Region of Interest Analysis as a Tool for Exploring Region of Interest Analysis as a Tool for Exploring

Adaptive IMRT Strategy for Cervix Cancer PatientsAdaptive IMRT Strategy for Cervix Cancer Patients

Young-Bin Cho1,2, Valerie Kelly1, Karen Lim1,2, James Stewart1, Anthony Fyles1,2, Kristy Brock1,2,

Jason Xie1, Anna Lundin3, Henrik Rehbinder3, Michael Milosevic1,2

1Radiation Medicine Program, Princess Margaret Hospital/Ontario Cancer Institute; 2University of Toronto; 3RaySearch Laboratories AB, Stockholm, Sweden

Introduction

Planning studies of whole pelvis IMRT for cervix

cancer have shown that organ motion and

deformation can lead to deviations between the

original treatment plan and the distribution of

dose delivered during treatment. Finding

strategies to respond to these anatomical

changes using an adaptive planning approach is

time consuming, however, due to the large

number of variable parameters. Assuming that

the dose gradient conforms well to the PTV, and

that geometrical expansion of the PTV can be

used to represent various isodose levels, a

region of interest (ROI) analysis can efficiently

explore and evaluate different planning

strategies.

This study shows (1) the effect of dose

conformity on target coverage and OAR sparing

and (2) the performance of various adaptive

planning strategies. (Results from our previous

adaptive planning studies were compared with

the estimation from ROI analysis).

Materials and Methods

A. Adaptive planning and dose accumulation

25 patients had baseline and weekly MR scans

performed during radiotherapy. These were

fused to the planning CT using bony alignment.

Perfect daily setup to bone was assumed. Each

image-set was fully contoured and PTV margins

generated. PTV D98 was 4750cGy (95%

prescription dose). Adaptive IMRT plans were

developed by a dosimetrist as well as via an

automated method using the ORBIT Workstation

from RaySearch laboratories. Dose delivered to

the deformed organs in each image-set was then

calculated (Fig 1 and Fig 2) . Isodose volume at

80-95% of the prescribed dose were then

compared with that of the PTV for each plan

using ROI analysis.

The optimal schedule for one time on-line

adaptive planning is during the 1st or 2nd week of

treatment. Any later would be too late to

compensate for areas of target under dosing. This

result agrees well with our previous studies.

The volume of OARs (rectum, sigmoid, bowel and

bladder) in the PTV was also reduced with

adaptation and OAR sparing was significantly

enhanced with more frequent on-line adaptation.

Conclusions

We found ROI analysis to be an elegant and

efficient tool for exploring and evaluating

adaptive planning strategies. Using this method,

on-line adaptation proved to be the most

important factor for target coverage with OAR

sparing. Off-line adaptation did not improve target

coverage, but did improve OAR sparing (due to

shrinking target volumes). At present, weekly

adaptation using the automated process adds an

additional 2mm expansion to the 95% isodose

line when compared to the single adaptive plan

generated by the dosimetrist. Conformality is an

important factor for successful adaptation. Less

conformal plans are less sensitive to target

motion but result in less OAR sparing.

Acknowledgements

The authors would also like to thank Philip Chan,

David Jaffray and Johann Löf.

B. ROI analysis

Various ROIs were analysed to compare different

adaptive planning strategies. CTV expansions of

of 3, 5, 7, 10, 15 and 30mm were made. The

3mm expansion was considered to be the PTV.

The 5mm expansion was regarded as a 2mm

additional expansion from the PTV (PTVexp_2)

and so on. Proportions of CTV and OARs lying

in/out of the PTV and PTVexp were assessed for

each weekly image-set (Fig 3). Various adaptive

planning strategies were simulated (no

adaptation, single mid-treatment adaptation, and

weekly adaptation with on-line and off-line

adaptation). Each margin and adaptation

schedule was assessed with regards to the

competing goals of target coverage and OAR

sparing.

Results and Discussion

Both GTV and CTV shrank to 25% and 65% of

their original volumes by the end of treatment. A

combination of weekly CTVs into a single CTV

union became 113% of the original volume due to

the inclusion of inter-fractional motion. This

clearly supports the need for planning margins.

Volumes of the PTV expansions increase rapidly

with margin size. The Volume of PTVexp_2

(2mm expansion from PTV) is 25% larger than

the PTV and PTVexp_7 is 81% larger than the

PTV. Conformity of adaptive plans done by the

dosimetrist as well as the unsupervised

automated method using ORBIT Workstation

were compared for all 25 patients (Fig 4). The

dosimetrist achieved better conformity and less

isodose volume (IDV) than automated version in

the same dose range. Volumes of PTVexp are

shown on the right vertical axis for comparison

with the IDV. Although the same PTVs were used

for planning, IDV of the automated method was

2mm larger than those of the dosimetrist.

Considering IDV95% is 2mm larger than PTV

(Fig 4) for IMRT plans by expert, the average

volumes of CTV outside of IDV95% were 2.6%

(blue dot in Fig 5) and 0.5% (red dot) for the

original PTV (IDV95% = PTV+2mm) and

enlarged PTV by 5mm (IDV95% = PTV+7mm)

when no adaptation was applied. Weekly (full)

on-line adaptation with no delay from planning to

delivery significantly improved CTV coverage,

however off-line adaptation (1 week delay from

planning to delivery in this study) could not.Figure 1

Slide from Karen Lim

Figure 2. Delivered dose is different from planned dose due to organ deformation & tumour shrinkage over the course of treatment.

Planned dose Delivered dose at Week 2

UterusCervix/ GTV

VaginaPTV

BowelRectumBladder

Week 0 Week 1 Week 2 Week 3 …

No

Ada

ptat

ion

Ada

pt

@ w

eek

2

Figure 3. CTV (orange), OAR (green), PTV (sold yellow) and PTVexp (expansion of PTV, dotted yellow)

Ada

pt

ever

y w

eek

Figure 4. Conformity of IMRT plans. An expert dosimetrist made IMRT plans with better conformity showing less amount of isodose volume than automatic tool does. Lower conformality is believed to be responsible for the inability of OAR sparing for on-line automatic adaptation.

80 85 90 950

0.5

1

1.5

2

2.5

3

Isodose to Prescription dose [ %]

Rat

io o

f Iso

do

se V

olu

me

to P

TV

PTV

2mm

4mm

7mm

12mm

Automatic optimization Expert optimization

Ideal dose distribution

Ad

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mar

gin

fro

m P

TV

Figure 6. Percentage of OAR inside PTV, IDV95% (47.5Gy, PTV+2mm) and IDV90% (45Gy, PTV+6mm). Adaptation helps to spare OAR for both on-line and off-line adaptation. Effect of PTV margin and conformity on OAR dose sparing is significant. 2nd week is the best time for single adaptive plan.

Figure 5. Larger PTV margin improves target coverage for both on-line and off-line adaptation. Either 1st or 2nd week is the best time for single on-line adaptation. Off-line adaptation does not significantly improve target coverage.

1%

2%

3%