Clinical application and dosimetric calibration procedure of the superficial and orthovoltage therapy unit Therapax DXT300.

ABSTRACT

AIMS AND BACKGROUND: The aim of this study was to configure a new superficial and orthovoltage therapy unit for clinical use and discuss the clinical indications for this kind of energy. There is renewed interest in radiotherapy using photon beams in the range from 40 to 300 kVp, in particular for the treatment of non-neoplastic diseases and superficial lesions. This is due to the fact that the latest units are of good quality, relatively cheap and reliable in comparison with alternative equipment. It is therefore necessary to document the steps involved in the clinical application of this therapeutic equipment. METHODS: The calibration procedure for a new superficial/orthovoltage therapy unit, Therapax DXT300 (Pantak, USA), installed at the Radiotherapy Department of the Mauriziano Hospital in Turin, was carried out by measurement of beam quality (half-value layer, HVL), percentage depth dose (PDD), profiles, internal dosimetry system, dose rate calibration and output factors. The Therapax DXT300 can be programmed to stop treatment either by time or dose, like a linear accelerator. It is equipped with one adjustable collimator (2 x 2 divided by 20 x 20 cm2) for orthovoltage therapy (focus skin distance, FSD = 50 cm) and six fixed applicators for superficial and orthovoltage therapy (FSD = 30 and 50 cm, respectively). Up to eight combinations of generating potential, tube current and added beam filter can be set. RESULTS: In the range of accelerating potentials from 40 to 300 kVp the Therapax provides beams with HVL ranging from 1.48 mm Al to 3.68 mm Cu. As regards the PDD curves, a slight difference between PDD values has been observed with potentials ranging from 200 to 300 kVp. The beam symmetry and flatness are almost the same in Y and X plane directions for accelerating potentials of less than 200 kVp. At higher kVp values these parameters are significantly worse in the anode-cathode axis direction, because the useful X-ray beam is extracted perpendicular to the anode-cathode axis direction as the energy increases, the bremsstrahlung becomes more forward directed, resulting in a higher dose rate on the anode side. CONCLUSION: The present paper suggests a methodological approach for the clinical application of an orthovoltage/superficial therapy unit and discusses its current indications. It is necessary to maintain the performance of the equipment within predefined limits in order to meet the requirements of an efficient quality assurance program. This unit is particularly useful in the treatment of superficial lesions and non-neoplastic diseases and in palliative care (eg for bone metastases).