AIR CAVITY EFFECTS ON THE RADIATION-DOSE TO THE LARYNX USING CO-60, 6 MV, AND 10 MV PHOTON BEAMS

Purpose: To determine the perturbation effect in the surface layers of lesions located in the air-tumor tissue interface of larynx using Co-60, 6 MV, and 10 MV photon beams. Methods and Materials: Thermoluminescent dosimeters (TLDs), were embedded at 16 measurement locations in slab no. 8 of a humanoid phantom and exposed to two lateral-opposed beams using standard small 7 X 7 cm fields. Similarly, radiographic and radiochromic films were placed between slabs no. 7 and no. 8 of the humanoid phantom and exposed to two lateral-opposed radiation beams. The dosimeters were irradiated with Co-60, 6 MV, and 10 MV photon beams. Computer tomography (CT) treatment planning without inhomogeneity correction was performed. Results: At the tissue-air interface, the average measured percentage dose (% dose(m)) is about (108.7 +/- 4.8)% with TLD data, (96.8 +/- 2.5)% with radiographic film data, and (100.8 +/- 4.9)% with radiochromic film data. Similarly, in the central part of the cavity, the % dose, is (98.4 +/- 3.1)% with TLD data, (94.3 +/- 3.3)% with radiographic film data, and (91.7 +/- 5.0)% with radiochromic film data. Using the CT-based generated dose distribution (without inhomogeneity correction), the average calculated percentage dose (% dose(c)) is (98.7 +/- 1.0)% at the tissue-air interface and 98% in the central part of the air cavity. Conclusion: For the beam energies studied, the variation from the % dose, at the tissue-air interface for a given dosimetry technique is relatively small [< 5% (TLD), < 3% (radiographic)] and < 5% (radiochromic)l and therefore should not be significant in clinical settings. The variation from the % dose, at the tissue-air interface is more significant for lower energies [8% (Co-60), 7.3% (6 MV)]. This variation is about 4.3% for 10 MV photon beam, therefore, while our institutional practice favors lower energy (Co-60 to 6 MV) for node-negative glottic cancers, physical/dosimetric evidence offers no disadvantage to the use of higher energy photons.