Ohio River Valley Spring 2011

Quality Assurance Utilizing 3D Dose
Reconstruction for Stereotactic Lung Radiotherapy
James Durgin, Michael Weldon,
Nilendu Gupta

Ohio River Valley AAPM
Spring Educational Symposium
March 5, 2011

Overview of Lung SBRT Program

 OSU Experience  Research Project
 Began in 2008  41 plans
 53 lesions, 43 patients  Both recurrences in
 Mean/Mode Rx: 9GyX5 data set
 1 biopsy-proven  Non-IMRT
recurrence  No wedges
 1 imaging-based  All heterogeneous
recurrence calculations using AAA
 Low toxicity profile in Eclipse
 6MV Siemens Oncor
accelerator

The Ohio State University Comprehensive Cancer Center –
Arthur G. James Cancer Hospital and Richard J. Solove
Research Institute 2

Compass Overview

Plan Data

Backprojected Measurements

Forward Calculated Dose


Purpose
How effective are current QA procedures?

Clinical Challenges

 Point calculations are
less than ideal
 Inhomogeneities
 Scatter
 Small field sizes
 Detector arrays are
calculated for
homogenous materials
 Effect of multiple entry
points unknown


Methods
Retrospective point calculations
Forward calculations with a collapsed cone algorithm
Backprojected 3D reconstructed dose

Point Calculation Analysis

 Accessed point calculations in patient chart
 RadCalc software
 Utilized equivalent path length, field size scaling
 Calculated non-weighted field average

Avg= -0.29%


Secondary Forward Calculation

 IBA’s Compass
software TPS Dose Forward Calculated Dose
 Collapsed cone
algorithm
 Incorporates
heterogeneity
calculations
 Subject to
commissioning
differences
DVH Comparison Dose Difference Map


Compass Beam Model Commissioning
 Same input data/physicist commissioning as Eclipse
 Good agreement down to 3X3cm in solid water
TPS Dose Forward Calculated Dose


Backprojected 3D Dose
 IBA’s Compass software, Matrixx hardware used
 Mean PTV dose and DVH statistics analyzed
TPS Dose Backprojected Dose


Comparison Methods

 Retrospective analysis using point measurements
 Average point calculation vs. Eclipse prescription
 Comparison of calculation differences
 Forward calculation vs. Eclipse for mean PTV
 Measured backprojected dose compared to TPS
 Backprojection vs. Eclipse for mean PTV
 Within Compass differences
 Backprojected vs. forward calculation for mean PTV


Percent Change from Eclipse Mean PTV
35

Point Calculation to TPS
30

25
Number of Plans

20

15

10

5

0
-5 to -4 -4 to -3 -3 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +3 +3 to +4 +4 to +5 +5 to +6
Percent Difference


35

30

Forward Calculated in Compass to TPS
25
Number of Plans

20

15

10

5

0
Percent Difference


35

30

25

Backprojected to TPS
Number of Plans

20

15

10

5

0
Percent Difference


35

30

25

Backprojected to TPS
Number of Plans

20

Backprojected to Forward Calculated
15

10

5

0
Percent Difference


Mean PTVs Aren’t the Whole Story Though


OAR/Coverage Statistics for Backprojected
Dose Compared to Eclipse

 OARs receiving >20% of Rx, dose maximum analyzed
 Max dose for 2 spinal cord structures increased >5%
 Max dose for 1 esophagus structure increased >5%
 Max dose for 1 heart structure increased >5%
 Max dose for 1 brachial plexus structure increased >5%
 Max dose for 0 skin structures increased >5%
 Coverage of 95% isodose line
 2 PTVs experienced a drop in 95% coverage of more
than 5%


Summary

 QA is a process of constant improvement
 Ultimately TPS determines dose
 What to trust determines the success of QA
 Measurements/reconstructed dose have value, but
resources must be used wisely
 3D reconstructed dose provides variability analysis
for plans that pass traditional QA procedures


The OhioG. James Cancer Hospital and Richard J.Center –
Arthur State University Comprehensive Cancer Solove
Arthur G. James Cancer Hospital and Richard J. Solove Research Institute

References

 Per-beam, Planar IMRT QA Passing Rates Do Not
Predict Clinically Relevant Patient Dose Errors, 2011
 Comparison of DVH data from multiple radiotherapy
treatment planning systems, 2010
 US Patent Application: Radiation Therapy Dose
Perturbation System and Method, 2009


Bonus 1: 3DVH
 Sun Nuclear software using dose error kernels
 Compared Compass to TPS differences >3% for
mean PTV
Percent Change Between QA Methods
35

3DVH Measured to TPS
30

Compass Measured to Forward Calculated
25
Number of Plans

Compass Measured to TPS
20

15

10

5

0
%Difference


Ohio River Valley Spring 2011

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