This work presents a Monte Carlo model of Leksell Gamma Knife Perfexion as well as the main parameters of the dose distribution in the standard phantom obtained using this model. The model is developed in the Geant4 simulation toolkit in a modular way which enables its reuse in other Perfexion studies. Large phase space files were created, containing particles that are entering the inner machine cavity after being transported through the collimation system. All 14 output factors of the machine and effective output factors for both the 4 mm (0.830 ± 0.009) and 8 mm (0.921 ± 0.004) collimators were calculated. Dose profiles along the main axes are also included for each collimator size. All results are compared to the values obtained from the treatment planning system, from experiments, and from other Monte Carlo models.

OBJECTIVE The purpose of this study was to evaluate long-term treatment results, radiation-related toxicity, and prognostic factors for the progression-free survival (PFS) of patients with pilocytic astrocytomas treated by means of stereotactic radiosurgery with a Leksell Gamma Knife. METHODS A total of 25 patients with pilocytic astrocytomas underwent Gamma Knife surgery during the period 1992–2002. The median target volume was 2700 mm3 (range 205–25,000 mm3). The 18 patients treated with 5 daily fractions received a median minimum target dose of 25 Gy. Doses for the 2 patients treated with 10 fractions over 5 days (2 fractions delivered on the same day at least 6 hours apart) were 23 and 28 Gy. For the 5 patients treated with a single fraction, the minimum target dose ranged from 13 to 20 Gy (median 16 Gy). RESULTS Complete regression occurred in 10 patients (40%) and partial regression in 10 patients (40%). The 10-year overall survival rate was 96% and the 10-year PFS rate was 80%. Target volume appeared to be a significant prognostic factor for PFS (p = 0.037). Temporary Grade 3 toxicity appeared in 2 patients (8%), and these patients were treated with corticosteroids for 2 months. Permanent Grade 4 toxicity appeared in 2 patients (8%) and was associated with neurocognitive dysfunction. In these 2 individuals, the neurocognitive dysfunction was also felt to be in part the result of the additional therapeutic interventions (4 in one case and 6 in the other) required to achieve durable control of their tumors. CONCLUSIONS Radiosurgery represents an alternative treatment modality for small residual or recurrent volumes of pilocytic astrocytomas and provides long-term local control. Target volume appears to be the most important factor affecting PFS.

Background/Objectives: The treatment of large arteriovenous malformations (AVMs) presents a challenge and the effectiveness of radiosurgery decreases with increasing treatment volume. Here, we analyzed and compared single-session treatment for AVMs >15 cm³ with staged treatment, when the volume of a large AVM was divided into 2 or more compartments, which were subsequently treated at intervals of a few months. In the event that complete obliteration was not achieved within 3 years, repeat radiosurgery was considered. Methods/Results: Between 1993 and 2011, gamma-knife radiosurgery was performed on 50 patients with large AVMs. A total of 27 of them, with volumes ranging from 15.1 to 27 cm³ (median = 17.8), were treated in a single session, with a marginal dose ranging from 10 to 17 Gy (median = 15). Complete obliteration was achieved in 7 (26%) of these; in 14 patients the treatment was repeated after a median of 38 months. The volume of AVMs subjected to repeat treatment regressed and ranged from 6.76 to 16.4 cm³ (median = 7.7), with the marginal dose ranging from 13 to 18 Gy (median = 17); 10 patients later achieved complete obliteration at a median of 79 months after the initial treatment. Overall, 63% of patients achieved complete obliteration of AVM after treatment. Rebleeding in a latent period was recorded in 3 patients (11%) and symptomatic collateral edema in 2 (7%); 23 patients were treated using the staged method, with the interval between staged treatments usually being 6 months. Their overall AVM volume ranged from 13.2 to 46.6 cm³ (median = 23.3), the volume for a single stage ranged from 4.5 to 31.3 cm³ (median 11.7), and the marginal dose ranged from 10 to 18 Gy (median = 17). Complete obliteration was achieved in 4 (17%) of them, in 8 patients the treatment was repeated at a median of 53 months after the first treatment, and a third retreatment was performed on 2 patients 98 and 102 months, respectively, after the first treatment. The volume for repeated treatment of AVMs ranged from 0.81 to 7.7 cm³ (median = 3.3), with the marginal dose ranging from 14 to 20 Gy (median 17.5). Of all the retreated patients, 5 AVMs subsequently achieved complete obliteration. Overall, 9 patients (39%) were totally cured. Rebleeding in a latent period was observed in 1 of these patients (4.3%) and symptomatic edema in another 1 (4.3%). Conclusions: Radiosurgery of large AVMs is a valuable treatment either as a single-session or staged treatment, with a reasonable chance of cure and a low risk of complications.

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