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 Houston, July 30, 2008
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 (Beacon patients:
1.25mm). 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.
•
CT and MR images are fused
according to bony anatomy using
All soft tissue structures are
contoured based on the MR
information while the external
contour and bony structures are
based on CT.
Seminal vesicles ?
Extracapsular extension
•Retrograde urethrograms are not performed.
Imaging modality may affect treatment regime
MR-CT fusion based on boney anatomy
MRI
CT
Contoured on CT
Mismatch arises from
time of scan differences
Prostate (CT)
Prostate (MR)
MR-based
prostate-rectum
interface
Contoured on MR
Imaging artifacts may affect contouring
2
MRI
CT
Overlap (not including PTV)
Note that the
prostate is in a
different position
relative to the
femoral heads
CT-based prostaterectum interface
MR prior to beacon
placement (> 1 week)
CT
-Fusion based on boney anatomy
-soft tissue based on MR
- plan calculated on CT base
(CT derived isocenter)
MRI
-Isodose lines generated based on
MR-defined target
-but the patient is aligned by
beacons (CT)
CT
MRI
Solution: fuse based on soft tissue
(prostate); alignment will be
uneffected
-may result in a geographical miss
3
PTV growth = 8mm in all
directions except
posteriorly where a 5mm
margin is typically used
A
The “effective margin” is defined as:
0%
Rt FH
Bladder
0%
S
I
CTV
1.
2.
0%
0%
3.
0%
Lt FH
P
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.
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
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
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)
4
DVH Acceptance
Criteria
Good DVH
PTV95 % ≥ 100% Rx
R65 Gy ≤ 17%V
Acceptance Criteria
PTV95 = 100%
R40 Gy ≤ 35%V
B65 Gy ≤ 25%V
B40 Gy ≤ 50%V
FH50 Gy ≤ 10%V
What is a good plan?
When can I stop planning?
R40 = 22.7%
R65 = 8.3%
R40 = 19%
B65 = 8.4%
Good plan example (axial)
100%
90%
80%
CTV
70%
DVH Acceptance
Criteria
DVH for bad plan
(meets DVH criteria)
PTV95 % ≥ 100% Rx
R65 Gy ≤ 17%V
PTV95 = 100%
R40 Gy ≤ 35%V
60%
B65 Gy ≤ 25%V
50%
B40 Gy ≤ 50%V
R40 = 31.5%
FH50 Gy ≤ 10%V
R65 = 13.4%
“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
B40 = 21.3%
B65 = 9%
5
Bad plan example (axial)
Routine prostate IMRT plan acceptance criteria at FCCC
include all of the following except:
100%
90%
80%
0%
CTV
70%
0%
60%
0%
50%
0%
1.
2.
3.
4.
0%
The 50% isodose line falls
outside the rectal contour
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%
10
Regions for dose constraint
Region
1
2
3
4
5
6
Limit
90% of target goal
80%
70%
50%
30%
20%
Minimum
Maximum
% volume ↑ limit
20
45% of target goal
Target Max
20
40%
90% of target goal
20
35%
75%
1
25%
55%
1
15%
35%
1
10%
25%
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%
Price et al. IJROBP 2003
6
Localization
Average Prostate Motion >5mm (from Calypso beacons)
70
BAT Alignment
60
Overall Average = 12 sec
Average Maximum = 102 sec
Time (sec)
50
“CT-on-rails”
Gold Seed and CBCT
40
30
20
Calypso
10
(localization & tracking)
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)
Nodal Irradiation
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
LN ext = external iliac, proximal obturator and proximal internal iliac
7
Prostate
Prostate
Distal SVs
Proximal SVs
Proximal SVs
Extended
Lymphatics
Prostate
Prostate
Regional
Lymphatics
Distal SVs
Proximal SVs
Regional
Lymphatics
Distal SVs
Proximal SVs
8
Extended
Lymphatics
No longer
a geometry
problem;
avoidance
is only
minimally
useful
Bladder
Prostate
Regional
Lymphatics
Distal SVs
Lymphatic irradiation study
• 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.)
Proximal SVs
Rectum
Price et al. IJROBP 2006
Bladder Dose
Rectal Dose
100
40
R65s
R65v
R40s
R40v
R65 limit
R40 limit
35
80
70
Volume (%)
Volume (%)
30
25
20
15
B65s
B65v
B40s
B40v
B65 limit
B40 limit
90
60
50
40
60% failure
30
10
20
5
10
0
0
1
2
3
4
5
6
7
8
9
Patient Index
10
1
2
3
4
5
6
7
8
9
10
Patient Index
Ln ext
Ln ext
9
Bowel Dose
450
400
10x10
10x5
10x10
10x5
10x10
10x5
10x10
10x5
10x10
10x5
350
300
250
Bowel40 (ext LN lat PTV)
Bowel40 (no growth)
Bowel40 limit
350
300
Volume (cc)
Volume of Bowel at 40Gy (cc)
400
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)
200
250
200
150
Only 40%
failure
150
60% failure
100
100
50
50
0
0
1
0
2
3
4
5
6
2
3
4
5
6
7
8
9
10
Varian 21 Ex & Siemens Primus
Bladder Dose (Ext LN treatment)
100
B65 (ext LN lat PTV)
B65 (no lat growth)
90
Volume (%)
1
Patient Index
Targeting Group
80
B40 (ext LN lat PTV
B40 (no lat growth
B65 limit
70
B40 limit
60
50
40
30
20
Only ~30%
failure
10
• 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?
0
1
2
3
4
5
6
7
8
9
10
Patient Index
MSFmod = MUIMRT/MU3D CRT
Price et al. JACMP 2003
10
Prostate
Bladder
Analysis
PTV
5 mm x 5 mm beamlets
10 mm x 5 mm beamlets (coll 90)
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
•
Average # of segments
•
Average # of MU
2055
•
Average MSFmod
7.0
Prostate
PTV
386
Average # of segments 197
(~49 % reduction)
• Average # of MU 1344
(~34.6 % reduction)
• Average MSFmod 4.6
(~34.3 % reduction)
•
10 x 5 mm2 beamlets
Collimator 0 degrees
Rectum
5x5 beamlet
5x10 beamlet
CTV
Significantly increased MU may result in all of the following
except:
100%
100%
CTV
PTV
PTV
0%
0%
50%
50%
0%
5x5 beamlet
100%
5x10 beamlet
0%
100%
0%
PTV
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
CTV
CTV
PTV
Reductions
50%
Segments from 141 to 81
MU 1420 to 886
50%
10
MSFmod 4.8 to 3.0
11
HooRay!!! Post Docs!!!
Teh Lin
Ahmed Eldib
Routine QA
Prostate (Measured vs Calculated)
Qianyi Xu
90%
300
80%
50%
Number of Patients
250
Alain Tafo
30%
200
150
100
50
Lihui Jin
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)
0.125 cc ion chamber
Post Docs Rule!!!
12