Abstract
The authors compared the relative dosimetric merits of Gamma Knife (GK) and CyberKnife (CK) in 15 patients with 26 brain metastases. All patients were initially treated with the Leksell GK 4C. The same patients were used to generate comparative CK treatment plans. The tissue volume receiving more than 12 Gy (V12), the difference between V12 and tumor volume (V12net), homogeneity index (HI), and gradient indices (GI25, GI50) were calculated. Peripheral dose falloff and three conformity indices were compared. The median tumor volume was 2.50 cm3 (range, 0. 044‐19.9). A median dose of 18 Gy (range, 15‐22) was prescribed. In GK and CK plans, doses were prescribed to the 40‐50% and 77‐92% isodose lines, respectively. Comparing GK to CK, the respective parametric values (median±standard deviation) were: minimum dose (18.2±3.4 vs. 17.6±2.4 Gy, p=0.395); mean dose (29.6±5.1 vs.20.6±2.8 Gy, p<0.00001); maximum dose (40.3±6.5 vs.22.7±3.3 Gy, p<0.00001); and HI (2.22±0.19 vs. 1.18±0.06, p<0.00001). The median dosimetric indices (GK vs. CK, with range) were: RTOG_CI, 1.76 (1.12‐4.14) vs. 1.53 (1.16‐2.12), p=0.0220; CI, 1.76 (1.15‐4.14) vs. 1.55 (1.18‐2.21), p=0.050; nCI, 1.76 (1.59‐4.14) vs. 1.57 (1.20‐2.30), p=0.082; GI50, 2.91 (2.48‐3.67) vs. 4.90 (3.42‐11.68), p<0.00001; GI25, 6.58 (4.18‐10.20) vs. 14.85 (8.80‐48.37), p<0.00001. Average volume ratio (AVR) differences favored GK at multiple normalized isodose levels (p<0.00001). We concluded that in patients with brain metastases, CK and GK resulted in dosimetrically comparable plans that were nearly equivalent in several metrics, including target coverage and minimum dose within the target. Compared to GK, CK produced more homogenous plans with significantly lower mean and maximum doses, and achieved more conformal plans by RTOG_CI criteria. By GI and AVR analyses, GK plans had sharper peripheral dose falloff in most cases.PACS number: 89.20.‐a
Highlights
Brain metastases significantly shorten the lives of cancer patients, with the majority of primary tumors originating from lung, breast, skin, kidney, and gastrointestinal organs
Prognosis for patients with brain metastases remains very poor, typically with median survival ranges from 2.3–7.1 months.[6]. Treatment options include expectant medical management, systemic chemotherapy, biological agents, surgery, whole-brain radiotherapy (WBRT), and local boost with Stereotactic radiosurgery (SRS).[7]. In patients with single brain metastasis, adding adjuvant WBRT after surgery decreased the rate of local recurrence.[8] up to 10% of patients receiving WBRT may experience cognitive deterioration, short-term memory loss, and radiation-induced dementia.[9]. Increasingly, radiation oncologists and neurosurgeons prefer using local techniques, such as SRS and surgery, as first-line treatments in patients with oligometastatic brain tumors, while deferring WBRT as a salvage option
In the RTOG 9005 study, higher rates of CNS toxicity were noted in patients with larger size of tumors, which was the most important predictor for radionecrosis.[19]. Other risk factors included increased volume receiving 10 Gy or more, higher radiation dose, repeated radiosurgical treatments to the same tumor, and increased size of erroneously irradiated normal brain tissue.[22,23,24] For Gamma Knife (GK), tissue volume enclosed by the 12 Gy isodose line correctly predicted complication risk in patients with AVMs and other non-AVM intracranial tumors.[25,26] Our results showed significantly different falloff profiles between GK and CK, as evident in gradient index, Average volume ratio (AVR), volume receiving more than 12 Gy (V12), and V12net calculations
Summary
Brain metastases significantly shorten the lives of cancer patients, with the majority of primary tumors originating from lung, breast, skin (melanoma), kidney, and gastrointestinal organs. It represents a significant clinical burden, with an incidence of at least 40% in advanced-stage cancer patients, and directly responsible for an estimated 20% of cancer deaths.[1] Economically, brain metastases represent a significant burden in total health-care expenditure for cancerrelated treatments.[2]. For patients with reasonable performance status and life expectancy, the American Society for Radiation Oncology (ASTRO) supports the use of WBRT with a radiosurgery boost to control up to four brain metastases. SRS has a role in treatment of previously resected cavities of brain metastases.[12]
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