Absorbed dose and transmission factor in blocked fields for radiotherapy treatments with 6 MV X-ray.

Absorbed dose and transmission factors were determined for 6 MV X-ray fields applied in four conventional external radiation treatments where protection blocks are used to shield organs at risk. Studied treatments are: head and neck (cancer of the larynx), breast (breast cancer), brain (brain tumours) and pelvis (cervix-uterine cancer). Experimental determinations were made using ionization chambers and thermoluminescent dosimeters, at different locations in phantoms, for blocked and unblocked fields. Calculated absorbed doses, using the treatment planning system and Clarkson method, were compared to those measured. Measured transmission factor values ranged from 5% to 18%. It was found that the planning treatment system may underestimate the absorbed dose under the blocks, up to 35%.