levels of 0.001, 0.03, 0.1, 0.3, 1, 3, 10 and 21% in the gas phase. The oxygen enhancement ratio (OER), evaluated at 2% cellular survival, exhibited a decrease with dose rate from a value of 3.2 at the acute dose rate, to 2.4 at the lowest dose rate. This observation results from a decreased dose-rate effect on hypoxic cells, which is attributed to the partial suppression of sublethal damage (SLD) repair under hypoxic conditions. Oxygen radiosensitisation at the acute dose rate agrees with the calculated values based on the oxygen fixation hypothesis. Direct application of the Howard-Flanders and Moore equation to results obtained at low dose rate is not appropriate due to the influence of pO2 on SLD repair which affects radiosensitivity at low dose rate. When cells are irradiated at 3.37 Gy/h under nutrient-deprived condition (i.e. in Hanks balanced salt solution without glucose), low levels of oxygen appear to be more radioprotective than extreme hypoxia. Specifically, cells irradiated with 0.03% and 0.1% O2 are more radioresistant than cells under N2, with enhancement factors of 0.7 and 0.8, respectively. This phenomenon can be understood in terms of the ability of moderately hypoxic cells (0.03%-0.1% O2), and inability of anoxic cells, to repair SLD under nutrient deprived conditions. Radiosensitisation by these low levels of oxygen is insufficient to offset the difference caused by the disparate SLD repair capabilities.
Cellular and molecular repair of X-ray-induced damage: dependence on oxygen tension and nutritional status.
Cellular and molecular repair was studied at 23 degrees C using split-dose recovery and alkaline elution techniques, respectively, as a function of cellular oxygen and nutrient conditions. Hypoxic cells (0.001% O2) in full medium showed a partial reduction in the level of sublethal damage (SLD) repair relative to aerated cells (21% O2, OER equal to 3.2 relative to 0.001% O2); the respective repair kinetics were similar with a common repair half-time of 30 min. Similarly, hypoxic cells showed a slight reduction in strand break rejoining capacity compared to aerated cells. Under nutrient deprivation, anoxic cells displayed no SLD repair or strand break repair, while aerated cells exhibited the same level of SLD and strand break repair as for well-fed cells. In addition, nutrient deprived cells at low O2 levels (0.03%, OER equal to 1.15) displayed normal SLD and strand break repair capability. These results indicate that both nutrient and O2 deprivation are necessary for complete inhibition of cellular and molecular repair, and low levels of O2 (0.03%) can effectively reverse this inhibition.
Cell killing, radiosensitization and cell cycle redistribution induced by chronic hypoxia.
Some of the biological changes associated with extreme hypoxia at 37 degrees C (less than 10 ppm pO2) were examined in Chinese hamster V79 cells. Specifically, extreme hypoxia caused an initial decrease in plating efficiency to 55% in 4 hr after the onset of hypoxia. Beyond this time, the decline in plating efficiency was more gradual reaching 35% of control at 20 hr. Flow microfluorimetry (FMF) studies, in which cells are sorted on the basis of DNA content and then assayed for viability, demonstrated that mid S phase cells were most sensitive to chronic hypoxia, with surviving fraction equal to 2.5% at 20 hr. Furthermore, the viability of G1 and G2/M cells, after 20 hr of hypoxic storage, was also reduced to 20 and 7.6%, respectively. Hypoxia also caused alterations in the cell cycle distribution of initially asynchronous cells, as determined by dual parameter FMF measurements of both cellular DNA content and incorporated BudR. In particular, G2/M cells completed mitosis, while G1 cells showed little or no movement. Lastly, cells stored in chronic hypoxia displayed an enhanced radiosensitivity when compared to acutely hypoxic cells. Possible reasons for these observations are discussed.
Dose-rate effect between 1 and 10 Gy/min in mammalian cell culture.
A dose-rate effect is observed between 1 and 10 Gy/min for cultured Chinese hamster V-79 fibroblast cells irradiated under pO2 levels of less than 10 ppm, 100 ppm, 300 ppm and 21%. The dose for a cellular surviving fraction of 0.1% increases by 10-15% as the dose rate is changed from 10 to 1 Gy/min. This observed dose-rate effect is consistent with the repair of sublethal damage taking place during the radiation delivery. A related finding is that acute hypoxia in cell culture does not inhibit sublethal damage repair.
Physical dosimetry of 125I seeds of a new design for interstitial implant.
The physical characteristics of a new 125I seed, consisting of radioactive iodine absorbed on a silver wire and contained in a sealed titanium shell, have been measured. Advantages of the new seed design are: increased radiopacity, possible determination of seed orientation in an implant for dosimetric calculations, and source strength specification traceable to the National Bureau of Standards. Spectroscopic analysis of the new seed using an intrinsic Ge detector revealed the 27.4, 31.4 and 35.5kev photons from the decay of 125I, and in addition, 22.1 and 25.2kev fluorescent X ray from the silver wire. Measured and calculated relative dose distribution along the perpendicular bisector of the new seed is similar to that of the existing seed, with a slightly more rapid fall-off due to the existence of the lower energy photons. The measured angular distributions of the seeds of the two designs are similar, exhibiting significant anisotropy. A protocol of source strength specification, choice of effective gamma constant value and dose calculation relative to 125I implants is suggested.
Contaminant electrons in the build-up region of a 4 MV photon beam.
The per cent ionisation curves for the 4 MV photon beam from a Clinac 4 exhibit pronounced enhancement with field size in the build-up region. The source and nature of this increase is investigated by analysing the radiation components of the 4 MV photon beam with a 0.23 T magnetic field. The build-up ionisation curves were measured at 86 cm SSD with a parallel-plate ionisation chamber embedded in a polystyrene phantom. The surface ionisation increases from 18% to 32% as the field size is increased from 4.3 cm X 4.3 cm to 9.7 cm X 32.3 cm. With the electromagnet placed beside the treatment head, the field size dependence reduces to 16% to 24%. This and similar results at 113.2 cm SSD, clearly demonstrate that electrons from the Clinac 4 treatment head are responsible for approximately half the observed increase in the surface ionisation with field size.