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 electrons close behind inhomogeneities in these radiation qualities. The dose was assessed in a solid water phantom using three plane-parallel ionization chambers (NE 2532/2, 'Markus' and 'Attix' chamber) and sheets of aluminium, copper, zinc, platinum, lead and bone equivalent material. The depth dose distribution directly behind the inhomogeneity was assessed using sheets of 15 microm thick polyethylene foil. A dose increase was found directly behind inhomogeneities of high atomic number with a rapid dose fall-off over the first 100 microm. The dose downstream of the inhomogeneity was found to increase with increasing beam quality from 120 kVp (HVL 2.8 mm Al) to 250 kVp (HVL 2 mm Cu). In the latter the dose was increased directly behind lead and platinum sheets by up to a factor of eight compared with a solid water depth of similar attenuation. The results of the study demonstrate the importance of using appropriate materials if shielding is in contact with the patient.