Rose Wekesa : Cancer Care Kenya, Nairobi

ELECTRONIC PORTAL IMAGING: Achieving accuracy and precision for external beam

radiation therapy

Image ComparisonDigitally Reconstructed Radiograph Portal Image

Radiotherapy goal

• The inherent goal of radiation therapy is to eradicate all the

cancer cells or palliate symptoms by delivering enough

doses to the tumour, while minimizing injury to normal

tissues

• This is normally described in terms of tumour control

probability (TCP) and normal tissue complication probability

(NTCP).

• Imaging for treatment planning and for verification of

treatment among other factors makes it possible to achieve

the radiotherapy goal.

• This presentation focuses on treatment verification by portal

imaging.

Suntharalingam N, Podgorsak EB & Hendry JH 2005. Basic radiobiology. IAEA. Vienna

TCP and NTCP Curves

Introduction

Radiation therapy is one of the safest and most effective ways

to treat cancer.

Errors do occur, but they are extremely rare, with a very small

fraction of treatments involving an incident that puts a patient

at risk of harm.

These errors can be avoided if we put certain measures in our

daily practice.

Sequential process of planning and delivering radiotherapy.

Purpose

• To evaluate patient set-up variations for oesophageal

cancers treated with External Beam Radiation Therapy

• Six anatomical landmarks were selected for comparison and

recorded; the X (Left-Right, L-R), Y(Superior-Inferior, S-I)

and Z (Anterior-Posterior, A-P) directions

• Data was used to set acceptable tolerance limits for portal

imaging of the oesophagus.

Methods and Materials• Fifty seven patients with locally advanced disease treated

with chemoradiation were randomly selected for this study over a period of two years.

• Patients were positioned using body tattoos.

• EPI's were performed prior to treatment and registered to the digitally reconstructed radiographs (DRR).

• DRRs were used to adjust patient setups before treatment delivery. A total of 163 EPI pairs were analyzed and errors calculated

• Orthogonal pair images were acquired, which were used to give displacement

• If a patient had a single field or two opposing fields, the accuracy of field placement was assessed in two dimensions only.

Treatment verification Process

PRE-TREATMENT: Acquire reference images

TREATMENT

Fraction 1 Acquire portal images

Fraction 1

Fraction 1

Acquire portal images

Acquire portal images

Online review of images against

reference

Continue with

current set up STOP: Revise

set up, re-image

Gross error?

Action level?

No Yes

Calculate the mean displacement

(MD) in all 3 axes

MD Action level?

Continue with

current set up Revise set up,

re-image X2 #s

No Yes

Fraction 4

Fraction 5

No imaging

No imaging

Portal Imaging

Portal Imaging

Calculate the mean displacement

(MD) in all 3 axes

Continue with

current set up STOP: Investigate

further

MD Action level? Yes No

Weekly Portal Imaging

2

3

1

Portal imaging by anatomical matching

• Isocenter variation in X and in Y directions were measured

on anterior (AP) portal images, whereas, in Z and Y direction

were measured on lateral portal images.

• After the anatomical matching was performed on the

treatment fields for an individual patient, variations were

recorded into a Microsoft Excel spreadsheet.

• The reported X, Y and Z displacement of isocenter between

simulation and treatment was applied into the appropriate

shifts along each body axis.

Radiotherapy Targeting!

• For treatment,

same position

as of simulation

is set

• Lasers,

immobilization &

tattoo's are used

to verify the correct position.

during Radiation therapy

Two questions

always emerge.

1. Are they

hitting the

right spot?

2. Am I getting

the right

dose?

Anatomical match structures used

RESULTS Displacement No of

Patients

Portal

image pairs

(Total=163)

Total

Mean

(Mpop)

mean (%)

Left-Right (L-R) 57 57 2.05 3.6

Supero- Inferior

(S-I)57 57 2.79 4.9

Antero-posterior

(A-P)57 49 3.09 6.3

DISCUSSION:• The systematic and random errors for EPI’s were;

2.05mm in L-R, 2.79mm in S-I and 3.09mm in A-P

directions.

• The population-based mean variation is less than 5 mm, thus the

set-up provides sufficient targeting for all of the patients.

• Consistent with recommendations by the RCR, Institute of

Physics & Engineering in Medicine & Royal college of

radiographers (in line with ICRU Reports 50 & 62)

• Variations were more significant during the first sessions in

treatment than the weekly portals.

• This could be attributed to patient relaxing in the course of the

treatment and getting over the anxiety.

CONCLUSION

• Radiotherapy verification is a process that helps us

ascertain that we are treating tumor volume as planned.

• EPI results in different position correction for verification of

radiotherapy in all malignancies.

• When protocols are formulated & used at specified intervals,

patients can benefit in terms of treatment accuracy since

most of the set up errors if not all can be corrected.

• Portal imaging is therefore a must in every radiation

treatment.

Thanks for your attention!

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