SU-E-T-155: Dose Response Curve of EBT2 and EBT3 Radiochromic Films to a Synchrotron-Produced Monochromatic X-Ray Beam.

PURPOSE: This work investigates the dose-response curves of Gafchromic EBT2 and EBT3 radiochromic films using synchrotron-produced monochromatic x-ray beams. These dosimeters are being utilized for dose verification in photoactivated Auger electron therapy at the LSU Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. METHODS: Monochromatic beams of 25, 30 and 35 keV were generated on the tomography beamline at CAMD. Ion chamber depth-dose measurements were used to calculate the dose delivered to films irradiated simultaneously at depths from 0.7 - 8.5 cm in a 10×10×10-cms polymethylmethacrylate phantom. AAPM TG-61 protocol was applied to convert measured ionization into dose. Calibrations of films at 4 MV were obtained for comparison using a Clinac 21 EX radiotherapy accelerator at Mary Bird Perkins Cancer Center. Films were digitized using an Epson 1680 Professional flatbed scanner and analyzed using the optical density (OD) derived from the red channel. RESULTS: For EBT2 film the average sensitivity (OD/dose) at 50, 100, and 200 cGy relative to that for 4-MV x- rays was 1.07, 1.20, and 1.23 for 25, 30, and 35 keV, respectively. For EBT3 film the average sensitivity was within 3 % of unity for all three monochromatic beams. CONCLUSIONS: EBT2 film sensitivity shows strong energy dependence over an energy range of 25 keV - 4 MV. EBT3 film shows weak energy dependence, indicating that it would be the better dosimeter for Auger electron therapy. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

Changes in plasma arginine vasopressin concentrations in cyclists participating in a 109-km cycle race.

OBJECTIVE: To evaluate the osmotic and non-osmotic regulation of arginine vasopressin (AVP) during endurance cycling. DESIGN: Observational study. Setting 109 km cycle race. PARTICIPANTS: 33 Cyclists. INTERVENTIONS: None. MAIN OUTCOME MEASUREMENTS: Plasma sodium concentration ([Na(+)]), plasma volume (PV) and plasma arginine vasopressin (AVP) concentration ([AVP](p)). RESULTS: A fourfold increase in [AVP](p) occurred despite a 2-mmol l(-1) decrease in plasma [Na(+)] combined with only modest (5%) PV contraction. A significant inverse correlation was noted between [AVP](p) Delta and urine osmolality Delta (r = -0.41, p<0.05), whereas non-significant inverse correlations were noted between [AVP](p) and both plasma [Na(+)] Delta and % PV Delta. Four cyclists finished the race with asymptomatic hyponatraemia. The only significant difference between the entire cohort with this subset of athletes was postrace plasma [Na(+)] (137.7 vs 133.5 mmol l(-1), p<0.001) and plasma [Na(+)] Delta (-1.9 vs -5.1 mmol l(-1), p<0.05). The mean prerace [AVP](p) of these four cyclists was just below the minimum detectable limit (0.3 pg ml(-1)) and increased marginally (0.4 pg ml(-1)) despite the decline in plasma [Na(+)]. CONCLUSIONS: The osmotic regulation of [AVP](p) during competitive cycling was overshadowed by non-osmotic AVP secretion. The modest decrease in PV was not the primary non-osmotic stimulus to AVP. Partial suppression of AVP occurred in four (12%) cyclists who developed hyponatraemia during 5 h of riding. Therefore, these results confirm that non-osmotic AVP secretion and exercise-associated hyponatraemia does, in fact, occur in cyclists participating in a 109 km cycle race. However, the stimuli to AVP is likely different between cycling and running.

Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling.

The aim of the study was to examine the effects of fluid replacement on thermoregulation and cycling performance in hot, humid conditions. Six male cyclists (PPO = 426 +/- 39 W) performed six 80 km time trials. Subjects replaced 0% (0); 33% (33); 66% (66); or 100% (100) of the weight lost during an "ad libitum" trial (Ad Lib). In another condition (WET), subjects rinsed their mouths at 10 km intervals. There was no trial effect on any thermoregulatory variables or on performance. When WET, 0, 33 ("LO") were compared to Ad Lib; 66, 100 ("HI"), power output was higher in HI (209 +/- 22 vs. 193 +/- 22 W, p < 0.05). Restricting fluid below ad libitum rates impaired performance (LO group). Rates greater than ad libitum did not result in further improvements. Ad libitum fluid ingestion is optimal as it prevents athletes from ingesting too little or too much fluid.

Hyponatraemic encephalopathy despite a modest rate of fluid intake during a 109 km cycle race.

OBJECTIVE: To report a case of exertional hyponatraemic encephalopathy that occurred despite a modest rate of fluid intake during a 109 km cycling race. METHODS: Men and women cyclists were weighed before and after the race. All subjects were interviewed and their water bottles measured to quantify fluid ingestion. A blood sample was drawn after the race for the measurement of serum Na(+) concentration. RESULTS: From the full set of data (n = 196), one athlete was found to have hyponatraemic encephalopathy (serum [Na(+)] 129 mmol/l). She was studied subsequently in the laboratory for measurement of sweat [Na(+)] and sweat rate. CONCLUSIONS: Despite a modest rate of fluid intake (735 ml/h) and minimal predicted sweat Na(+) losses, this female athlete developed hyponatraemic encephalopathy. The rate of fluid intake is well below the rate currently prescribed as optimum. Drinking to thirst and not to a set hourly rate would appear to be the more appropriate behaviour.

The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment.

AIM: The purposes of this study were to determine (i) the effects of different facing air velocities on body temperature and heat storage during exercise in hot environmental conditions and (ii) the effects of ingesting fluids at two different rates on thermoregulation during exercise in hot conditions with higher air velocities. METHODS: On five occasions nine subjects cycled for 2 h at 33.0 +/- 0.4 degrees C with a relative humidity of 59 +/- 3%. Air velocity was maintained at 0.2 km h(-1) (0 WS), 9.9 +/- 0.3 km h(-1) (10 WS), 33.3 +/-2.2 km h(-1) (100 WS) and 50.1 +/- 3.2 km h(-1) (150 WS) while subjects replaced 58.8 +/- 6.8% of sweat losses. In the fifth condition, air velocity was maintained at 33.7 +/- 2.2 km h(-1) and subjects replaced 80.0 +/- 6.8% of sweat losses (100.80 WS). RESULTS: Heat storage, body temperature and rating of perceived exertion were higher in 0 and 10 WS compared with all other conditions. There were no differences in any measured variable between 100 and 150 WS, or between 100 and 100.80 WS. Thus, the evaporative capacity of the environment is increased with higher air velocities, reducing heat storage and body temperature. At higher air velocities, a higher rate of fluid ingestion did not influence heat storage, body temperature or sweat rate. CONCLUSION: The finding of previous laboratory studies showing a beneficial effect of high rates of fluid ingestion on thermoregulation during exercise in hot, humid, windstill conditions cannot be extrapolated to out-of-doors exercise in which facing air velocities are seldom lower than the athlete's rate of forward progression.

The effect of pre-exercise cooling on high intensity running performance in the heat.

The purpose of this study was to determine the effect of pre-exercise cooling on high intensity, moderate duration running performance and thermoregulatory responses in a hot environment (38 degrees C, 40 %RH). On separate days, 11 male subjects completed two treadmill runs to exhaustion at 100% of maximal aerobic power with (CL) and without (CT) pre-exercise cooling. Cooling consisted of 20 min of standing rest in a 22 degrees C environment with fan cooling (4.0 m x sec -1) and water spraying (50 ml x min -1) applied to both anterior and posterior body surfaces. Core temperature (T(c)) was determined with an esophageal T(es) probe, and skin temperatures (T(sk)) were measured using surface thermistors positioned at four sites. Finger prick blood samples were taken before and after exercise for the determination of blood lactate. Heart rates and ratings of thermal sensations and comfort were also recorded. Time to exhaustion was significantly shorter in the CL condition (368.9 +/- 56.2) compared to the CT condition (398.8 +/- 55.5 sec). Peak T(es) (37.51 +/- 0.57 vs. 38.56 +/- 0.30 degrees C for CL and CT, respectively), T(sk) (34.18 +/- 1.22 vs. 36.15 +/- 0.70 degrees C for CL and CT, respectively), rates of heat gain (0.20 +/- 0.05 vs. 0.28 +/- 0.05 degrees C x min -1 for CL and CT, respectively), and net heat storage (238.4 +/- 109.6 vs. 531.9 +/- 78.3 kJ for CL and CT, respectively) were all lower in the CL compared to CT throughout the treadmill runs. There were no differences in lactate accumulation between the two conditions. Based on these data, it can be concluded that pre-exercise cooling influences thermoregulatory responses during high intensity, moderate duration exercise; however, performance is impaired compared to a control trial in which no cooling procedures were employed.

The influence of different external cooling methods on thermoregulatory responses before and after intense intermittent exercise in the heat.

The purpose of this study was to determine the effect of different cooling methods on thermoregulation before and after intermittent anaerobic exercise in the heat (38 degrees C). On separate days, 10 men completed 4 conditions consisting of 2 sets of six 30-second sprints (with 30 seconds of rest) at 125% of maximal aerobic power with each set of sprints followed by a cooling procedure. The 4 conditions were the following: passive cooling at room temperature (22 degrees C; PRC), fan cooling (4.0 m x s(-1), 22 degrees C; FAC), fan cooling with water spraying (50 ml x min(-1); FWC), and a noncooling passive recovery in the heat chamber (38 degrees C; PCC). Each set of 6 sprints was followed by a 12-minute cooling period; after the second 12-minute period, cooling continued until esophageal temperature (Tes) was reduced by 1.0 degrees C. Tes and mean skin temperatures (Tsk) were taken before and during exercise and during all cooling phases. Cooling rates (mean +/- SEM) after the second set of sprints (based on Tes) were greater (p < 0.05) in PRC (0.043 +/- 0.007) than in the other conditions (FWC = 0.027 degrees +/- 0.005 degrees, FAC = 0.03 degrees +/- 0.004 degrees, and PCC = 0.021 degrees +/- 0.003 degrees C per minute). Overall decreases in heat content, however, were greater in the FWC (-332.2 +/- 27.8) and FAC (-129.9 +/- 14.7 kJ) conditions compared with the PRC condition (29.0 +/- 14.9 kJ). The time required to lower Tes by 1.0 degrees C with PRC (22.8 +/- 1.8) was less than with FAC (30.4 +/- 2.7 minutes). Finally, the rate of increase in Tes during the second set of sprints was less in the FAC and FWC conditions (0.15 degrees +/- 0.01 degrees and 0.11 degrees +/- 0.01 degrees C per minute) compared with the PCC and PRC conditions (0.19 degrees +/- 0.01 degrees and 0.18 degrees +/- 0.01 degrees C per minute), suggesting differences in pre-exercise cooling. Based on cooling rates and the time required to lower Tes by 1.0 degrees C, PRC was the most effective method of cooling. The conclusion is different, however, when taking into account changes in heat content since the FAC and FWC conditions were more effective in dissipating heat and in preventing heat gain during the second set of sprints