An investigation of eye lens dose for gamma knife treatments of trigeminal neuralgia Lijun Ma, a, * Lawrence Chin, b Mehrdad Sarfaraz, a David Shepard, a and Cedric Yu a a Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD 21201 b Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, MD 21201 Received 17 April 2000; accepted for publication 20 June 2000 Stereotactic Gamma Knife radiosurgery has been widely used for treating trigemi- nal neuralgia TN. A single large fractional dose of 7000 to 9000 cGy is com- monly prescribed as the maximum dose for these treatments. For this reason, if a small percentage of the prescribed dose such as 2–3 % scattered to the eye, it could reach or even exceed the tolerance dose of the lens. For several TN cases, we found that the Leksell Gamma Plan system calculates the lens dose about 0.5–2 % of the maximum dose independent of the use of eye shielding. These dose values are significantly high and it motivated us to investigate the lens dose for the TN pa- tients treated with stereotactic Gamma Knife radiosurgery. Phantom studies and in vivo dosimetry measurements were carried out for six patients treated at our insti- tution. The average dose to the lens ipsilateral to the treated nerve was measured to be 7.70.6 cGy. Based on the biological model of Lyman and Emami Int. J. Radiat. Oncol. Biol. Phys. 21, 109–122 1991, the probability of the lens com- plication cataract was determined to be 0.1%. Our findings suggest that few TN patients would develop cataracts after receiving Gamma Knife radiosurgery. © 2000 American College of Medical Physics. PACS numbers: 87.53.-j, 87.66.-a Key words: gamma knife, stereotactic radiosurgery, trigeminal neuralgia, dosimetry I. INTRODUCTION Stereotactic Gamma Knife radiosurgery is considered to be one of the safest and most effective treatment modality for trigeminal neuralgia TN. 1–3 Since the procedure reimbursement has been accepted by Medicare, there has been a dramatic increase in the number of patients receiving Gamma Knife radiosurgery for trigeminal neuralgia. According to a recent survey by the Leksell Gamma Knife Society, trigeminal neuralgia is one of the most commonly treated indications for Gamma Knife procedures in the United States. To carry out an effective TN procedure, it is common that a single large fractional dose of 7000 to 9000 cGy is used to irradiate the affected nerve. 1 Because of the large delivered dose, a scatter dose even 2% of the total dose could reach or even exceed the tolerance dose of the eye lens 5 about 150 cGy. Based on the predictions of the Gamma Knife treatment planning system, we found that the dose to the eye lens sometimes exceeds 100 cGy. This value was significantly higher than expected and it motivated us to carry out clinical physics studies to investigate the lens dose for trigeminal neuralgia patients treated with stereotactic Gamma Knife radiosurgery. II. MATERIALS AND METHODS Phantom studies and in vivo dosimetry measurements on six patients were carried out to measure the dose to the eye lens. All patients were treated at the University of Maryland Gamma JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 1, NUMBER 4, FALL 2000 116 1526-9914Õ2000Õ1„4…Õ116Õ4Õ$17.00 © 2000 Am. Coll. Med. Phys. 116