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Significant Cord and Esophagus Dose Reduction by 4p Non-Coplanar Spine Stereotactic Body Radiation Therapy and Stereotactic Radiosurgery

V Yu

V Yu*, A Tran , D Nguyen , K Woods , M Cao , T Kaprealian , R Chin , D Low , K Sheng , UCLA, Los Angeles, CA


TH-EF-BRB-3 (Thursday, August 4, 2016) 1:00 PM - 2:50 PM Room: Ballroom B

To demonstrate significant organ-at-risk (OAR) sparing achievable with 4π non-coplanar radiotherapy on spine SBRT and SRS patients.

Twenty-five stereotactic spine cases previously treated with VMAT (n = 23) or IMRT (n = 2) were included in this study. A computer-aided-design model of a Linac with a 3D-scanned human surface was utilized to determine the feasible beam space throughout the 4π steradian and beam specific source-to-target-distances (STD) required for collision avoidance. 4π radiotherapy plans integrating beam orientation and fluence map optimization were then created using a column-generation algorithm. Twenty optimal beams were selected for each case. To evaluate the tradeoff between dosimetric benefit and treatment complexity, 4π plans including only isocentrically deliverable beams were also created. Beam angles of all standard and isocentric 4π plans were imported into Eclipse to recalculate the dose using the same calculation engine as the clinical plans for unbiased comparison. OAR and PTV dose statistics for the clinical, standard-4π, and isocentric-4π plans were compared.

Comparing standard-4π to clinical plans, particularly significant average percent reduction in the [mean, maximum] dose of the cord and esophagus of [41%, 21.7%], and [38.7%, 36.4%] was observed, along with global decrease in all other OAR dose statistics. The average cord volume receiving more than 50% prescription dose was substantially decreased by 76%. In addition, improved PTV coverage was demonstrated with a maximum dose reduction of 0.93% and 1.66% increase in homogeneity index (D95/D5). All isocentric-4π plans achieved dosimetric performance equivalent to that of the standard-4π plans with higher delivery complexity.

4π radiotherapy significantly improves stereotactic spine treatment dosimetry. With the substantial OAR dose sparing, PTV dose escalation is considerably safer. Isocentric-4π is sufficient to achieve the dosimetric gain. The successful implementation of 4π using an FDA approved planning system paves the way for a prospective clinical trial.

Funding Support, Disclosures, and Conflict of Interest: Varian Medical Systems, NIH R43CA183390 and R01CA188300, NSF graduate research fellowship DGE-1144087

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