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The relevance of dosimetry in animal models of cochlear irradiation
Mario A Mujica-Mota1, Slobodan Devic, Sam J Daniel
1*McGill Auditory Sciences Laboratory; †Medical Physics Unit, McGill University; ‡Department of Radiation Oncology, Jewish General Hospital; and §Department of Otolaryngology, The Montreal Children's Hospital, Montréal, QC, Canada.
Objective:
To compare dose measurements of three different irradiation setups for an animal model of unilateral cochlear irradiation using radiochromic films positioned at the cochlear plane. A method of dosimetry is proposed.
Methods:
Radiation field simulation was performed to locate the cochlear plane for irradiation experiments using CT scan images. Fifteen film pieces were irradiated at the cochlear plane with 3 different irradiation field sizes. A 12-mm diameter field (n = 5), 5.7 mm diameter field (n = 5), and a 6.5 × 7.2 mm field (n = 5). After obtaining an ideal irradiation field size, 15 film pieces were used to compare dosimetry between tissue substitute materials (PVC n = 5 and PVC + Teflon, 5 each) and real tissue (frozen animal, n = 5). Auditory brainstem responses at 3 frequencies (8, 16, 20, and 25 kHz) were performed on 7 guinea pigs after a cycle of fractionated unilateral irradiation.
Results:
Dosimetry in real tissue demonstrated an asymmetric dose distribution at the cochlear plane and ultimately a lower dose deposition (30%) when compared with tissue substitute materials. Auditory brainstem responses of ears subjected to radiotherapy demonstrated progressive hearing loss in long-term assessment.
Conclusion:
Asymmetric dose deposition at the cochlear plane highlights the need of comprehensive real tissue dosimetry in animal studies of cochlear irradiation. To avoid misleading discrepancies in dose-deposition between different studies using the same animal model, appropriate planning and confirmatory dosimetry systems are highly desirable.
