patient specific qa for monte carlo lung sbrt on...

Patient Specific QA for Monte Carlo Lung SBRT on Cyberknife:

Is It Necessary?J. Fabien, MS; Y. Zhang, MS; J. Brindle, PhD; D. Dobbins, CMD;

T. Podder, PhD; B. Wessels, PhD

University Hospitals Seidman Cancer CenterCase Western Reserve University Medical Center

The Lung SBRT Issue

• Originally, all Cyberknife planning used Ray Tracing algorithm

• 2012: Started using Monte Carlo algorithm

• RTOG clinical trials exclude Ray Tracing

• Realized a 10-20% deficiency in dose and resulting loss of prescription coverage

• After dosimetry effects were seen, ALL lung and T-spine patients were calculated with MC

Monte CarloRay Tracing

The Lung SBRT Issue

54 Gy

Ray Tracing

The Lung SBRT Issue

Monte CarloRay Tracing

The Lung SBRT Issue

Ray Tracing

The Lung SBRT Issue

Monte CarloRay Tracing

The Lung SBRT Issue

Monte CarloRay Tracing

PTV = 98.6%CTV = 100%

PTV = 2.2%CTV = 19.8%

The Lung SBRT Issue

Ray Tracing

The Lung SBRT Issue

Monte CarloRay Tracing

The Lung SBRT Issue

Ray Tracing

Ray tracing calculation using equivalent path length:

• TPR (FS, Deff)

• OCR (FS, R800, Deff)

• OF (FS, SAD)

OFTPRSAD

OCRMUD

2800

Ray TracingSecond check using MuCheck (Oncology Data Systems):

Ray Tracing dose = 6000 cGyMuCheck dose = 6026 cGy

Ray Tracing

Ray Tracing dose = 6000 cGyMuCheck dose = 6026 cGy

• Much more complex

• Much more accurate with heterogeneities

• No software second check exists

• Phantom measurement is necessary to verify dose

Monte Carlo

The Method

• Use heterogeneous phantom

• Overlay chamber position in low gradient area

• Calculate mean MC dose to chamber volume

• Deliver patient plan to phantom, measure dose to chamber

1. Transfer patient plan onto hetero phantom

2. Calculate low resolution dose to check & adjust chamber position

3. Calculate high resolution

4. Calculate with Monte Carlo

MC chamber mean dose = 85.30 Gy

5. Deliver patient plan to hetero phantom with ion chamber

MC chamber mean dose = 85.30 GyMeasured chamber dose = 84.14 Gy -1.4%

-10.00%-9.00%-8.00%-7.00%-6.00%-5.00%-4.00%-3.00%-2.00%-1.00%0.00%1.00%2.00%3.00%4.00%5.00%6.00%7.00%8.00%9.00%

10.00%

0 5 10 15 20 25 30 35 40 45 50 55 60

Pt #

% D

iffer

ence

Delivery QA Results

Avg. MC delivery % Error = -2.50%

N = 60

Measured dose vs. MC calculated dose for 60 patients, % difference:

Is it necessary?

• Patient specific QA varies across Cyberknife users• We sought verification of MC accuracy using

chamber measurements• Use hetero phantom to simulate patient anatomy

to uncover dose discrepancies• Reduce frequency once baseline is established

Yes … initially.

Current Clinical Implementation

• Verified sufficiently the MC algorithm is working, and more accurate for hetero anatomy

• 60 patients measured, average difference -2.5%• In radiation oncology we nominally require a 2nd

check calculation• Currently we use a calculated MC vs. RT QA in a

homogenous phantom to identify gross planning, collimator or alignment errors

• 20 patients calculated, average difference -2.0%

Calculated QA Method

• Use homogenous phantom

• Overlay patient plan in center of phantom

• Calculate max point dose with RT, then MC for same point

• Export RT beam list for 2nd check

1. Transfer patient plan onto homog. phantom

2. Calculate low resolution to verify position

RT Max dose = 85.30 Gy

3. Calculate high resolution Ray Tracing dose

RT Max dose = 85.30 GyMC (RT Max) dose = 83.03 Gy

4. Calculate Monte Carlo dose

-2.7%

5. Compare with MUCheck results

% Diff. = +0.1%

MC dose = 83.03 GyRT dose = 85.30 Gy

MUCheck dose = 85.41 Gy

% Diff. = -2.7%

Monte Carlo Calculated QA Results

-10.00%-9.00%-8.00%-7.00%-6.00%-5.00%-4.00%-3.00%-2.00%-1.00%0.00%1.00%2.00%3.00%4.00%5.00%6.00%7.00%8.00%9.00%

10.00%

0 5 10 15 20

Pt #

% D

iffer

ence

Monte CarloMuCheck

Avg. MuCheck % Error = +0.07%

Avg. MC-RT % Error = -2.07%

N = 20

Calculated QA ResultsRT vs. MC calculated dose & RT vs. MuCheck 2nd check

% difference (20 patients):