Effect of irradiated volume on lung damage in pigs

The volume dependence of radiation induced lung damage is of important clinical concern. The aim of the present study was to investigate the effect of irradiated volume on radiation pneumonitis and lung fibrosis in pigs after fractionated irradiation.

Twenty-five animals were irradiated with 10 x 14 cm2 fields to the whole right lung (about 530 cm3), while 19 animals received irradiation only to the lower part of the right lung (10 x 8 cm2 fields, about 260 cm3). The irradiations were given in five fractions over 5 days with total doses ranging from 14.3-38.0 Gy (whole lung) or 21.3-31.2 Gy (half lung) using a 60cobalt unit and an isocentric technique. Early lung damage was assessed during the first 8 weeks after the start of treatment by weekly chest radiographs and by twice weekly determinations of the breathing rate in resting animals. Fibrosis was quantified at autopsy (after 8 weeks or after 5-8 months) by histological evaluation and by determination of the hydroxyproline content of the lung tissue. Based on reference values obtained in 17 untreated control animals the experimental data were converted to quantals for probit analysis.

The data did not indicate any significant differences for the incidence of lung damage after half lung and whole lung irradiation when injury was assessed by radiography, histology, or hydroxyproline content. However, using an increase in breathing rate as experimental endpoint, significant differences of radiation induced morbidity were observed. While none of the animals after half lung irradiation showed pathological breathing rates even after doses of 29 Gy and 31 Gy, a clear if shallow dose-response relationship with an ED50-value of 27 +/- 11 Gy (SD) was obtained after whole lung irradiation (P < 0.01).

A volume effect can only be demonstrated for functional lung morbidity whereas induction of structural lung damage is independent of the volume irradiated.