Skin Reactions and Tissue Heterogeneity in Electron Beam Therapy. Part II: In Vivo Dosimetry

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Abstract
With 18 Mev electrons a marked dosage contribution to the underlying lung tissue is detected even at considerable depths. The percentage depth dose delivered to the pulmonary parenchyma is much greater than that which would be expected on the basis of depth dose figures determined in a water phantom. Electron beam energy of 9 Mev does not insure a minimal dosage to pulmonary parenchyma in a thin chest wall such as may be encountered following radical mastectomy. In- creased dosage, however, is confined to lung tissue lying close to the irradiated chest wall. If no attempt is made to correct for the shielding effect of the mandible, bone in the path of the beam leads to some underdosage of the tonsillar area when a unilateral electron beam field is used. The average percentage decrease in dose delivered to the tonsillar area was approximately 10%. The possible usefulness of a correction factor: 0.65 x cm bone + depth (cm) = effective depth (cm) has been explored. The correlation between the water phantom depth dose at the effective depth and the in vivo determination has been good. In vivo dosimetry with lithium fluoride to investigate effects of tissue heterogeneity on electron beam dose is a reasonable approach, yielding reproducible results within a range of accuracy clinically acceptable.

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