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LINAC-Based Stereotactic Radiosurgery Dosimetric Outcomes Dictated by Isocenter Settings

J Zhang1*, S Lee2, W Inouye2, W Lien1, (1) Southern California Permanente Medical Group, Los Angeles, CA, (2) Va Long Beach Healthcare System,Long Beach, CA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

LINAC-based stereotactic radiosurgery has gained popularity in treating intracranial tumors. This study aims to compare dosimetric outcomes of different SRS methods for typical 2 brain tumors of various sizes and various isocenter number.

Hypothetical 4 sets of target lesions of 3 different sizes were contoured on brain MRI images of an index patient. There were 4 locations of tumors. And each time one base tumor were selected together with one tumor from other three tumors with similar volume to make a two-tumor treatment case. These two target lesions were in different relative locations with different distance between 4.38cm and 8.69cm. The PTV were constructed to be within 3 respective categories (<0.5, 0.5-1.0, 1.0-4.0 cc), in inverse proportion with prescription dose range of 18-22.5 Gy. Varian Truebeam 6X FFF beams were used for Brainlab Element treatment planning system. Each plan has five non-coplanar dynamic conformal arcs. At least 95% of PTV volume were covered by prescription dose. For each sets of 2 lesions, SRS plans were analyzed independently between (1) one-isocenter vs. two-isocenter, and (2) different tumor relative locations and distance.

The degree of normal tissue sparing was evaluated with the amount of brain volume receiving >12 Gy (V12) and total MUs. Mean differences of the dosimetric outcomes determined for all 4 sets of lesions of respective volumes.

The 3 types of PTV volumes contoured were: 0.46, 0.81 and 1.33 cc, respectively.
Compared to one-isocenter plans, two-isocenter plans have higher V12 by 0.56±0.27cc as 7.37%±3.47% and much more MU number by 54.67%±13.96%.

This study showed that (1) one-isocenter plans may provide better brain sparing and incur much less MUs for shorter beam-on time and less radiation leakage to patient comparing with two-isocenter plans.


Stereotactic Radiosurgery


TH- External Beam- Photons: intracranial stereotactic/SBRT

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