HIGH-DOSE BEHIND INHOMOGENEITIES DURING MEDIUM-ENERGY X-RAY-IRRADIATION

Medium-energy x-rays from orthovoltage treatment units are used for a variety of radiotherapy treatments ranging from cutaneous malignancies in the head and neck region to bone metastases. It was the aim of the present study to investigate the dose distribution due to secondary e lectrons close behind inhomogeneities in these radiation qualities. Th e dose was assessed in a solid water phantom using three plane-paralle l ionization chambers (NE 2532/2, 'Markus' and 'Attix' chamber) and sh eets of aluminium, copper, zinc, platinum, lead and bone equivalent ma terial. The depth dose distribution directly behind the inhomogeneity was assessed using sheets of 15 mu m thick polyethylene foil. A dose i ncrease was found directly behind inhomogeneities of high atomic numbe r with a rapid dose fall-off over the first 100 mu m. The dose downstr eam of the inhomogeneity was found to increase with increasing beam qu ality from 120 kVp (HVL 2.8 mm Al) to 250 kVp (HVL 2 mm Cu). In the la tter the dose was increased directly behind lead and platinum sheets b y up to a factor of eight compared with a solid water depth of similar attenuation. The results of the study demonstrate the importance of u sing appropriate materials if shielding is in contact with the patient .