June Kim MD, Presenter: Nothing to Disclose

Andrew Fink MD, Abstract Co-Author: Nothing to Disclose

Lori Copsey BS,RT, Abstract Co-Author: Nothing to Disclose

Joseph Ott MS, Abstract Co-Author: Nothing to Disclose

Rachel Dayton MPH, Abstract Co-Author: Nothing to Disclose

Penny Tatman MPH, Abstract Co-Author: Nothing to Disclose

To quanitfy the dose variation between the Monte Carlo (MC) and the conventional Ray-Tracing (RT) dose calculation algorithms in CyberKnife Stereotactic Radiosurgery (CK SRS) planning for lung tumors.

From 5/1/10 to 3/1/11, 17 patients underwent CK SRS planning CT for a total of 18 lung lesions.  Each lesion was designated as either central or peripheral based on the guidelines of the Lung Cancer Stereotactic Radiosurgery vs. Surgery Trial.  Peripheral lesions were further grouped into pleural-based vs. intraparenchymal.  For each lesion, an SRS plan was developed using the RT algorithm then recalculated using MC algorithm.  The dose distribution to the target and organs at risk (OAR) were compared between the two algorithms.

Five lesions were located centrally and 13 lesions peripherally.  Among the 13 lesions, 3 lesions were completely surrounded by normal lung tissue and were designated as intraparenchymal.  There was a significant decrease in radiosurgery dose to the planning treatment volume (PTV) and gross tumor volume (GTV) when the MC algorithm was utilized for dose calculation as compared to the RT algorithm with p-values <0.001 in both cases.  The mean percent reduction to the PTV was 24.6% and 6.7% to the GTV.  With the MC algorithm dose reduction was 8.7% for central lesions and 30.6% for peripheral lesions.  The dose reduction was 24.6% for the pleural-based lesions vs. 50.7% for the intraparenchymal lesions.  However, due to the small number of patients in the intraparenchymal group, we were unable to calculate a p-value.  There was a significant decrease in dose received by 5% of the volume (D5%) of the OAR when the MC algorithm was used for the spinal cord, trachea and normal lung tissue.  There was no significant difference in D5% for the esophagus or great vessels of the mediastinum.

There is a significant dose reduction to the PTV, GTV and OAR adjacent to air cavities and bony structures when the MC algorithm is utilized in the SRS plan for lung tumors.  Further studies on this topic are warranted to establish consensus regarding adjustment to prescription dose to the target and guidelines for normal tissue dose.

There is a significant dose difference in the intraparenchymal lesions and organs at risk surrounded by air cavities and bony structures when using MC compared to RT algorithms for CK SRS.

Kim, J, Fink, A, Copsey, L, Ott, J, Dayton, R, Tatman, P, CyberKnife Stereotactic Radiosurgery Dose Calculation Comparison: Monte Carlo vs Ray-tracing.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11012046.html