RADIOLABELED ANTIBODY COMBINED WITH EXTERNAL RADIOTHERAPY FOR THE TREATMENT OF HEAD AND NECK-CANCER - RECONSTRUCTION OF A THEORETICAL PHANTOM OF THE LARYNX FOR RADIATION-DOSE CALCULATION TO LOCAL TISSUES

We propose to use radiolabeled antibodies in combination with external beam radiotherapy to improve locoregional control of head and neck ca ncer, In this case radiation toxicity to mucosa may become a dose-limi ting factor and a calculation of the possible compensatory decrease to the external beam radiotherapy would be needed, For this purpose, the following theoretical phantom of a representative organ of this anato mic region, the larynx, was reconstructed and local dosimetric data we re derived for a selection of beta-emitting isotopes. The phantom was reconstructed as cylindrical concentric tubes using the established va lues of an outer diameter of 38 mm and a height of 44 mm, Published me an adult larynx weight (28 g) and cartilage weight (14.7 g) were used. Mean mucosa weight from 5 mucosa samples of our patients was calculat ed to be 2.0 +/- 0.4 (SD) g, The remaining weight was apportioned to a fat/muscle compartment (11.3 g), The specific gravity of cartilage (1 .10 g/cm(3)), mucosa (1.04 g/cm(3)), and fat/muscle (1.04 g/cm(3)) wer e used to cross-check the volume/mass disparity of the theoretical tub ular tissue shells, The established maximum glottic diameter of 24 mm was used to calculate the central air column volume, Mean laryngeal tu mor volume from 8 representative laryngeal tumors was 4.4 +/- 3.1 cm(3 ), Tissue compartment thickness was 660 mu m for mucosa, 3100 mu m for muscle/fat, and 3320 mu m for cartilage, These values allowed the cal culation of dose absorbed fractions for a number of theoretical radioi mmunoconjugates by extending the established calculation of absorbed f ractions for spheres of known diameter to absorbed fractions of tissue planes (annuli) of known thickness, We calculated a D-eq for the resp ective tissues in the larynx for I-131-, Re-186, Re-188, Cu-67-, Y-90- , and Sm-153-labeled HMFG1, Compensatory decrease to the external radi otherapy dose is 1.1 Gy for each injection of the radioimmunoconjugate we propose to use (I-131-HMFG1), This would be best implemented throu gh the modification of the external radiotherapy fractions falling wit hin 2 effective half-lives of this radioconjugate in the mucosa.