Implementation Of Hypofractionated And Adaptive Radiotherapy For Large Brain Metastases Using A Relocatable Head Frame On Gamma Knife Perfexion

Keywords: technique, gamma knife, radiotherapy, brain metastasis, stereotactic frame

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     For the treatment of large brain metastases not amenable to high-dose radiosurgery, we designed a clinical trial of hypofractionated, stereotactic radiotherapy (SRT) with an extended inter-fraction interval in order to measure and adapt to patient-specific response.
     Here we report early experiences in implementation of hypofractionated adaptive (HFA) SRT using a relocatable head frame (RHF) on Gamma Knife® (GK) PerfexionTM(PFX).
     Patients with large (>2cm) recurrent brain metastases after WBRT were eligible for enrollment.
     SRT was delivered in 3 fractions on days 1, 7, and 21 according to dose escalation level (8, 10, 12, and 14Gy/fraction). Immobilization accuracy of the RHF was measured after vacuum bite block fabrication, before/after CT scans, and before/after treatments using the repositioning check tool. Gross target volumes (GTV) were contoured on co-registered MRI in Leksell Gamma Plan (LGP 8.3.1 and 9.0), and planning target volumes (PTV) were generated with margins of 2.0mm SI, 1.5mm RL/AP using PinnacleÒ (Philips, USA) and an in-house program. Treatment plans were generated for each fraction to achieve the objectives: GTV V100>99% and PTV V95>95%. Collision avoidance was confirmed using an in-house virtual simulator, the collision check tool and a dry run.
     Two patients have been enrolled to date at the 8Gy dose level, for a total of 5 metastases treated. GTVs ranged from 0.8-24.6cc, and PTVs were 160-230% of GTV. Mean dose gradient from GTV to PTV was 0.5-1.1Gy/mm, and the conformity index was 1.42-2.13. Fraction treatment time was 33-58min at 2.7Gy/min. Prescription isodose range from 46-75% and was determined by PTV objectives. Collision QA using virtual simulator was efficient and accurate. Repositioning measures met the 1mm tolerance for all fractions. No treatment interruptions occurred. Symptoms and steroid requirements improved through the course of treatment for both patients. Resolution of edema displaced one GTV by 3mm, where substantial under-dosage (V100=90%) would have occurred without adaptive replanning.
     This is a retrospective study.
     HFA-SRT has been implemented using a RHF on PFX. LGP support of PTV generation would streamline planning procedures
     Acute toxicity outcomes are promising, but the impact of an adaptive approach to HSRT over 21 days remains to be determined.


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