The r.b.e. of fast fission neutrons (2 MeV) for chronic radiation damage of the large bowel of rats after single dose and fractionated irradiation

Superoxide dismutase mimic, MnTE-2-PyP(5+) ameliorates acute and chronic proctitis following focal proton irradiation of the rat rectum

Radiation proctitis, an inflammation and damage to the lower part of colon, is a common adverse event of the radiotherapy of tumors in the abdominal and pelvic region (colon, prostate, cervical). Several Mn(III) porphyrin-based superoxide dismutase mimics have been synthesized and successfully evaluated in preclinical models as radioprotectants. Here we report for the first time the remarkable rectal radioprotection of frequently explored Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, MnTE-2-PyP(5+). A batch prepared in compliance with good manufacturing practice (GMP), which has good safety/toxicity profile, was used for this study. MnTE-2-PyP(5+) was given subcutaneously at 5 mg/kg, either 1 h before or 1 h after irradiation, with additional drug administered at weekly intervals thereafter. MnTE-2-PyP(5+) ameliorated both acute and chronic radiation proctitis in male Sprague-Dawley rats irradiated with 20-30 Gy protons delivered to 2.5 cm span of rectum using spread-out Bragg peak of a proton treatment beam. Focal irradiation of the rectum produced acute proctitis, which healed, followed by chronic rectal dilation and symptomatic proctitis. MnTE-2-PyP(5+) protected rectal mucosa from radiation-induced crypt loss measured 10 days post-irradiation. Significant effects were observed with both pre- and post-treatment regimens. However, only MnTE-2-PyP(5+) pre-treatment, but not post-treatment, prevented the development of rectal dilation, indicating that proper dosing regimen is critical for radioprotection. The pre-treatment also prevented or delayed the development of chronic proctitis depending on the radiation dose. Further work aimed at developing MnTE-2-PyP(5+) and similar drugs as adjunctive agents for radiotherapy of pelvic tumors is warranted. The present study substantiates the prospects of employing this and similar analogs in preserving normal tissue during cancer radiation as well as any other radiation exposure.

Chronic radiation damage in the rat rectum: an analysis of the influences of fractionation, time and volume

PURPOSE

Analysis of four different sets of experiments performed by the G.S.F. group in Munich investigating the late tolerance of the rat rectum to external or intracavitary irradiation.

MATERIAL AND METHODS

The endpoint was late rectal stenosis in female Wistar rats. The raw data were fitted to the linear-quadratic model by means of a likelihood maximization method (Direct Analysis). The model was altered to allow for repopulation, incomplete repair, and varying irradiated lengths of the rectum.

RESULTS

Fractionation sensitivity was high or intermediate (alpha/beta ratio values [95% confidence limits] ranging from 2.67 [0.86, 4.80] to 6.65 [2.21, 11.73] Gy). Significant repopulation occurred when treatments were longer than 5 days (Dprolif equal to 0.61 [0.20, 1.47] and 1.08 [0.58, 1.90] Gy/day, in fractions of 4 Gy). Another interpretation is that radiosensitivity changed during treatment. Repair half-time estimates ranged between 1.84 [1.52, 2.34] and 5.02 [2.83, 21.7] h. Finally, the present analysis indicated that the smallest surviving compartment capable of tissue rescue was about 1/50 to 1/100 of a 1 cm high cylinder of the rectum wall.

CONCLUSIONS

The radiobiological features of late stenosis in the rats are consistent with combined injuries of early and late responding components of the rectal wall. This raises some concerns about the possible danger of hyperfractionated treatments, where the beneficial impact of fraction size reduction may be obviated for interfraction intervals that are too short. Also, accelerated irradiation may result in more late complications because of increased early reactions.

Early and late injuries in mouse rectum after fractionated X-ray and neutron irradiation

PURPOSE

to assess mouse rectum tolerance to fractionated X-ray and neutron irradiation.

MATERIALS AND METHODS

doses per fraction ranged between 0.25 and 35 Gy for X-rays, 0.05 and 12 Gy for neutrons. Neutron top-up doses were added when the fractionated irradiation was given in fractions less than 2 Gy of X-rays or 0.35 Gy of neutrons in order to bring the damage into the detectable range. The early endpoints were the nadir of weight loss occurring within the first 2-3 weeks following irradiation and lethality by 2 months. The late endpoints were the peak of weight reached at maturity of the mice, the proportion of short feces in the daily fecal output at 10 months and lethality by 12 months. The linear-quadratic (LQ) model was fitted to the data (direct "one-step" analysis) and the estimated parameters were used to calculate relative biological effectiveness (RBE) values.

RESULTS

alpha/beta ratio estimates were for X-rays: 19.9 Gy [95% confidence limits: 15.2, 27.0] for weight nadir. 13.4 Gy [9.3, 19.5] for early lethality, 6.4 Gy [3.6, 11.0] for peak weight, and 6.9 Gy [4.2, 10.8] for late lethality, for neutrons 19.9 Gy [9.5, 61.0] for peak weight. The fecal-deformity data were poorly fitted by the LQ model. The RBE was slightly higher for acute endpoints than for the late ones when X-ray fraction sizes were equal to or larger than 10 Gy. However, the change in RBE with decreasing X-ray dose per fraction was much steeper for the late endpoints, so that it became equal to or even higher than for acute reactions at doses per fraction of 5 Gy or less.

CONCLUSION

Our results were consistent with those obtained from previously published studies using the same experimental system but larger doses per fraction.

Development of a shielded 241Am applicator for continuous low dose rate irradiation of rat rectum

This paper describes the development of sources, applicators, and techniques that can be used to irradiate rat rectums with continuous irradiation at dose rates of interest in brachytherapy, either with the full circumference of the rectum irradiated, or with half of the circumference shielded from the radiation. The system uses encapsulated 241Am sources, to irradiate rat rectum with 60 keV photons continuously at a dose rate of up to 50 cGy/hr. Details of the design and fabrication of the 241Am sources, the rectal applicator, the dosimetry of the system, and the protocols for preparing and irradiating the rats, and for detecting early rectal injury using histological examination of irradiated rectum are presented. Highly effective shielding (attenuation factors as low as 0.04) of half of the circumference of the rat rectum was achievable. Unidirectional 241Am irradiators for intracavitary brachytherapy offer a unique tool for examining the effects of shielding a portion of the circumference of the rat rectum, on the radiation tolerance of the rectum.

Radiobiology of pions in comparison with other heavy particles

Our recent data on late effects of pions in spinal cord, lung and rectum in rodents is presented with reference to other high-LET radiations. Unlike high-LET radiations such as neutrons and neon ions, the RBE for late effects of pions (up to 1.5) is not found to be significantly different from acute effects. Because of the potential of matching treatment volume (especially by using dynamic treatments) to the target volume by using pions and heavy ions, it is of utmost importance to study the tolerance of normal tissues to late effects as a function of volume. Such knowledge combined with pion and heavy ion dynamic treatment could lead to a further step in heavy particle radiotherapy.

Radiation tolerance of the rat rectum to fractionated X-rays and pi-mesons

A 2.5 cm segment of the rectum of female F344 rats was irradiated with up to 10 fractions of X-rays or pions. Transient signs of acute proctitis were followed by chronic rectal injury starting at about 2 months. Recto-vaginal fistulas were frequently observed in animals with severe chronic injury. Two patterns of chronic injury were observed, an early type consisting of deep ulcers and fistulas, and a late type characterized by vascular injury, fibrosis and mucosal cysts. In a 4-fraction X-ray experiment, the influence of a low-residue diet was compared with a regular diet, showing no significant differences in pathology or survival. Isoeffective doses were determined for the occurrence of severe rectal injury at 250 days. Fitting the data to an LQ-model yielded an alpha/beta ratio of 13 Gy for pions, and 6.5 Gy for X-rays. Increasing the overall treatment time from 9 days to one month gave a significant rise in isoeffective doses for chronic injury. This suggests that the more delayed types of injury did not develop independently from the acute mucosal changes. The RBE of pions for rectal injury at 250 days was 1.2 for single doses, increasing to about 1.4-1.5 at a dose per fraction of 4 Gy. These values are similar to those obtained in other acute and late responding tissues.