Extracranial Doses during Stereotactic Radiosurgery and Fractionated Stereotactic Radiotherapy Measured with Thermoluminescent Dosimeter in vivo

INTRODUCTION Stereotactic radiotherapy has been majorly applied to the intracranial targets as primary or metastatic brain tumors, benign tumors, vascular malformations as arteriovenous malformations(AVM) or angiographically occult vascular malformations(AoVM), and functional neurologic diseases as neuralgias since 1951, when Dr. Leksell, a Swedish neurosurgeon, used radiation making intentional lesions or destruction of the certain focus in the brain for the relief of pain instead of neurosurgery. The idea was contrary to the idea of advantage of fractionated radiotherapy in protracted time periods and almost forgotten until its clinical effectiveness and physical accuracy was accepted in 1980’s. Leksell’s Gamma Unit using 179 or 201 fine sources of Co-60, Synchrocyclotron, or linear accelerator(Linac) have been used for the clinical application of the technique. The Gamma Unit was the only apparatus in 1960’s but Linac systems prevail the these days with the rapid development and imaging techniques as computerized axial tomography(CT) or magnetic resonance imaging(MRI) and of medical engineering (1, 2). This technique can be applied just once or many times. Stereotactic radiosurgery(SRS) is an alternative to neurosurgery and single high-dose delivery of large dose of radiation from many non-coplanar directions toward well define targets of small volume usually less than 4 cm in diameter. But when the above technique is combined with fractionation, to get two birds at once with a stone, we can achieve physical and biological advantages. We can have a new way of taking advantages in beam delivery within 2 mm by precision immobilization for stereotaxy and also in reducing of normal tissue toxicity by fractionated beam delivery. That is the fractionated stereotactic radiotherapy(FSRT). Recently the usage of 3-dimensional non-coplanar radiotherapy technique is increasing toward intracranial and extracranial targets mainly for intrahepatic, pulmonary, or retroperitoneal/abdominal targets. (3) We developed our own hardware and software system(Green Knife) for intracranial stereotactic radiotherapy using invasive stereotaxy in 1994 and system using for non-invasive fractionated stereotaxy in 1997. But the high dose can reach the extracranial normal tissue or organs after transmitting intracranial targets and thus unwanted damages can be elicited, because extracranial organs are usually not considered in planning of beam delivery. Thus it is very necessary to confirm the quantity and factors affecting that and make every efforts to reduce its affection if it is substantial. We have used the both systems for the clinical application. We measured the extracranial dose and its distribution during the above medical procedures to estimate effect of exit doses of stereotactic radiosurgery(SRS) and fractionated stereotactic radiotherapy (FSRT) of the intracranial target lesions using a linac system. We’d like to integrate and compare the doses by further extending of previous works (4,5,6)