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

RESUMEN

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ABSTRACT

Absorbed dose and transmission factors were determinedfor 6 MV X-ray fields applied in four conventionalexternal radiation treatments where protectionblocks are used to shield organs at risk. Studiedtreatments are: head and neck (cancer of the larynx),breast (breast cancer), brain (brain tumours) andpelvis (cervix-uterine cancer). Experimental determinationswere made using ionization chambers andthermoluminescent dosimeters, at different locationsin phantoms, for blocked and unblocked fields.Calculated absorbed doses, using the treatmentplanning system and Clarkson method, were comparedto those measured. Measured transmissionfactor values ranged from 5% to 18%. It was foundthat the planning treatment system may underestimatethe absorbed dose under the blocks, up to 35%