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. 2017 Nov 25;12(1):187.
doi: 10.1186/s13014-017-0919-4.

Optimization of the prescription isodose line for Gamma Knife radiosurgery using the shot within shot technique

Perry B Johnson  1 , Maria I Monterroso  2 , Fei Yang  2 , Eric Mellon  2
Affiliations

Optimization of the prescription isodose line for Gamma Knife radiosurgery using the shot within shot technique

Perry B Johnson et al. Radiat Oncol. .

Abstract

Background: This work explores how the choice of prescription isodose line (IDL) affects the dose gradient, target coverage, and treatment time for Gamma Knife radiosurgery when a smaller shot is encompassed within a larger shot at the same stereotactic coordinates (shot within shot technique).

Methods: Beam profiles for the 4, 8, and 16 mm collimator settings were extracted from the treatment planning system and characterized using Gaussian fits. The characterized data were used to create over 10,000 shot within shot configurations by systematically changing collimator weighting and choice of prescription IDL. Each configuration was quantified in terms of the dose gradient, target coverage, and beam-on time. By analyzing these configurations, it was found that there are regions of overlap in target size where a higher prescription IDL provides equivalent dose fall-off to a plan prescribed at the 50% IDL. Furthermore, the data indicate that treatment times within these regions can be reduced by up to 40%. An optimization strategy was devised to realize these gains. The strategy was tested for seven patients treated for 1-4 brain metastases (20 lesions total).

Results: For a single collimator setting, the gradient in the axial plane was steepest when prescribed to the 56-63% (4 mm), 62-70% (8 mm), and 77-84% (16 mm) IDL, respectively. Through utilization of the optimization technique, beam-on time was reduced by more than 15% in 16/20 lesions. The volume of normal brain receiving 12 Gy or above also decreased in many cases, and in only one instance increased by more than 0.5 cm3.

Conclusions: This work demonstrates that IDL optimization using the shot within shot technique can reduce treatment times without degrading treatment plan quality.

Keywords: Gamma Knife; Gradient index; Metastases; Optimization; Radiosurgery.

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Conflict of interest statement

Ethics approval and consent to participate

This work was performed with approval from the University of Miami – IRB B, exempt review.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Fig. 1
Fig. 1
Dose gradient for the X/Y (left figure) and Z (right figure) dimensions, 8 mm collimator setting
Fig. 2
Fig. 2
Graphical representation of the optimization space provided by the 10,000+ beam configurations using the shot within shot technique plotted with colors indicating (a) prescription IDL or (b) treatment time
Fig. 3
Fig. 3
Overlap region where prescribing to a higher IDL maintains the same prescription isodose diameter in the axial plane but decreases the distance between the prescription and half-prescription line
Fig. 4
Fig. 4
Twin peaks representing the time savings predicted when using shot within shot optimization. The comparison is made between shot within shot plans prescribed at the 50% IDL (SS50%) and shot within shot plans optimized using collimator weighting and different selections of the prescription IDL (SSopt). The optimal plan provided an equivalent diameter of the prescription IDL in the axial plane, a gradient distance (factor = 0.5) no worse than 3% from the original, and a minimized beam-on time
See this image and copyright information in PMC

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