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J Roper1*, J Chang2*, S Lim3*, A Goenka2*, (1) Sarah Cannon, Nashville, TN, (2) Northwell Health, Lake Success, NY, (3) Memorial Sloan-Kettering Cancer Center, New York, NY


(Tuesday, 4/2/2019) 8:00 AM - 10:00 AM

Room: Osceola Ballroom C

Ongoing clinical trials show promising outcomes for ablative stereotactic radiation therapy in patients with oligometastatic disease and there is a growing trend in clinical practice towards aggressive local control using stereotactic radiosurgery (SRS) or stereotactic body radiotherapy (SBRT) techniques. The increased use of SRS/SBRT technique poses new challenges to physicists and other clinical staff due to the labor intensive and time consuming treatment.

Icon is the latest Gamma Knife (GK) model to address these challenges. It is equipped with an on-board imager for cone beam computed tomography so that patient setup can be verified with three dimensional tomographic images in real time. With this capability, both framed and frameless setup/treatment can be performed on GK for patients requiring brain SRS/SBRT. However, the workflow and treatment planning system are modified considerably for GK Icon to accommodate the improved flexibility and efficiency. These changes might make it difficult for clinics familiar with the previous GK models to adopt this new technology.

The single isocenter for multiple targets (SIMT) technique has gained clinical acceptance for the linear accelerator based (linac-based) SRS/SBRT treatment of multiple brain metastases as it allows multiple targets to be treated simultaneously, even at different dose levels, using radiation beams that are at most centered on only one of the targets. The potential benefits of a more efficient SIMT treatment, however, are not without potential pitfalls. There are important differences between SIMT and conventional SRS, which require different considerations during treatment planning and delivery. SIMT aperture shaping is more complex and has considerable influence on plan quality. Additionally, rotational setup errors that have only a minimal effect on target coverage with conventional SRS can have a substantial effect on SIMT coverage, in particular when targets are separated by large distances.

Another new challenge is that both modalities use small fields for SRS/SBRT treatment. Traditionally, linac-based treatment planning systems are optimized for field size larger than 30x30 mm2. These models can potentially produce substantial dose calculation errors when the SIMT technique is used to treat small targets (< 10 mm in diameter). This effect can be amplified when the small targets are located further away from the isocenter. Similarly for GK, the field sizes are intrinsically small (maximal cone size is 16 mm in diameter). Special attentions are needed for commissioning the treatment planning system. With increased interests in implementing SRS/SBRT procedures, good understanding of small field dosimetry is becoming a crucial responsibility for clinical physicists.

The purpose of this symposium is to address these new challenges in SRS/SBRT. We will first present clinical advantages and potential pitfalls of GK-based and linac-based brain SRS/SBRT. This will be followed by a review of various treatment planning strategies and workflow managements developed for both modalities. Finally, commissioning of treatment planning system for SRS/SBRT using small fields will be discussed.

Learning Objectives:
1. Understand the clinical advantages and potential pitfalls of GK-based and linac-based brain SRS/SBRT.
2. Be acquainted with workflow managements developed for frameless treatment using GK.
3. Conceptualize the effects of rotational setup errors, quantify dosimetric consequences and explore different strategies to compensate for these errors during treatment planning and delivery using SIMT technique.
4. Get familiar with the commissioning of treatment planning system for SRS/SBRT using small fields.


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