Title: Portal Imaging in Radiotherapy: A Comprehensive Exploration of Techniques, Applications, and Advancements Introduction Portal imaging is a critical component of modern radiotherapy, playing a pivotal role in the verification and precision of radiation treatment delivery. This technique involves the acquisition of X-ray images during or immediately after a patient's radiotherapy session, providing valuable information on the alignment of the treatment field with the intended target and surrounding critical structures. In this comprehensive exploration, we delve into the principles, techniques, clinical applications, challenges, and future prospects of portal imaging in the context of radiotherapy. 1. Principles of Portal Imaging Portal imaging is rooted in the principles of verifying and ensuring the accuracy of radiation therapy delivery. Before each treatment fraction, the patient's position is verified to ensure it aligns precisely with the treatment plan. Portal images are acquired using specialized imaging devices, usually in the form of electronic portal imaging devices (EPIDs) or film-based systems. These images serve as a real-time snapshot of the radiation field, allowing clinicians to assess the actual treatment setup against the planned position. 2. Techniques of Portal Imaging 2.1 Electronic Portal Imaging Devices (EPIDs) Electronic portal imaging devices, or EPIDs, have become a standard tool in portal imaging due to their real-time imaging capabilities and digital nature. EPIDs consist of a detector panel that captures the transmitted radiation through the patient during treatment. The resulting electronic images are immediately available for review, facilitating prompt decision-making regarding the need for adjustments in patient positioning or treatment parameters. 2.2 Film-Based Portal Imaging Film-based portal imaging, while less commonly used today, has historical significance and is still employed in certain clinical settings. It involves exposing X-ray film positioned behind the patient during treatment. The film is then developed, and the resulting image is analyzed to verify the alignment of the treatment field. Though the process is not as immediate as with EPIDs, film-based systems may still offer advantages in certain situations. 3. Clinical Applications of Portal Imaging Portal imaging is integral to the success of radiotherapy across various cancer types and treatment modalities. 3.1 Treatment Verification and Positioning The primary application of portal imaging is to verify the accuracy of patient positioning and the alignment of the treatment field with the intended target volume. Any discrepancies detected through portal images allow for immediate adjustments to be made, ensuring that the radiation is delivered precisely to the targeted area while minimizing exposure to adjacent healthy tissues. 3.2 Tumor Localization and Changes in Anatomy Portal imaging aids in localizing tumors, particularly

1. PORTAL IMAGING

2. Goal of Radiation therapy
Deliver a prescribed radiation dose
to the target volume accurately
while sparing the surrounding
normal and critical structures

3. goal of radiotherapy
High-precision RT
try to spread these
curves apart

4. • Small changes in the doses of 7 % to
15 % can reduce tumour control
significantly or increase rate of normal
tissue complications
• ICRU 50 Recommendations
Accuracy in dose delivery +/- 5 %
Only if field placement is precise during
entire course

5. • Compromise in Geometric accuracy =
geographical mess
• Discrepancies occur frequently esp.
complicated setups
• Influence outcome of treatment
• Reduced by frequent monitoring of patient
positioning
• Several studies suggest patient position to
be checked daily

6. Trends in radiation oncology
• Steep dose gradients;
90%-30% over 3mm
• Typically narrow margins
around GTV

7. • Efforts to develop more convenient
methods to image the patient during
radiation treatment ,the process known as
portal imaging
• Aim : verify that radiation portal delivered
is same as that prescribed ( by imaging
the portal during radiation)

11. Physics
• Contrast :result of differences in x- ray
attenuation ;describes how much a object
stands out from its surroundings
• Photoelectric effect dominates at lower
energies which is proportional to the
atomic number
• at higher energies Compton effect
predominates which is dependent on the
the electron density

12. Signal to noise ratio
Detectability of object depends not only on how large the
difference in attenuation is b/n the object and the
surroundings
But also on how large this signal is compared to the
uncertainty in the signal
Quantum efficiency
Measure of how efficient the imaging system is at
transferring the information contained in the radiation
beam incident upon the detector
Spatial resolution
Measure of how the image signal is blurred by the imaging
system

13. Problems with a port film
• Time consuming
• Labor intensive
• Reduce throughput in a busy department
• Quantitative interpretation of geometric
discrepancies is difficult and tedious to
perform

14. Electronic portal imaging devices
• Since 1980’s
• Two categories
• Scanning systems :radiation detector
subtends only a small fraction of the
radiation beam
• Area systems: entire area

15. Matrix ion chamber
• Two sets of electrodes (256 wires each)
that are oriented perpendicular (matrix) to
each other separated by a 0.8 mm gap
which is filled with a fluid (2,2,4-trimetyl
pentane ; a.k.a liquid on chamber )which is
ionized when the device is irradiated

16. Adv
Compact size
Geometric reliability
Less time 5.5 s to
read out an image
Disadv
Only 1 electrode active at
a time
higher doses required to
generate images
sensitivity of each
chamber varies -
spurious signals
Susceptible to artifacts -
dose rate changes

17. Camera based EPID’s
• Metal plate and a phospur (gd2O2S)
screen viewed b a camera using a 45
degree mirror
• Major limitation :
light collecting
efficiency
of the system
reduces image
quality

20. Commissioning and QA
Mechanical stability
Electrical stability
Calibration
Dose control
Image quality
Software analysis

21. Clinical application
Primary application
• Verification of patient set up
• Assessment of target and organ motion

22. EPID IMPLEMENTATION
• Certain specific goals and protocols for the
use of EPIDs must be established before
they are succesfully brought into the clinic
• Questions to be discussed before
implementation ??????

23. WHAT IS THE PUPOSE /AIM
OF INSTALLING EPID IN THE
CLINIC ?
• simple film replacement/routine QA
• Accurate and efficient patient set up and
repositioning
• Asses random and systematic errors in
treatment
• Inter and intra fraction motion studies

24. • Inter fraction errors
• Occur when two radiation treatments are
compared
• Intra fraction errors
• Occur in one radiation treatment

25. Systematic () and Random Errors ()
0
Displacement over days
Pt. 1 2 3 4
0
Mean displacement
Systematic error
for patient #1
Pt. 1 2 3 4
Random errors
for patient #1
0
Subtract daily displacements
from mean displacement
Pt. 1 2 3 4
0
Pt.1 2 3 4
Compute SD of means of
different patients
Population ()
systematic error
0
Pt. 1 2 3 4
Compute SD of random
errors
Population ()
random error
-5mm
+5mm
0mm
Pt. 1 2 3 4
Displacement, 1st day

26. • What is the frequency of imaging ?
• Weekly
• Daily
• Depending on site or patient
• Dependent on the statistics of set p error
or decision rules

27. • What image acquisition modes are
available on the EPID ?
• Single exposure
• Multiple exposure
• Movie loops

28. • Single exposure :Single image is applied for a
short period of time typically at the start of
treatment
• double exposure: one single exposure image
and one open film are combined using a
weighted sum to produce a single image
• Movie loops :continuous images or online
fluroscopy

29. • What is the choice of reference image ?
• DRR
• Conventional simualtion film
• First approved EPID image

30. • When will you intervene /adjust setup?
• Offline interfraction correction: where image data
are acquired and reviewed ,analysed and acted
on at later time
Eq.weekly portal filming using hard copy
• Online intrafraction correction acquiring image
data ,doing analysis and taking action during the
fraction the patient is treated

31. • What image analysis protocol will be used
?
• Visual inspection only
• Manual tools
• Semi automated
• Automated

32. • How will EPID system communicate with
existing systems (simulator, TPS,PACS)?
• Digital image communication (DI COM)
• Proprietary tools

33. • Which patients will EPID used on for
treatment verification ?
• All patients
• Special cases that are difficult to set up
• Specific disease site

34. Software tools
• Image acquisition
• Image enhancement tools
image can be manipulated to improve landmark
visibility and image interpretation
1.Contrast enhancement
2.Non linear mapping of pixel values within th
image based on redistributing intensity values to
normalize the shape of intensity histogram
3.High pass filter :convolution of a filter and image
to produce the feature enhanced image

35. Image registration tools
• Interactive registration :use of computer
graphics that allow users to overlay one
image on the top of another and
interactively translate ,rotate and scale the
overlaid image until the anatomic features
in both images coincide
• Most commonly used :line drawing
technique

44. Point pair registration
• Identify locations of common anatomical
landmarks on the simulator and portal images
• Co-ordinates on portal image can be
transformed until they best match co-ordinates
on simulator image
• Points / Curves
• Anatomical landmarks
• External fiducial markers
• Implanted fiducial markers

45. • Cross - correlation
• Hybrid registration /Chamfer matching
Involves registrating a drawing generated from a
simulator film with a cost function image
generated from portal image
The goodness of fit between the drawing and cost
function is assessed by determining the cost or
penalty for the drawing being at a specific
location with specific orientation and scale to the
cost function image

46. • 1.line drawing created from simulator film
• 2.portal film processed using a edge
detection algorithm to identify edges of
anatomical structures
• 3.extraction image generated
• 4.transformation of this yields cost function
image which is used to determine the best
match

47. EPID CLINICAL PROTOCOL
1.For each patient ,enter patient
Demographic data ,Field data and also
image acquisition single/double exposure
data
If EPID part of integrated information
system,data automatically input
2. With patient in treatment position and with
appropriate immobilizing device
EPID is put in imaging position

48. Patient and field selected and acquisition
parameters loaded
Image the patient with aprropriate monitor
units(4 + 4 MU for double exposure )
before the treatment
As protocol directs ,take a action which
may include doing nothing ,performing
online or offline correction
If EPID is a part of the information system
,recording ,storing and retrieving the
image may be simplified

49. Offline EPID use
1.Simple offline – to replace weekly port film, used
to generate a hard copy
Benefts –faster ,image enhancement
Error detection done manually
2.Monitoring type –shows cumulative effect of daily
FPE on course of radiotherapy in individual
patient
To determine time trends
Movie loops for target and normal organ motion

50. 3.Statistical models /decision rules
From the inaccuracies ,immobilization
techniques can be modified
Appropriate margin of PTV can be decided

51. Margin Recipes for generating PTV
Several recipes; the ‘Dutch’ recipe is most common
An assessment of the standard deviation of the systematic error (setup) and
the standard deviation of the random error (setup ) is required
• CTV to PTV margin may be given by 2 + 0.7 (Stroom )
or 2.5 + 0.7 (van Herk)
• These generally guarantee that there is a 90% probability that 99% of the
CTV will be encompassed by 95% isodose (assuming that the PTV is
encompassed by the 95% isodose!)
• This sort of margin expansion may not be always feasible in the context of
organs at risk, and so, require an element of judgment

52. Online EPID USE
• Allows reduction of both random and
sysytematic errors but doesn’t differentiate
between them
• Results of studies -50 % of initail fields are
judged in error and corrected
• Rate of correction is anatomical site
dependent and observer dependent
• Final offline analysis -15 % of setups still
in error by more than 5 mm

53. ADVANCED APPLICATIONS
1.TREATMENT QA
2.EXIT DOSIMETRY
3.ELECTRON BEAM DOSIMETRY