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Surface Area Matters: The SRS/SRT Intermediate Dose Spill Depends On PTV Surface Area (not PTV Volume)

D Desai1, I Cordrey2*, E Johnson3, S Srinivasan4, H Elasmar5, (1) Memorial Hospital, Hixson, TN, (2) Hillsdale Wildflower Refuge, Cookeville, TN, (3) Univ Kentucky Medical Ctr, Lexington, KY, (4) Memorial Health Care System, Hixson, TN, (5) Memorial Health Care System, Chattanooga, TN

Presentations

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

Room: AAPM ePoster Library

Purpose:
What PTV characteristics affect the intermediate dose spill in an SRS/SRT treatment? We hypothesize that the PTV Surface Area is a stronger determinant of the intermediate dose spill in SRS/SRT than the PTV volume.


Methods:
Using the IROC Head Phantom®, a series of PTVs of various volumes and surface areas are created and planned on the Eclipse® TPS using RapidArc 5-hemi-arc plans for delivery on a Varian TrueBeam STx® with 120 leaf HD MLC. All plans are prescribed such that 99% of the PTV volume receives 18 Gy in one fraction. Plans are created for PTVs of volumes from 1.8cc to 16.8cc, with surface areas from 10cm² to 31.7 cm².


Results:
We assess the intermediate dose spill using the metric R50% = (Volume of 50% isodose) / (PTV volume). Given the same volume, the higher surface area PTV always has a higher R50%. Given the same surface area, the higher volume has a lower R50%. We find an even better correlation between the true 50% Isodose Volume (PIV50%) and the PTV Surface Area. The graph of PIV50% vs PTV Surface Area has a correlation coefficient, r = 0.985. The PIV50% vs PTV Volume has r = 0.963. The Surface Area graph brings all PTVs of any shape into a single straight line plot, while the Volume graph shows a differentiation between different PTVs shapes

Conclusion:
This work demonstrates a strong dependence of the SRS/SRT intermediate dose spill upon the PTV surface area. Further, PTV surface area is a better indicator of intermediate dose spill than PTV volume. The metric PIV50% shows the best correlation with PTV surface area. Because of this strong correlation we encourage all TPS vendors to include surface area as a reported PTV characteristic (like PTV volume) in future versions of their software.

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Keywords

Stereotactic Radiosurgery, Dosimetry

Taxonomy

TH- External Beam- Photons: intracranial stereotactic/SBRT

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