A STUDY TO EVALUATE SINGLE PORT, CENTRAL AXIS DOSIMETRY AND VERIFICATION TECHNIQUES FOR VETERINARY RADIOTHERAPY, USING THE CANINE NASAL CAVITY AS A MODEL TARGET

Ten normal dogs received single doses of Co-60 radiation to their nasa l cavity. The minimum nasal cavity dose was estimated using three diff erent techniques: manual calculation from percent depth dose tables, c omputerized planning for a homogenous field, and computerized planning utilizing field heterogeneity factors. The doses delivered were measu red by LiF thermoluminescent dosimetry (TLD) and by verification film densitometry. The radiation dose resulting from manual calculated pres criptions correlated well to those from computerized planning for a ho mogenous soft tissue field (corr. coeff. .87; p = .001). However, for both of these methods the estimated dose differed from the measured do se by about 12%. Computerized planning utilizing field heterogeneity f actors showed no improvement over the previous two methods, (corr. coe ff. .88; p = .0009) but the results were biased by the dose measuremen ts being central axis doses in a normal nasal cavity. Both manual and computerized verification film densitometry correlated well to TLD mea surements (corr. coeff. .97 and .94 respectively, p = .00001 and p = . 001 respectively), differing from TLD measurements by about 5-7%. Both manual and computerized verification film densitometry correlated wel l to manual calculated prescriptions (corr. coeff. .86 and .85 respect ively, p = .0015 and .0016 respectively). Also manual and computerized verification film densitometry correlated well to computer calculatio ns using homogenous tissue fields (corr. coeff. .86 and .85 respective ly, p = .0015 and .0016 respectively) and to computer calculations usi ng heterogeneity factors (corr. coeff. .89 and .89 respectively, p = . 005 and .0005 respectively). Film densitometry was found to be a good method to spot check dose delivery to the nasal cavity, a body part fo r which accurate dosimetry is difficult to achieve.