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Acute and late damage in the mouse small intestine following multiple fractionations of neutrons or X-rays
lnt. J. Rudiution
Onwlogy
Biol. Phvs.. 1977, Vol. 2. pp. 693496.
Pergamon Pres\.
Printed in the U.S.A.
??Original Contribution
ACUTE
AND LATE DAMAGE IN THE MOUSE SMALL INTESTINE FOLLOWING MULTIPLE FRACTIONATIONS OF NEUTRONS OR X-RAYS?
J. P. GERACI,Ph.D., K. L. JACKSON,Ph.D., G. M. CHRISTENSEN, Ph.D., P. D. THROWER,B.S. and B. J. WEYER, B.S. Department of Radiology, Division of Radiation Biology, SB-30 University of Washington, Seattle, WA 98195, U.S.A. The neutron RBE for acute intestinal damage as measured by decrease in the intestine DNA content and the LD50,5ds,following whole-body exposure of mice is compared with the RBE for late effects in this organ. Late damage, primarily fibrosis, ulceration, perforation, and obstruction, was produced by irradiating a segment of the intestine. A significant sparing effect of fractionation was observed for all forms of gut injury with neutrons and X-rays. However, there was less sparing with neutron-induced injury, resulting in an increase in RBE with fractionation. The RBE for late effects changed more rapidly than the RBE for acute effects. This resulted in a significantly larger RBE for late effects relative to acute effects as the neutron dose per fraction approached that used clinically. RBE, Neutrons,
Intestine,
Acute effects, Late effects, Fractionation.
Whole-body
INTRODUCTION
METHODS
AND
Partial intestine irradiation
MATERIALS
Animals
Under Sodium Pentobarbital anesthesia, 4 cm segments of the ileum were marked with a silk suture through the omentum; they were exteriorized and exposed to graded equal daily doses of X-rays or
Female CD-l mice (25-30 g) from the investigators’ mouse colony were used in this study. The animals were given Purina Chow and water ad libitum. tThis work was supported by N.I.H.
irradiation
Mice were retained in Plexiglass boxes of sufficient size to allow them to turn around during irradiation. Groups of 8 mice were exposed to graded doses of neutrons or X-rays. X-rays were generated by a General Electric Maxitron therapy unit (dose rate about 100 rad/min; 50 cm tube target to animal distance; 250 kVp; 0.5 mm Cu, lmm Al added filtration). The absorbed X-ray doses were measured with thermoluminescent dosimeters (TLD) which were inserted surgically in the abdomen of test animals. Neutrons (8 MeV mean energy) were produced at the University of Washington cyclotron by 22 MeV deuteron bombardment of a beryllium (Be) target which absorbed 70% of the incident deuterons. The animals were irradiated 125 cm from the Be target. Neutron dosimetry was carried out with a tissue equivalent ionization chamber.’ The gamma contribution to the total dose was about 5%. Doses are reported as the total neutron and -y-radiation doses. Neutron dose rates were approximately 35 rad/min.
In cancer radiotherapy of the abdominal and pelvic regions the partially irradiated intestine is a critical normal tissue at risk. An excessive radiation dose delivered to a portion of the intestine can produce local mucosal ulceration, fibrosis and stricture; this leads to luminal obstruction and death if it is not corrected surgically. Because of the paucity of information concerning late neutron damage to this organ, dose and fractionation schemes presently being used in neutron therapy, where the intestine is dose limiting, are based on acute Relative Biological Effectiveness (RBE) values as measured by the 50% lethal dose within 5 days of irradiation (LDSo,5day)or mucosal stem cell survival following whole-body irradiation. In this regard there is concern that the RBE for late damage may exceed the RBE for acute damage when fractionation of the exposure is employed. This has been studied in the present investigation using local irradiation of a portion of the intestine.
Grant CA 12441. 693
694
Radiation
Oncology 0 Biology 0 Physics
neutrons. The silk sutures were used as topographic markers, allowing repeated irradiation of the same segment of intestine. For X-irradiation, 8 animals were positioned across the side of 15 x 15 cm2 aperture in a 0.6 cm thick sheet of lead with the 4cm segment of intestine located in the primary beam. The exteriorized intestine was 50 cm from the X-ray tube. The X-ray dose rate was about lOOrad/min as determined by TLD. A similar technique was employed for neutron irradiation. A 15 x 15 cm* field was produced by means of a borated water extended polyester collimator. The segment of intestine was located 105 cm from the Be target. At the intestine position the neutron dose rate was about 60rad/min as determined with a tissue equivalent ionization chamber. In order to keep the intestine moist and to supply radiation dose “build up” material the exteriorized segment of intestine was sandwiched between several thicknesses of saline soaked gauze during neutron or X-irradiation. The dose to the shielded portion of the animal was less than 2% of the primary field dose for X-irradiation and 5% for neutron irradiation. Following irradiation the intestine was returned to the abdomen and the wound closed with surgical “auto clips” (Clay-Adams). The silk sutures were removed after the last exposure.
July-August
1977. Volume 2, No. 7 and No. 8
cording to the procedure Wilcoxon.
described
by Litchfield
and
RESULTS Table 1 presents the LDSOjSday,RBE and their 95% confidence intervals for acute intestinal mortality following 1, 2, 3 and 5 daily whole-body fractions of neutrons or X-rays. These results show a significant increase in the LDs0/5day resulting from fractionation following both neutron and X-irradiation. The neutron RBE values did not differ significantly with fractionation. Figures 1 and 2 show the dose-response curve for a decrease in intestinal DNA content 2 days after 1,2, 3 and 5 daily fractions of neutrons or X-rays. As was the case with the LD5015dsya significant increase in the dose
100
I Small Intestine Neutron
Intestine DNA Two days after the last whole-body exposure, the mice were sacrificed, the small intestines removed, quick-frozen in liquid N2, and stored at - 20°C. For determination of DNA content the thawed intestine was homogenized and extracted with 5% (W/V) trichloroacetic acid by the method of Schneider.’ DNA was measured by combining the double wave lengths spectrophometric method of Dische’ with Burton’s’ method of color development.
Statistics LDSO and RBE were calculated
and their 95% confidence intervals by graphical probit analysis ac-
4Ot
Dose
Fig. 1. DNA content of the small intestine 2 days after the last neutron exposure as a function of the total neutron dose. Values have been normalized to the zero dose value. Each point represents the mean of 8 mice. The dose effect curve is a least squares best fit of the mean intestinal DNA content following 1, 2, 3 or 5 daily fractions of neutrons.
Table 1. Acute intestinal No. of fractions 1 2 3 5
death
No. of mice
Radiation
LDso,sdayt (rad)
160 160 80 80 96 80 96 80
X-ray Neutron X-ray Neutron X-ray Neutron X-ray Neutron
1228 (1167-1292) 520 (477-567) 1420 (1367-1475) 600 (550-655) 1700 ( 1609- 1796) 715 (655-780) 2340 (2153-2543) 825 (775-878)
Dose/fraction (rad) 1228 520 710 300 567 238 468 165
RBES 2.4 (2.2-2.6) 2.4 (2.2-2.6) 2.4 (2.2-2.6) 2.8 (2.5-3.1)
tLDsojSd,, in rad followed by 95% confidence limits. SThe ratio of LD50,5dayfor X-rays and Neutrons followed by 95% confidence
limits.
Acute
and late damage
in the mouse
i Small X-ray
Intestine
50 % Effect Ifx 660 2 fx 1040 3fx1540 5 f x 2400
0 J. P. GERACI et al.
was necessary to produce a 50% decrease in intestinal DNA with increasing number of fractions of either neutrons or X-rays. However, this dose increment was significantly less with neutrons, resulting in an increase in the RBE from 1.8 for a single dose to 2.4 for 5 fractions. Table 2 gives the LDSO,Wday, RBE and their 95% confidence intervals following 1, 2, 3 and 5 equal daily fractions of segmental intestine irradiation. The data show a sparing effect of fractionation as indicated by an increase in the LDTOIqO dk,, with either neutrons or X-rays. The RBE for this late damage, however, increased from 1.9 for a single dose to 3.4 for 5 fractions resulting from less sparing of the neutron-induced injury. The data presented in Fig. 3 show the RBE for different forms of acute and late gut damage as a function of neutron dose per fraction. There was a trend for increase in RBE at low neutron doses per fraction for all forms of gut damage. However, the RBE for late gut damage as measured by the LD (O,YOdi,y increased more rapidly than acute damage. At low neutron doses per fraction the RBE for late
100
90.
small intestine
rod rod rad rod
Dose
Fig. 2. DNA content of the small intestine 2 days after the last X-ray exposure as a function of total X-ray dose. DNA values have been normalized to the zero dose value. Each point represents the mean of 8 mice. The dose effect curves are a least squares best fit of the mean intestinal DNA content following 1, 2, 3 or 5 daily fractions of X-rays.
Table 2. RBE for late damage to the intestine No. of fractions 1
2 3 5
No. of mice
Radiation
L&I,, d,,t (rad)
255 210 215 228 80 80 80 7.5
X-ray Neutron X-ray Neutron X-ray Neutron X-ray Neutron
2260 (2132-2395) 1180 (1109-1255) 2520 (2270-2798) 1240 (1145-1342) 3040 (2828-3267) 1290 (1192-1396) 4.550 (4322-4790) 1340 (1230- 1460)
Dose/fraction (rad) 2260 1180 1260 620 1013 430 910 268
RBES 1.9 (I .7-2.1) 2.0(1.7-2.3) 2.4 (2.1-2.7) 3.4 (3.1-3.7)
in rad followed by 95% confidence ILDm,wday
limits. for X-rays and Neutrons followed by 95% confidence *The ratio of LD1OIWday
limits.
4.0 / Late
b
A
LD50/90d
’
LD50/5d
0
50%
decrease
In gut DNA
0
Neutron
Dose per Fraction
Fig. 3. RBE for different forms of gut damage as a function
of neutron
dose per fraction.
696
Radiation Oncology 0 Biology 0 Physics
damage was significantly acute damage.
greater
than the RBE for
DISCUSSION Previous work has shown that most of the animals which succumb following segmental intestinal irradiation die between 6 and 90 days post-exposure.4 The mechanism of death is associated with bowel obstruction, perforation, stricture, fibrosis, adhesions, and ulceration of the intestine at the irradiated site.4 This damage is similar to the late clinical complication observed following abdominal radiation therapy. A measure of the dose required to produce this injury in mice is the LDSOIWday. In the present study it has been observed that as the neutron dose per fraction approaches 268 rad, the RBE, as measured by the LDr,,,9ndayproduced by segmental gut irradiation, is substantially larger than the RBE for acute injury as measured by the LDsolsday or decrease in intestinal DNA content (Fig. 3). This difference between late and acute effects is expected to be even more pronounced at the lower neutron doses per fraction (SO-150 rad) used clinically. It is the RBE for late gut effects that should be used to plan
July-August
1977, Volume 2, No. 7 and No. 8
treatment doses in neutron therapy where the intestine is the tissue at risk if late complications are to be avoided. Also, since it is possible that the RBE for late gut injury may exceed the tumor RBE, this may preclude use of neutron therapy where there is a high probability that the same segment of intestine will be repeatedly exposed during treatment. Further work is needed, however, with larger numbers of fractions to prove unequivocally that the RBE for late effects exceeds that for acute effects in the intestine at the lower neutron dose per fraction used in therapy. Nevertheless, the observation that the neutron RBE at 268 rad for late damage is larger than the RBE for acute damage in the same normal tissue is the first animals. such reported observation in experimental This finding is statistically significant; it is based on the responses of 155 mice used to measure the RBE
for late damage at 268 rad of neutrons (Table 2). In the only other similar study, carried out with skin, Field3 has reported that the RBE for acute and late effects is approximately the same. However, the present results agree with the clinical observation that the late neutron radiation sequelae in normal tissue (primary fibrosis and ulceration) are greater than that predicted by the severity of acute reactions.“*
REFERENCES 1. Burton, K.: A study of the conditions and mechanism of the diphenylamine reaction for calorimetric estimation of deoxyribonucleic acid. Biochem. J. 62: 315-323, 1956. 2. Dische, Z.: Color reactions of nucleic acid components. In The Nucleic Acids, ed. by Chargaff, E., Davidson, J.N., New York, Academic Press, 1955, Vol. I, pp. 285-305. 3. Field, S.B.: Early and late reactions in skin of rats following irradiation with X-rays or fast neutrons. Radiology 92: 381-384, 1%9. 4. Geraci, J.P., Jackson, K.L., Christensen, G.M., Parker, R.G., Fox, M.S. Thrower, P.D.: The relative biological effectiveness of cyclotron fast neutrons for early and late damage to the small intentine of the mouse. Europ. J. Cancer 10: 99-102, 1974. 5. Hussey, D.H., Fletcher, G.H., Caderao, J.B.: Experience
6.
7.
8. 9.
with fast neutron therapy using the Texas A & M variable energy cyclotron. Cancer 34: 65-77, 1974. Litchfield, J.T., Wilcoxon, F.: A simplified method of evaluating dose-effect experiments. J. Pharmocol. Expt. Therapy 96: 99-113, 1949. Schneider, W.C.: Phosphorous compounds in animal tissue; extraction and estimation of desoxypentose nucleic acid and of pentose nucleic acid. J. Biol. Chem. 161: 293-303, 1945. Stone, R.S.: Neutron therapy and specific ionization. Am. J. Roentgenol. 59: 771-785, 1948. Wootton, P., Alvar, K., Bichsel, H., Eenmaa, J., Nelson, J.S.R., Parker, R.G., Weaver, K.A., Williams, D.L., Wyckoff, W.G.: Fast neutron beam radiotherapy at the University of Washington. J. Can. Assoc. Rad. 26: 44-53 1975.
Onwlogy
Biol. Phvs.. 1977, Vol. 2. pp. 693496.
Pergamon Pres\.
Printed in the U.S.A.
??Original Contribution
ACUTE
AND LATE DAMAGE IN THE MOUSE SMALL INTESTINE FOLLOWING MULTIPLE FRACTIONATIONS OF NEUTRONS OR X-RAYS?
J. P. GERACI,Ph.D., K. L. JACKSON,Ph.D., G. M. CHRISTENSEN, Ph.D., P. D. THROWER,B.S. and B. J. WEYER, B.S. Department of Radiology, Division of Radiation Biology, SB-30 University of Washington, Seattle, WA 98195, U.S.A. The neutron RBE for acute intestinal damage as measured by decrease in the intestine DNA content and the LD50,5ds,following whole-body exposure of mice is compared with the RBE for late effects in this organ. Late damage, primarily fibrosis, ulceration, perforation, and obstruction, was produced by irradiating a segment of the intestine. A significant sparing effect of fractionation was observed for all forms of gut injury with neutrons and X-rays. However, there was less sparing with neutron-induced injury, resulting in an increase in RBE with fractionation. The RBE for late effects changed more rapidly than the RBE for acute effects. This resulted in a significantly larger RBE for late effects relative to acute effects as the neutron dose per fraction approached that used clinically. RBE, Neutrons,
Intestine,
Acute effects, Late effects, Fractionation.
Whole-body
INTRODUCTION
METHODS
AND
Partial intestine irradiation
MATERIALS
Animals
Under Sodium Pentobarbital anesthesia, 4 cm segments of the ileum were marked with a silk suture through the omentum; they were exteriorized and exposed to graded equal daily doses of X-rays or
Female CD-l mice (25-30 g) from the investigators’ mouse colony were used in this study. The animals were given Purina Chow and water ad libitum. tThis work was supported by N.I.H.
irradiation
Mice were retained in Plexiglass boxes of sufficient size to allow them to turn around during irradiation. Groups of 8 mice were exposed to graded doses of neutrons or X-rays. X-rays were generated by a General Electric Maxitron therapy unit (dose rate about 100 rad/min; 50 cm tube target to animal distance; 250 kVp; 0.5 mm Cu, lmm Al added filtration). The absorbed X-ray doses were measured with thermoluminescent dosimeters (TLD) which were inserted surgically in the abdomen of test animals. Neutrons (8 MeV mean energy) were produced at the University of Washington cyclotron by 22 MeV deuteron bombardment of a beryllium (Be) target which absorbed 70% of the incident deuterons. The animals were irradiated 125 cm from the Be target. Neutron dosimetry was carried out with a tissue equivalent ionization chamber.’ The gamma contribution to the total dose was about 5%. Doses are reported as the total neutron and -y-radiation doses. Neutron dose rates were approximately 35 rad/min.
In cancer radiotherapy of the abdominal and pelvic regions the partially irradiated intestine is a critical normal tissue at risk. An excessive radiation dose delivered to a portion of the intestine can produce local mucosal ulceration, fibrosis and stricture; this leads to luminal obstruction and death if it is not corrected surgically. Because of the paucity of information concerning late neutron damage to this organ, dose and fractionation schemes presently being used in neutron therapy, where the intestine is dose limiting, are based on acute Relative Biological Effectiveness (RBE) values as measured by the 50% lethal dose within 5 days of irradiation (LDSo,5day)or mucosal stem cell survival following whole-body irradiation. In this regard there is concern that the RBE for late damage may exceed the RBE for acute damage when fractionation of the exposure is employed. This has been studied in the present investigation using local irradiation of a portion of the intestine.
Grant CA 12441. 693
694
Radiation
Oncology 0 Biology 0 Physics
neutrons. The silk sutures were used as topographic markers, allowing repeated irradiation of the same segment of intestine. For X-irradiation, 8 animals were positioned across the side of 15 x 15 cm2 aperture in a 0.6 cm thick sheet of lead with the 4cm segment of intestine located in the primary beam. The exteriorized intestine was 50 cm from the X-ray tube. The X-ray dose rate was about lOOrad/min as determined by TLD. A similar technique was employed for neutron irradiation. A 15 x 15 cm* field was produced by means of a borated water extended polyester collimator. The segment of intestine was located 105 cm from the Be target. At the intestine position the neutron dose rate was about 60rad/min as determined with a tissue equivalent ionization chamber. In order to keep the intestine moist and to supply radiation dose “build up” material the exteriorized segment of intestine was sandwiched between several thicknesses of saline soaked gauze during neutron or X-irradiation. The dose to the shielded portion of the animal was less than 2% of the primary field dose for X-irradiation and 5% for neutron irradiation. Following irradiation the intestine was returned to the abdomen and the wound closed with surgical “auto clips” (Clay-Adams). The silk sutures were removed after the last exposure.
July-August
1977. Volume 2, No. 7 and No. 8
cording to the procedure Wilcoxon.
described
by Litchfield
and
RESULTS Table 1 presents the LDSOjSday,RBE and their 95% confidence intervals for acute intestinal mortality following 1, 2, 3 and 5 daily whole-body fractions of neutrons or X-rays. These results show a significant increase in the LDs0/5day resulting from fractionation following both neutron and X-irradiation. The neutron RBE values did not differ significantly with fractionation. Figures 1 and 2 show the dose-response curve for a decrease in intestinal DNA content 2 days after 1,2, 3 and 5 daily fractions of neutrons or X-rays. As was the case with the LD5015dsya significant increase in the dose
100
I Small Intestine Neutron
Intestine DNA Two days after the last whole-body exposure, the mice were sacrificed, the small intestines removed, quick-frozen in liquid N2, and stored at - 20°C. For determination of DNA content the thawed intestine was homogenized and extracted with 5% (W/V) trichloroacetic acid by the method of Schneider.’ DNA was measured by combining the double wave lengths spectrophometric method of Dische’ with Burton’s’ method of color development.
Statistics LDSO and RBE were calculated
and their 95% confidence intervals by graphical probit analysis ac-
4Ot
Dose
Fig. 1. DNA content of the small intestine 2 days after the last neutron exposure as a function of the total neutron dose. Values have been normalized to the zero dose value. Each point represents the mean of 8 mice. The dose effect curve is a least squares best fit of the mean intestinal DNA content following 1, 2, 3 or 5 daily fractions of neutrons.
Table 1. Acute intestinal No. of fractions 1 2 3 5
death
No. of mice
Radiation
LDso,sdayt (rad)
160 160 80 80 96 80 96 80
X-ray Neutron X-ray Neutron X-ray Neutron X-ray Neutron
1228 (1167-1292) 520 (477-567) 1420 (1367-1475) 600 (550-655) 1700 ( 1609- 1796) 715 (655-780) 2340 (2153-2543) 825 (775-878)
Dose/fraction (rad) 1228 520 710 300 567 238 468 165
RBES 2.4 (2.2-2.6) 2.4 (2.2-2.6) 2.4 (2.2-2.6) 2.8 (2.5-3.1)
tLDsojSd,, in rad followed by 95% confidence limits. SThe ratio of LD50,5dayfor X-rays and Neutrons followed by 95% confidence
limits.
Acute
and late damage
in the mouse
i Small X-ray
Intestine
50 % Effect Ifx 660 2 fx 1040 3fx1540 5 f x 2400
0 J. P. GERACI et al.
was necessary to produce a 50% decrease in intestinal DNA with increasing number of fractions of either neutrons or X-rays. However, this dose increment was significantly less with neutrons, resulting in an increase in the RBE from 1.8 for a single dose to 2.4 for 5 fractions. Table 2 gives the LDSO,Wday, RBE and their 95% confidence intervals following 1, 2, 3 and 5 equal daily fractions of segmental intestine irradiation. The data show a sparing effect of fractionation as indicated by an increase in the LDTOIqO dk,, with either neutrons or X-rays. The RBE for this late damage, however, increased from 1.9 for a single dose to 3.4 for 5 fractions resulting from less sparing of the neutron-induced injury. The data presented in Fig. 3 show the RBE for different forms of acute and late gut damage as a function of neutron dose per fraction. There was a trend for increase in RBE at low neutron doses per fraction for all forms of gut damage. However, the RBE for late gut damage as measured by the LD (O,YOdi,y increased more rapidly than acute damage. At low neutron doses per fraction the RBE for late
100
90.
small intestine
rod rod rad rod
Dose
Fig. 2. DNA content of the small intestine 2 days after the last X-ray exposure as a function of total X-ray dose. DNA values have been normalized to the zero dose value. Each point represents the mean of 8 mice. The dose effect curves are a least squares best fit of the mean intestinal DNA content following 1, 2, 3 or 5 daily fractions of X-rays.
Table 2. RBE for late damage to the intestine No. of fractions 1
2 3 5
No. of mice
Radiation
L&I,, d,,t (rad)
255 210 215 228 80 80 80 7.5
X-ray Neutron X-ray Neutron X-ray Neutron X-ray Neutron
2260 (2132-2395) 1180 (1109-1255) 2520 (2270-2798) 1240 (1145-1342) 3040 (2828-3267) 1290 (1192-1396) 4.550 (4322-4790) 1340 (1230- 1460)
Dose/fraction (rad) 2260 1180 1260 620 1013 430 910 268
RBES 1.9 (I .7-2.1) 2.0(1.7-2.3) 2.4 (2.1-2.7) 3.4 (3.1-3.7)
in rad followed by 95% confidence ILDm,wday
limits. for X-rays and Neutrons followed by 95% confidence *The ratio of LD1OIWday
limits.
4.0 / Late
b
A
LD50/90d
’
LD50/5d
0
50%
decrease
In gut DNA
0
Neutron
Dose per Fraction
Fig. 3. RBE for different forms of gut damage as a function
of neutron
dose per fraction.
696
Radiation Oncology 0 Biology 0 Physics
damage was significantly acute damage.
greater
than the RBE for
DISCUSSION Previous work has shown that most of the animals which succumb following segmental intestinal irradiation die between 6 and 90 days post-exposure.4 The mechanism of death is associated with bowel obstruction, perforation, stricture, fibrosis, adhesions, and ulceration of the intestine at the irradiated site.4 This damage is similar to the late clinical complication observed following abdominal radiation therapy. A measure of the dose required to produce this injury in mice is the LDSOIWday. In the present study it has been observed that as the neutron dose per fraction approaches 268 rad, the RBE, as measured by the LDr,,,9ndayproduced by segmental gut irradiation, is substantially larger than the RBE for acute injury as measured by the LDsolsday or decrease in intestinal DNA content (Fig. 3). This difference between late and acute effects is expected to be even more pronounced at the lower neutron doses per fraction (SO-150 rad) used clinically. It is the RBE for late gut effects that should be used to plan
July-August
1977, Volume 2, No. 7 and No. 8
treatment doses in neutron therapy where the intestine is the tissue at risk if late complications are to be avoided. Also, since it is possible that the RBE for late gut injury may exceed the tumor RBE, this may preclude use of neutron therapy where there is a high probability that the same segment of intestine will be repeatedly exposed during treatment. Further work is needed, however, with larger numbers of fractions to prove unequivocally that the RBE for late effects exceeds that for acute effects in the intestine at the lower neutron dose per fraction used in therapy. Nevertheless, the observation that the neutron RBE at 268 rad for late damage is larger than the RBE for acute damage in the same normal tissue is the first animals. such reported observation in experimental This finding is statistically significant; it is based on the responses of 155 mice used to measure the RBE
for late damage at 268 rad of neutrons (Table 2). In the only other similar study, carried out with skin, Field3 has reported that the RBE for acute and late effects is approximately the same. However, the present results agree with the clinical observation that the late neutron radiation sequelae in normal tissue (primary fibrosis and ulceration) are greater than that predicted by the severity of acute reactions.“*
REFERENCES 1. Burton, K.: A study of the conditions and mechanism of the diphenylamine reaction for calorimetric estimation of deoxyribonucleic acid. Biochem. J. 62: 315-323, 1956. 2. Dische, Z.: Color reactions of nucleic acid components. In The Nucleic Acids, ed. by Chargaff, E., Davidson, J.N., New York, Academic Press, 1955, Vol. I, pp. 285-305. 3. Field, S.B.: Early and late reactions in skin of rats following irradiation with X-rays or fast neutrons. Radiology 92: 381-384, 1%9. 4. Geraci, J.P., Jackson, K.L., Christensen, G.M., Parker, R.G., Fox, M.S. Thrower, P.D.: The relative biological effectiveness of cyclotron fast neutrons for early and late damage to the small intentine of the mouse. Europ. J. Cancer 10: 99-102, 1974. 5. Hussey, D.H., Fletcher, G.H., Caderao, J.B.: Experience
6.
7.
8. 9.
with fast neutron therapy using the Texas A & M variable energy cyclotron. Cancer 34: 65-77, 1974. Litchfield, J.T., Wilcoxon, F.: A simplified method of evaluating dose-effect experiments. J. Pharmocol. Expt. Therapy 96: 99-113, 1949. Schneider, W.C.: Phosphorous compounds in animal tissue; extraction and estimation of desoxypentose nucleic acid and of pentose nucleic acid. J. Biol. Chem. 161: 293-303, 1945. Stone, R.S.: Neutron therapy and specific ionization. Am. J. Roentgenol. 59: 771-785, 1948. Wootton, P., Alvar, K., Bichsel, H., Eenmaa, J., Nelson, J.S.R., Parker, R.G., Weaver, K.A., Williams, D.L., Wyckoff, W.G.: Fast neutron beam radiotherapy at the University of Washington. J. Can. Assoc. Rad. 26: 44-53 1975.