Maya Mathew MD, Presenter: Nothing to Disclose

Sebastien Gros, Abstract Co-Author: Nothing to Disclose

Haoming Qiu MD, Abstract Co-Author: Nothing to Disclose

Iris Rusu, Abstract Co-Author: Nothing to Disclose

Edward Melian MD, Abstract Co-Author: Nothing to Disclose

Anil Sethi PhD, Abstract Co-Author: Nothing to Disclose

Stereotactic Radiosurgery (SRS) is often delivered with 5-6 cone based circular arcs. Although circular arc technique is ideal for spherical targets, it gives suboptimal results for large, irregularly shaped volumes. In SRS, treatment plan quality is evaluated based on several dosimetric indices: conformality index @100% (CI) and 50% (CI50) isodoses, homogeneity index, OAR doses etc. We investigate improvement in SRS plan quality with MLC based dynamic conformal arcs.

Thirty patients treated for brain metastasis with SRS were included in this study. All patients received 22Gy single fraction dose to the planning target volume (PTV). 24 patients were treated with circular arcs and 6 with conformal beams. All patients underwent MR scans (T1-Axial, T2-Axial and T1 Coronal) followed by contrast aided CT scan. Composite PTV was outlined on the fused CT/MR data sets. Cone based treatments were re-planned with dynamic conformal arcs using micro-MLC(3mm) without changing dose constraints and arc parameters. Treatment plans were evaluated based on dosimetric indices: Dmin, Dmax, Dmean , D90, D95, CI, CI50, and homogeneity index, HI = Dmax/Dmin. For each target, a shape index (SI) was computed as the ratio of the smallest sphere volume enclosing the given target to PTV. Dependence of dose parameters on PTV and SI was investigated.

Target size ranged from 0.05 to 3.16 cc (average 0.538 ±0.645 cc) with an average shape index of 1.52 ±0.49. Transitioning from cone based plans to MLC plans improved CI from 2.16 ±0.55 to 1.59±0.29 (p<0.001). With increasing PTV, both MLC and cone based CI and CI50 showed improvement. For larger PTVs, the degree to which the CI and CI50 could be improved by MLC planning also increased. Both CI and CI50 were found to significantly improve (p<0.001) with MLC based dynamic conformal arcs especially for irregularly shaped larger (>1cm diameter) targets.

For large irregular targets SRS plan quality can be significantly improved using MLC based dynamic arc technique.

We present a guide for designing and evaluating optimal and consistent SRS treatment plans for brain metastasis as a function of target and shape.  

Mathew, M, Gros, S, Qiu, H, Rusu, I, Melian, E, Sethi, A, Factors Influencing Treatment Plan Quality in Stereotactic Radiosurgery of Brain Metastasis.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14018241.html