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Dosimetric Effect of Plan Homogeneity On Intracranial Stereotactic Radiosurgery (SRS) Treatment Planning Using Volumetric Modulated Arc Therapy (VMAT)

T Tseng*, R Sheu , S Green , K Dharmarajan , S Sharma , Y Lo , Mount Sinai Medical Center, New York, NY


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To evaluate dosimetric differences of intracranial stereotactic radiosurgery (SRS) planned with volumetric modulated arc therapy (VMAT) when different levels of dose homogeneity are achieved.

Methods: Eleven patients with single (n=8) or multiple (n=3) lesions were selected. Total eighteen lesions (0.18–16.27 cc) were planned with three to seven coplanar and non-coplanar volumetric modulated arcs in Eclipse® (AAA, Millennium 120 MLC). Three plans with comparable GTV coverage but different levels of homogeneity index (HI), HI_LOW (mean:1.15, range: 1.12–1.19), HI_MED (mean:1.24, range:1.18–1.30), HI_HIGH (HI mean:1.36, range:1.28–1.40) were generated by optimizing with different maximum dose constraints to GTV. The lower HI represents the better homogeneity of the plan. Plan quality was evaluated with dosimetrical parameters: conformity index (CI), Gradient Index (GI), normal brain V12Gy, V10Gy, and V3Gy. Wilcoxon signed rank test was used for statistical comparison between HI_LOW and HI_MED, also HI_MED and HI_HIGH.

Results: The mean CI was improved when dose HI increase from HI_LOW to HI_MED, but stayed comparable when further increase to HI_HIGH (CI(HI_LOW)=1.42±0.29, CI(HI_MED)=1.34±0.24 and CI(HI_HIGH)=1.35±0.25). The mean GI decreased consistently when inferior dose homogeneity was achieved (GI(HI_LOW)=5.40±3.02, GI(HI_MED)=4.93±2.61 and GI(HI_HIGH)=4.53±2.45)(p(HI_LOW-HI_MED)<0.001 and p(HI_MED-HI_HIGH)<0.001). V12Gy and V10Gy, the indices used to predict symptomatic radionecrosis, both were also significantly lower when dose homogeneity decreased (V12Gy(HI_LOW)=6.62±3.76 cc, V12Gy(HI_MED)=5.91±3.55 cc and V12Gy(HI_HIGH)=5.31±3.32 cc, p(HI_LOW-HI_MED)=0.003 and p(HI_MED-HI_HIGH)=0.003)(V10Gy(HI_LOW)=10.21±5.70 cc, V10Gy(HI_MED)=9.12±5.41 cc and V10Gy(HI_HIGH)=8.28±5.12 cc, p(HI_LOW-HI_MED)=0.003 and p(HI_MED-HI_HIGH)=0.003). Low dose spread, expressed as the volume receiving 3Gy (V3Gy), was only improved between the plans with HI_LOW and H_MED, but did not show significant change between the plans with HI_MED and HI_HIGH (V3Gy(HI_LOW)=88.38±64.01 cc, V3Gy(HI_MED)=80.13±57.54 cc and V3Gy(HI_HIGH)=77.36±61.76 cc).

Conclusion: Our study shows that by releasing maximum dose constraint during VMAT optimization and accepting plans with inferior dose homogeneity help to achieve faster dose fall off and decrease normal brain V12Gy and V10Gy.


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