IMRT for Prostate Cancer
All patients are simulated in the supine position. Reproducibility is
achieved using a custom alpha cradle cast that extends from the
mid-back to mid-thigh. The feet are positioned in a custom
plexiglas foot-holder. The patient is told to have a 1/2- 3/4 bladder
because during treatment a full bladder is difficult to maintain.
foot
holder
Robert A. Price Jr., Ph.D.
Philadelphia, PA
AAPM Minneapolis, July 24, 2007
alpha cradle
Simulation (Positioning and Immobilization )
CT Scans
Bad rectum
• The patient is asked to
empty the rectum using an
enema prior to simulation.
Also, a low residue diet the
night before simulation is
recommended to reduce gas.
If at simulation the rectum is
>3 cm in width due to gas or
stool, the patient is asked to
try to expel the rectal
contents.
•
Scans are acquired from approximately
2 cm above the top of the iliac crest to
approximately mid-femur. This scan
length will facilitate the use of noncoplanar beams when necessary.
•
Scans in the region beginning 2 cm
above the femoral heads to the bottom
of the ischial tuberosities are acquired
using a 2.5 mm slice thickness and 2.5
mm table increment. All other regions
may be scanned to result in a 1 cm slice
thickness.
6 cm
4.5 cm
Good rectum
3 cm
2.5 cm
1
MRI
CT
MR Scans
1.5 T, GE Medical Systems
•
All prostate patients also undergo MR
imaging within the department,
typically within one half hour before or
after the CT scan. Scans are obtained
without contrast media. The resultant
images are processed using a gradient
distortion correction (GDC) algorithm.
•
CT and MR (after GDC) images
are fused according to bony
anatomy using either chamfer
matching or maximization of
mutual information methods. All
soft tissue structures are contoured
based on the MR information
while the external contour and
bony structures are based on CT.
MRI
Seminal vesicles ?
Extracapsular extension
•Retrograde urethrograms are not performed.
Imaging modality may affect treatment regime
PTV growth = 8mm in all
directions except
posteriorly where a 5mm
margin is typically used
CT
A
Rt FH
Bladder
S
Prostate (CT)
I
CTV
Prostate (MR)
Lt FH
P
Imaging artifacts may affect contouring
Rectum
The “effective margin” is defined by the
distance between the posterior aspect of
the CTV and the prescription isodose line
and typically falls between 3 and 8 mm.
2
The “effective margin” is defined as:
0%
0%
1.
2.
0%
0%
3.
0%
4.
5.
The distance between the posterior aspect
of the CTV and the anterior rectal wall
The distance between the posterior aspect
of the CTV and the prescription isodose
line
The distance between the posterior aspect
of the CTV and the posterior rectal wall
The average 3D PTV margin
The difference between take-home pay and
what your better half allows you to spend
10
Regions
• 26 previously treated
patients (6 and 10 MV)
• The average number of
beam directions decreased
by 1.62 with a standard
error (S.E.) of 0.12.
• The average time for
delivery decreased by
28.6% with a S.E. of 2.0%
decreasing from 17.5 to 12.3
minutes
• The amount of nontarget tissue receiving
D100 decreased by
15.7% with a S.E. of
2.4%
• Non-target tissue
receiving D95, D90, D50
decreased by 16.3, 15.1,
and 19.5%, respectively,
with S.E. values of
approximately 2%
Number of Beam Directions
In the interest of
delivery time we
typically begin with 6
and progress to ≤ 9
Simpler plans such as
prostate only or prostate
+ seminal vesicles
typically result in fewer
beam directions than
with the addition of
lymphatics
3
Typical Dose
Routine treatments
• Prostate + proximal sv
(80 Gy @ 2.0 Gy/fx)
• Distal sv, lymphatics
(56 Gy @ ~1.4 Gy/fx)
Post Prostatectomy
• Prostate bed
(64-68 Gy @ 2.0 Gy/fx)
Acceptance Criteria
What is a good plan?
When can I stop planning?
Good plan example (axial)
DVH Acceptance
Criteria
100%
Good DVH
90%
PTV95 % ≥ 100% Rx
80%
R65 Gy ≤ 17%V
PTV95 = 100%
R40 Gy ≤ 35%V
CTV
70%
60%
B65 Gy ≤ 25%V
50%
B40 Gy ≤ 50%V
FH50 Gy ≤ 10%V
R40 = 22.7%
R65 = 8.3%
R40 = 19%
“Effective margin”
The 50% isodose line should
fall within the rectal contour on
any individual CT slice
The 90% isodose line should
not exceed ½ the diameter of
the rectal contour on any
slice
B65 = 8.4%
4
Bad plan example (axial)
DVH Acceptance
Criteria
100%
DVH for bad plan
(meets DVH criteria)
PTV95 % ≥ 100% Rx
90%
80%
R65 Gy ≤ 17%V
PTV95 = 100%
R40 Gy ≤ 35%V
CTV
70%
60%
B65 Gy ≤ 25%V
50%
B40 Gy ≤ 50%V
R40 = 31.5%
FH50 Gy ≤ 10%V
R65 = 13.4%
The 50% isodose line falls
outside the rectal contour
B40 = 21.3%
B65 = 9%
Localization
Routine prostate IMRT plan acceptance criteria at FCCC
include all of the following except:
0%
0%
0%
0%
1.
2.
3.
4.
0%
5.
The volume of either femoral head receiving
50 Gy ≤ 20%
The volume of bladder receiving 65 Gy ≤ 25%
The 50% isodose line should fall within the rectal
contour on each individual CT slice
The 90% isodose line should not exceed ½ the
rectal diameter on any CT slice
The volume of rectum receiving 65 Gy ≤ 17%
BAT Alignment
“CT-on-rails”
Gold Seed and CBCT
Calypso
(localization & tracking)
10
5
Average Prostate Motion >5mm (from Calypso beacons)
70
60
Overall Average = 12 sec
Average Maximum = 102 sec
Time (sec)
50
Nodal Irradiation
40
30
20
10
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Patient Index
Targeting Progression
Intermediate risk (group 1)
High risk (group 2)
PTV = prostate + proximal sv
PTV1 = prostate + proximal sv
PTV2 = distal sv (no lymph nodes)
High risk (group 3)
Prostate
PTV1 = prostate + proximal sv
PTV2 = distal sv
PTV3 = periprostatic + peri sv LNs
High risk (group 5)
High risk (group 4)
PTV1 = prostate + proximal sv
PTV1 = prostate + proximal sv
PTV2 = distal sv
PTV2 = distal sv
PTV3 = periprostatic + peri sv LNs
PTV3 = periprostatic + peri sv LNs + LN ext
+ LN ext
+ presacral/perirectal LN
Proximal SVs
LN ext = external iliac, proximal obturator and proximal internal iliac
6
Prostate
Prostate
Regional
Lymphatics
Distal SVs
Distal SVs
Proximal SVs
Proximal SVs
Extended
Lymphatics
Prostate
Extended
Lymphatics
Prostate
Regional
Lymphatics
Distal SVs
Regional
Lymphatics
Distal SVs
Proximal SVs
Proximal SVs
Rectum
7
Extended
Lymphatics
Bladder
Prostate
Extended
Lymphatics
No longer
a geometry
problem;
avoidance
is only
minimally
useful
Bladder
Prostate
Regional
Lymphatics
Distal SVs
Regional
Lymphatics
Distal SVs
Proximal SVs
Proximal SVs
Rectum
Rectum
Presacral Nodes
Lymphatic irradiation study
Rectal Dose
40
R65s
R65v
R40s
R40v
R65 limit
R40 limit
35
30
Volume (%)
• 10 patient data sets
• Generate plans for each stage in targeting progression
• Evaluate effect of nodal irradiation on our routine prostate
IMRT plan acceptance criteria
• Evaluate effect on bowel
• Treatment time (logistical concerns as well as patient comfort)
• Physics concerns (dose per fraction vs. “cone downs”,
increased “hot spots”, PTV growth and localization technique,
rectal expansion and inclusion of presacral nodes, etc.)
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
Patient Index
Price et al. IJROBP 2006
Ln ext
8
Extended Lymphatics (good)
100%
90%
80%
70%
60%
50%
Bladder Dose
100
B65s
B65v
B40s
B40v
B65 limit
B40 limit
90
80
Volume (%)
70
56 Gy
60
76 Gy
50
40
60% failure
30
20
10
0
1
2
3
4
5
6
7
8
9
10
Patient Index
56 Gy
Ln ext
Extended Lymphatics (good)
100%
90%
80%
70%
60%
50%
Extended Lymphatics (bad)
100%
90%
80%
70%
60%
50%
56 Gy
76 Gy
B40 = 40.5%
B65 = 8.2%
R40 = 29.1%
R65 = 10.2%
56 Gy
100%
90%
80%
70%
60%
50%
9
Extended Lymphatics (bad)
Bowel Dose
450
10x10
10x5
10x10
10x5
10x10
10x5
10x10
10x5
10x10
10x5
400
Volume of Bowel at 40Gy (cc)
B40 = 74.3%
B65 = 35.3%
350
300
250
200
150
60% failure
100
R40 = 34.4%
50
0
R65 = 12.4%
1
0
2
3
4
5
6
Targeting Group
What if we limit nodal
PTV growth laterally
with a corresponding
limit in the lateral shift
based on daily
localization?
Bowel Dose (ext LN treatment)
400
Bowel40 (ext LN lat PTV)
Bowel40 (no growth)
Bowel40 limit
350
Bladder Dose (Ext LN treatment)
100
B65 (ext LN lat PTV)
B65 (no lat growth)
90
80
B40 (ext LN lat PTV
B40 (no lat growth
B65 limit
70
B40 limit
250
Volume (%)
Volume (cc)
300
200
150
60
50
40
30
Only 40%
failure
100
20
Only ~30%
failure
10
50
0
1
0
2
3
4
5
6
7
8
9
10
Patient Index
1
2
3
4
5
6
7
8
9
10
Patient Index
10
Prostate
Bladder
Varian 21 Ex & Siemens Primus
• 1 cm leaf width vs 5 mm leaf width
• 10 x 10 mm2 minimum beamlet vs 5 x 5 mm2
• We limit to 6-9 beam directions (primarily due to
treatment time)
• Corvus treatment planning
• Increased MU
Increase leakage
secondary
malignancies?, shielding concerns?
PTV
10 x 5 mm2 beamlets
Rectum
Collimator 90 degrees.
Bladder
Prostate
This places the short
axis of the beamlet
~perpendicular to the
prostate-rectal interface
PTV
5 x 5 mm2 beamlets
Bladder
Rectum
Prostate
PTV
10 x 5 mm2 beamlets
Collimator 0 degrees
MSFmod = MUIMRT/MU3D CRT
Rectum
Price et al. JACMP 2003
5x5 beamlet
5x10 beamlet
Analysis
100%
100%
CTV
CTV
PTV
•
Average # of segments
•
Average # of MU
2055
•
Average MSFmod
7.0
PTV
10 mm x 5 mm beamlets (coll 90)
5 mm x 5 mm beamlets
386
•
Average # of segments 197
(~49 % reduction)
• Average # of MU 1344
(~34.6 % reduction)
• Average MSFmod 4.6
(~34.3 % reduction)
50%
50%
5x5 beamlet
PTV
100%
5x10 beamlet
100%
CTV
CTV
PTV
Reductions
50%
Segments from 141 to 81
50%
MU 1420 to 886
MSFmod 4.8 to 3.0
11
Significantly increased MU may result in all of the following
except:
0%
0%
0%
0%
1.
2.
3.
4.
5.
Increased leakage radiation
Potential increase in the occurrence of secondary
malignancies
Shielding concerns
Increased treatment time
A decrease in beam stability
Routine QA
Prostate (Measured vs Calculated)
90%
300
80%
50%
250
Number of Patients
0%
30%
200
150
100
50
0
[<-3.5]
[-2.5,-3.5] [-1.5,-2.5] [-0.5,-1.5] [-0.5,0.5] [0.5,1.5]
[1.5,2.5]
[2.5,3.5]
[>3.5]
Frequency Bins (% difference)
10
0.125 cc ion chamber
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12