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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

Department of Clinical Oncology [A. M., J. S. W. S., A. A. E.], ICRF Tumour Targeting Laboratory [A. M., G. R-B., A. A. E.], and Department of Medical Physics [M. M.], Royal Postgraduate Medical School, Hammersmith Hospital, London W12 OHS, and Department of Ear, Nose and Throat Surgery, St. Mary's University Hospital, Praed Street, London W2 1NY [N. S.], United Kingdom

We propose to use radiolabeled antibodies in combination with external beam radiotherapy to improve locoregional control of head and neck cancer. In this case radiation toxicity to mucosa may become a dose-limiting 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 anatomic region, the larynx, was reconstructed and local dosimetric data were derived for a selection of ß-emitting isotopes.

The phantom was reconstructed as cylindrical concentric tubes using the established values of an outer diameter of 38 mm and a height of 44 mm. Published mean adult larynx weight (28 g) and cartilage weight (14.7 g) were used. Mean mucosa weight from 5 mucosa samples of our patients was calculated 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/cm³), mucosa (1.04 g/cm³), and fat/muscle (1.04 g/cm³) were used to cross-check the volume/mass disparity of the theoretical tubular tissue shells. The established maximum glottic diameter of 24 mm was used to calculate the central air column volume. Mean laryngeal tumor volume from 8 representative laryngeal tumors was 4.4 ± 3.1 cm³. Tissue compartment thickness was 660 µm for mucosa, 3100 µm for muscle/fat, and 3320 µm for cartilage. These values allowed the calculation of dose absorbed fractions for a number of theoretical radioimmunoconjugates by extending the established calculation of absorbed fractions for spheres of known diameter to absorbed fractions of tissue planes (annuli) of known thickness. We calculated a D_eq for the respective tissues in the larynx for ¹³¹I-, ¹⁸⁶Re-, ¹⁸⁸Re-, ⁶⁷Cu-, ⁹⁰Y-, and ¹⁵³Sm-labeled HMFG1. Compensatory decrease to the external radiotherapy dose is 1.1 Gy for each injection of the radioimmunoconjugate we propose to use (¹³¹I-HMFG1). This would be best implemented through the modification of the external radiotherapy fractions falling within 2 effective half-lives of this radioconjugate in the mucosa.

¹ A. M. acknowledges the financial support of the A. G. Leventis Foundation. To whom requests for reprints should be addressed.

Received 7/11/94. Accepted 1/ 3/95.

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