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Effect of the Maximal PTV Dose Constraint On the SRS Plan Quality for the Single Isocenter-Multiple-Target (SIMT) Technique

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K Joseph

K Joseph*, J Baker , Y Ahmed , J Antone , L Vijeh , J Alfaro , J Chang , Northwell Health, Lake Success, NY


SU-I-GPD-T-589 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the effect of PTV (planning target volume) maximal dose (PTVmax) on the SRS (stererotactic radiosurgery) plan quality for the single isocenter-multiple target (SIMT) technique.

Methods: Fifteen SRS cases with multiple PTVs (ranging from 1-6) were randomly selected and re-planned using the SIMT technique for a prescription dose of 20 Gy. Two plans were developed for each case with different constraints imposed on the PTVmax – (A) PL125: PTVmax was ~125% (between 124% and 128%) of the prescription, and (B) PL167: PTVmax was unconstrained when <167% of the prescription dose. All beam parameters were replicated when re-planned; including the number of beams, beam angles, dose rate, start and stop angles, collimator angles, and couch angles. VMAT (volumetric modulated arc therapy) optimizations adhered to UAB recommendations for brain SRS SIMT technique. Plans were normalized to ensure that 97% to 100% of the PTV was covered by at least 99% of the prescription dose and that the PTVmax constraint was met. Plan quality indexes included conformity indexes (CI), mean dose of PTV (PTVmean), tissue mean dose (Tmean), V12 Gy/VPTV and V10 Gy/VPTV were calculated.

Results: Paired sample t-test was performed to determine if the mean difference between the plan indexes (PL167- PL125) was statistically significant (p-value < 5%). As expected, the PTVmean of PL167 plans were higher than that for PL125 plans (+11.3% with p-value=0.000006). PTV coverage was slightly better for PL125 plans with a higher V100 (+0.6% with p-value=0.008). Plan indexes for assessing normal tissue sparing were generally better for PL165 plans than for PL125 plans (V12Gy/VPTV: -34.3%, p=0.0002; V10Gy/VPTV: -37.8%, p=0.0002). However, differences in CI (1.4%, p= 0.266) and Tmean (-2.9%,p= 0.210) were not statistically significant.

Conclusion: The plan quality was found to be significantly better for SRS plans with unconstrained maximal PTV dose using the SIMT technique.

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