**2.2.3 Tumor volume and radiation dose**

Administration of maximal dose to a minimum volume without damaging normal tissue is one of the major goals of SRS. The probability of direct and indirect surrounding tissue damage due to radionecrosis and edema increases in proportion to the tumor volume and radiation dose. However the SRS dose above 15Gy is known as a good prognostic criterion for LGA, high TCR rate (94.7%) with low dose SRS for PA has also been reported (Boethius et al., 2002). Tumor volume less than 6-8 cc is significantly related with better prognosis (Park et al, 2010). Despite the lack of a definite dose range for LGA, doses ranging between 10-15 Gy are currently used. Dose modification or reduction should be considered for patients who have undergone fractionated cranial RT before SRS (Wang et al., 2006).

### **2.2.4 Histological grade and age**

Pilocytic astrocytoma has a better prognosis than grade II astrocytomas. Grade II astrocytoma carries a potential to transform into malignant glioma. Pilocytic astrocytoma also has better prognosis in children than in adults. Median 1, 3 and 5 year PFS rates are 91.7%, 82.8% and 70.8% for pediatric PA; and 83.8%, 31.5% and 31.5 for adult PA respectively (Kano et al., 2009a, 2009b). SRS as an alternate to RT has been found very effective for PA patients in whom the re-resection is not feasible or with early recurrence. But the place of SRS in multimodal treatment of grade II astrocytoma is controversial. 91.3%, 54.1% and 37.1% PFS rates for 1, 5 and 10 years respectively has been reported for radiosurgical treatment of residual or recurrent grade II astrocytomas (Park et al, 2010). More studies are needed for determining definite indications and criteria of SRS for LGA. Prognostic factors of SRS for LGA are listed below (Table 5).


Table 5. Prognostic factors of SRS for LGA

Stereotactic Radiosurgery for Gliomas 287

be considered particularly for lesions located in eloquent areas and dose should be reduced. Aggressive irradiation might result in excessive edema and radionecrosis requiring additional procedures such as emergent decompression or shunting (Smith et al., 2008). Radiation induced tumors is another potential complication of SRS. Several sporadic reports of GBM formation in long term following high dose SRS are already present. However long term follow up is needed to assess this potential, incidence seems less than 1:100.000 for now

**Case 1.** 25 years old male presented with progressive headache. Cranial MRI showed an intraxial mass lesion in close proximity to pineal region. Patient refused biopsy and considered for gamma knife. Mass disappeared at 6th month post-SRS and didn't recur

Fig. 2. Left: pre-SRS axial contrast enhanced MRI view. Middle: 6 months after SRS. Right: 6

**Case 2.** 37 years old male presented with complete loss of vision at the right eye and progressive loss of vision on the left eye for months. MRI scan revealed an optic glioma located on the right half of the chiasm. Patient underwent low dose fractionated SRS to avoid the damage to the chiasm and optic nerve. (Figure 3) Patient was followed up 66 months following SRS, and neither tumor progression, nor visual deterioration was seen.

(Berman et al., 2007; Salvati et al., 2003).

during 6 year follow ups. (Figure 2)

**4. Case illustrations** 

years after SRS.

(Figure 4)

Finally, the best candidates for SRS treatment are the pilocytic astrocytomas if previously resected, well circumscribed, and located in critical or deep areas or re-resection is not feasible, or if there is an early recurrence.
