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Dosimetric Characterization of a High Throughput Multiple Brain Metastases Gamma Knife Program

Y Lee

Y Lee*, A Sarfehnia , B Chugh , C Yeboah , H Soliman , S Myrehaug , C Tseng , M Tsao , L Holden , A Sahgal , M Ruschin , Sunnybrook Health Science Center, Toronto, ON


SU-I-GPD-T-622 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The treatment of multiple(>=4) metastases with single-session stereotactic radiosurgery (SRS) is an emerging technique with limited treatment planning and normal tissue dose tolerance guidelines. The aim of the present work is to characterize SRS plans arising from a high throughput Leksell Gamma Knife(LGK) program to facilitate intra- and inter-modality dosimetric comparisons (such as with linac-based SRS).

Methods: Within first 79 days of clinical operation of a newly installed LGK Perfexion(PFX), 105 patients (95 with brain metastases and 10 with benign tumors) comprising of 382 individual targets (371 brain metastases and 11 benign tumors) were treated. For all targets, 3D T1-weighted post-Gadolinium images with 1mm slice thickness were used for target delineation and pre-planning. Our institutional planning objectives were to strive for >=99% coverage, gradient index (GI)<3.5 and Paddick Conformity Index (CI-Paddick)>0.7. Other planning metrics included tissue V12Gy, organ-at-risk (OAR) dose, and beam-on time. Only the data from metastases are presented here.

Results: 33 patients had >=4 metastases (average (range) number of targets were 3.9(1-29)). The median (range) of metastases volumes was 0.117(0.004-11.570)cm³. Prescription doses ranged from 14-20Gy and the doses were prescribed to 45-95%(median=54.5%). Coverage of all targets was >97.3% (mean=99.4%) except one brainstem case with coverage of 57.3%. GI was variable for targets <0.5cm³ but converged toward 3.5 for larger targets. Mean brain V12Gy was 5.1cm³. OAR doses were kept low (lens doses <1Gy). Median (range) beam-on times per target was 13.6(2.9-55.1) minutes (nominal dose rate=3.55Gy/min).

Conclusion: We have characterized PFX dosimetric performance for the first 371 brain metastases treated at our centre using a consistent protocol. Within a year we expect over 1,000 individual brain metastases treated, which will serve as a basis for a robust dosimetric PFX model, which in turn can be used to compare against our previous linac-based SRS program and against other modalities/institutions.

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