geometric verification procedures

Geometric verification procedures.

Contents

Page

1. Introduction 3

2. Documentation 3

3. Equipment 3

4. Imaging Schedules 3

4.1 Conformal verification flowchart (not including supine pelvis) 44.2 Supine pelvis verification flowchart 54.3 Weekly verification flowchart 6

5. Acquisition 7

6. Assessment of portal images 7

6.1 Gross error assessment 76.2 Offline portal image assessment 8

7. Site specific verification

7.1 Verification specific to Brain and CNS 97.2 Verification specific to Head and Neck 117.3 Verification specific to Thorax and Mediastinum 137.4 Verification specific to Breast 157.5 Verification specific to Pelvis 177.6 Verification specific to Spine 197.7 Verification specific to Limbs 21

8. Geometry of displacements for orthogonal portal images 23

9. Geometry of displacements for non-orthogonal portal images 24

9.1 The Displacement Calculator 27

10. Geometry of displacements for single, oblique portal images 28

11. Couch height setting 29

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Geometric Verification procedures1. Introduction

Portal imaging is the process by which we can be certain that we are treating the volume that has been planned; portal imaging is concerned only with the geometric verification of the plan and not the dosimetric verification. The aim of portal image verification is to ensure the geometric accuracy of radiotherapy delivery, that it is within the accepted tolerances for that particular technique or plan (RAC09-01), that systematic deviations are accounted for and that random deviations are kept to a minimum, usually by the efficient use of immobilisation devices. Systematic deviations are those which occur in the same direction and of similar magnitude throughout a course of treatment whereas random errors can occur in a variety of directions in a variety of magnitudes throughout a course of treatment. Random errors are usually minimised by good use of immobilisation devices and an accurate, reproducible patient setup. On each fraction that portal images are acquired, the resulting deviations arising from verification will contain the components of both systematic and random errors. By assessing portal images over a range of days, repeating the acquisition regularly and averaging our results, we can correct for these composite errors more accurately.

In order for portal imaging to be an effective system, we need to ensure that we have a plan and treatment setup appropriate to the technique and patient and that the reference images and portal images are of good quality and show sufficient anatomy to allow accurate verification.

2. Documentation

RAC07-0 Guidelines for Portal imaging at QEH Birmingham.RAC07-1 Guidelines concerning the use of Theraview. RAC09-01 Action levels in portal image assessment.RAC15.0 Portal imaging requirements radiotherapy external beam treatment. RAF16-1 Regular imaging form. RAF16-2 Portal Imaging Daily Results. RAF16-3 Breast Verification form. RAF16-4 Portal Imaging Standard Results form. RAG15-2 Guidelines concerning the use of Portal vision. RAGGTTM Radiographers guidelines for the use of IVIEW GT. RAP20 Portal Imaging Departmental Protocol.RAW PI CHART Portal imaging for radical treatments - flow chart. RAW PI PALL Palliative portal imaging flow chart. RAW09-01 Using the portal imaging displacement calculator. RAW09-04 Checking an EPID via Pipspro.RAW11-3 Pelvic conformal therapy imaging protocol.

3. Equipment

Within this department, images acquisition and online assessment for gross errors is performed using Varian Portal Vision on Rooms 1 and 11, Elekta Theraview on Room 9 and Elekta iView GT on Rooms 2, 5, 7 and 10. Room 5 also has the Synergy XVI platform for verification using cone beam CT.

4. Imaging schedules

Within this department, the main imaging schedules in use are the Conformal Verification schedule (4.1), the Supine Verification Schedule (4.2) and the Weekly Verification Schedule (4.3). Each schedule demonstrates the process involved including acquisition, online review, offline review, corrective action and regular review.

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4.1 Conformal verification flowchart(not including supine pelvis)

No

Yes

Acquire portal image


Review online for gross errors

WithinTolerance?

STOP, Investigate,

Revise setup Re-image

Continue

Fraction 1

Fraction 2

Yes

No

No

Yes

No

Yes

Calculate mean displacements in

lateral and longitudinal axis

Displacement 2mm or above?

Adjust setup





WithinTolerance?

Repeat portal image

WithinTolerance

?

Recalculate setup

Fraction 3

Fraction 8

Fraction 14

Weekly Continue

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4.2 Supine Pelvis verification flowchart

Yes

No

No

Yes

No

Yes

No

Yes




WithinTolerance?

STOP, Investigate,


Continue

Fraction 1

Fraction 2

Calculate and set Ideal Couch

Height (page 29)

Calculate mean displacements in

lateral and longitudinal axis

Displacements 2mm or above?

Adjust setup





WithinTolerance?

Repeat portal image

WithinTolerance

?

Recalculate setup

Fraction 3

Fraction 8

Fraction 14

Weekly Continue

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4.3 Weekly verification flowchart

Yes

No

No

Yes

No

Yes



WithinTolerance?

STOP, Investigate,


Continue

Fraction 1

Offline portal image

assessment

Acquire portal

imagesweekly

WithinTolerance?

Repeat portal image

WithinTolerance

?

Recalculate setupContinue

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5. Acquisition

Guidelines concerning the use of Portal Vision (RAC07-0), Theraview (RAC07-01) and iView GT (RAGGTTM) can be found in the Quality Assurance manual on each relevant treatment unit.

The Portal Imaging Requirements document (RAC15-0) contains information relating to the frequency of imaging for each treatment site, whether double exposure portal images are required and any variations to the standard technique.

In all cases, in order to reduce any concomitant imaging dose, treatment beams are used for verification, not orthogonal images. Double exposure portal images are only used on the occasions where there is insufficient anatomical reference within the treatment field to ensure accurate verification.

Image acquisition takes place at the start of each treatment beam and reviewed for online review for gross errors before the remainder of that beam is completed. Each set of images is sent to the Tesla server from where they can be retrieved by Portal Imaging staff for offline review.

6. Assessment of portal images

Due to the poor contrast involved when imaging using a megavoltage beam, the visualisation of the target volume itself is often impossible. Portal image verification relies on the use of ‘surrogates’ such as bony landmarks within the treatment area or radio-opaque markers in order to ascertain the accuracy of field placement. It is therefore important to ensure that the surrogates used during each image registration are appropriate to the position of the target volume and not simply the most visible structures on the image, regardless of position.

6.1 Gross error assessment

A gross error is an unacceptably large setup error that could underdose part of the target volume or overdose an area outside of the target volume. These are errors which are too large to be accounted for by the margin between the clinical target volume and planning target volume.

Gross errors need to be investigated, corrected and re-imaged before any treatment can commence.

Gross error review begins with the treatment setup, noting the orientation and position of each field on the patient’s skin in relation to whole body images from the Prosoma printout in addition to recording and monitoring the FSD (focus to skin distance) for each field.

Portal image gross error review can be performed simply on the treatment unit by comparison of the positions of bony landmarks or radio-opaque markers on the reference DRR (digitally reconstructed radiograph) and on the portal image, noting their relative positions to field edges, individual MLC (multileaf collimators) blades or shielding. Examples of stable bony landmarks can be found on the site specific pages later in this document.

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6.2 Offline portal image assessment

Offline portal image assessment is performed by a team of portal imaging staff remotely from the treatment rooms using PipsPro image registration software to give quantitative displacements (RAW09-04) for the purpose of calculating corrective action. It is important to ensure that the anatomical features (surrogates) used during each image registration are appropriate to the position of the target volume and not simply the most visible structures on the image, regardless of position. It is therefore essential that the person performing the image registration has gathered all relevant information regarding the treatment and has an understanding of where the target volume is situated in relation to neighbouring anatomical landmarks.

Examples of stable, bony landmarks can be found on the following site specific pages.

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7. Site specific verification

7.1 Verification specific to Brain and CNS

The imaging equipment and methods used.

Portal Vision on Rooms 1 and 11iView on Rooms 2, 5, 7 and 10Theraview on Room 9

The treatment techniques, treatment equipment and immobilisation devices which that particular part of the protocol applies to.

Usual technique for brain is 3 fields of lateral (or lateral oblique) fields with a superior oblique field. May also incorporate boost fields.

Usual technique for whole CNS in with lateral fields to the cranium and matched spinal fields. A ‘moving junction’ technique applies.

Immobilisation with supine or prone S-frame BDS, standard or customised head curves, shims, knee support or ankle support.

The frequency and timing of imaging. (Geometric Verification should be carried out pre-treatment and within the treatment course.)

On the first fraction of each phase, to be repeated weekly.

Recommended anatomical reference points.

Vertebrae, Hard Palate, Base of skull (See diagram)

Tolerances, action levels and corrective strategies, including re-imaging, who to inform and what action to take.

0.4cm tolerance.

If out of tolerance or close to tolerance level, repeat. If still out of tolerance or close to tolerance level, proceed with corrective action based upon mean measurements.

Measurement of set up errors.-Gross error. -Systematic and random error.

Gross error assessment to be performed online by band 6 radiographer or above prior to the commencement of treatment.

Offline assessment to be carried out following treatment in the Portal Imaging Suite by trained radiographers of Band 6 or above to determine systematic errors, random errors and calculate corrective action.

For prone BDS, once treatment position is verified as within tolerance, the couch parameters are ‘locked down’ to a 0.5cm tolerance (RAW12-1).

The seniority and/or the professional discipline of the personnel who should be involved.

Verification to be carried out by fully trained radiographers of band 6 or above.

The specific responsibility and accountability of the personnel for the specific parts of the protocol.

To be performed only by band 6 radiographers or above who have received training in portal image assessment and had their competencies signed off.

How the working practice of the department prevents the check being omitted.

All portal image scheduling is performed by the band 6 or 7 radiographer in charge of the treatment room as part of the initial checks for all new patients. Scheduling is checked by a member of the portal imaging staff.

How the check achieves an active rather than a passive response.

By correction of all significant systematic errors.

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Verification specific to Brain and CNS

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Frontal sinus

Ethmoid bone

Hard Palate

Skull Table(Occipital bone)

Surgical Pathology

Skin surface

Cervical spine

Frontal sinus

Skull table (Parietal bone)

Sphenoid bone

Ethmoid bone

7.2 Verification specific to Head and Neck




Usual technique is lateral (or lateral oblique) fields to the neck with an anterior neck / supraclavicular field.

Immobilisation with S-frame BDS, standard or customised head curves, shims, knee support; There is possible use of mouth-bites.


On the first fraction of each phase, to be repeated weekly.


Vertebrae, Hard Palate, Base of skull (See diagram)


0.4cm tolerance.





The seniority and/or the professional discipline of the personnel who should be involved.








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Verification specific to Head and Neck cancer

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Base of skull

Occiput

Posterior arch of C1

Hard Palate

Anterior arch of C1

Anterior aspect of vertebrae

7.3 Verification Specific to the Thorax and Mediastinum




Usual technique is 3 fields which may also incorporate boost fields.

Immobilisation with chest board, knee support and heel support.


On fractions 1, 2, 3, 8, 14 then repeated weekly. Adjustment for systematic errors made after fraction 2.


Rib head and facets, sternoclavicular joint, pedicles, spinous processes, intervertebral spaces, soft tissue (heart, mediastinal mass, tumour)


0.7cm tolerance.





Adjustment for systematic errors made after fraction 2.The seniority and/or the professional discipline of the personnel who should be involved.








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Verification Specific to the Thorax and Mediastinum

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Clavicle

Lateral extent of chest wall

Pedicles

Rib heads and facets

Anterior aspect of vertebral

body

Intervertebral space

Pedicles

Spinous process

Transverse process


Sternoclavicular joint

Rib heads and facets

Lateral extent ofChest wall

7.4 Verification specific to the Breast




Usual technique is 2 tangential fields which may also incorporate boost fields. For 3 and 4 field techniques, a supraclavicular field and post-axillary field may be included.

Double exposure portal image may be required to verify small supraclavicular fields.

Immobilisation with breast board, bum stop and heel support.The frequency and timing of imaging. (Geometric Verification should be carried out pre-treatment and within the treatment course.)

On first fraction.


For breast only; Axillary fold, skin surface, inframammary fold, anterior extent of chest wall.For supraclavicular fossa; proximal end of first rib, proximal end of clavicle, pedicles, spinous processes (if double exposure used).


0.5cm tolerance for all geometric deviations with a minimum of 1.2cm coverage anteriorly and inferiorly.













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Verification specific to the Breast

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Spinous Process

Pedicles

Proximal end of first rib

Proximal end of clavicle

Axillary fold

Skin surface

Inframammary fold

Anterior extent of

chest wall

7.5 Verification specific to the Pelvis




Usual supine technique is 3 or 4 fields which may also incorporate boost fields. Supine immobilisation with head scoop, knee support and heel support.

Usual prone technique is 3 fields. Prone immobilisation with prone pillow and reversed knee support under ankles.


For supine techniques, on fractions 1, 2, 3, 8, 14 then repeated weekly. Adjustment for systematic errors made after fraction 2.For prone techniques, on first fraction then repeated weekly.


Sacrum, coccyx, pelvic brim, ischial crests, obturator foramen, pubic bone, Symphysis pubis, Kohler’s teardrop, acetabulum, femoral head.


0.7cm tolerance for conformal techniques, 1.0cm for non-conformal techniques.













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Acetabulum

Fovea

Femoral Head

Kohler’s Teardrop

Superior ridge ofObturator Foramen

Symphysis Pubis

Sacrum and Coccyx

Pelvis Brim

Ischial crest

Obturator foramen

Pubic bone

Verification specific to the Pelvis

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Ant Sup IliacSpine

Femoral Head

Iliac Spine

Pubic Tubercle

Femoral Shaft

Sacrum

Acetabulum

Greater Sciatic Notch

Ischial Spine

Ischial Tuberosity

7.6 Verification specific to the Spine




Usual technique may involve a single applied field, parallel opposed fields or a plan of 3 or more fields.

Supine immobilisation with head scoop, knee support and heel support.Prone immobilisation with prone pillow and reversed knee support under ankles.


For simple techniques, on first fraction to be repeated weekly. For conformal techniques, on fractions 1, 2, 3, 8, 14 then repeated weekly with adjustment for systematic errors made after fraction 2.


Proximal facets of ribs, intervertebral spaces, spinous processes, transverse processes, pedicles.















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Verification specific to the Spine

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Pedicle

Transverse processes

Posterior facet of ribs

Anterior aspect of vertebrae


Proximal facet of ribs


Spinous process

Transverse process

Pedicle

7.7 Verification Specific to Limbs




Usual technique may involve oblique, tangential fields or a plan of 3 or more fields.

Supine immobilisation with head scoop and individual vacbag.Prone immobilisation with prone pillow and individual vacbag.


On the first fraction to be repeated weekly.


E.g. Femur (more commonly occurring); Femoral condyles, patella, shaft of femur, skin surface, surgical pathology.















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Verification Specific to Limbs

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Femoral Condyles

Patella

Distal shaft of femur

Skin surface

Shaft of femur

Patella

Femoral Condyles

Distal shaft of femur

Skin surface

Shaft of femur

8. Geometry of displacements for orthogonal portal images

Where portal image registration is performed for orthogonal treatment beams (i.e. the field arrangement contains fields which are at cardinal angles and 90o apart), any displacements which are measured during registration will be in absolute lateral, longitudinal and vertical directions.

For example, this pair of fields at gantry = 0o and gantry = 90o:

Gantry = 0o Gantry = 90o

In the example of orthogonal portal images, over the initial period of imaging as set out in Figures 1-3, the mean values of the displacement results, if above the action level, can be translated directly into corrective couch moves.

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270 90

0

Right

Inf

Left

Sup

Gntry0 Ant

Inf

Post

Sup

Gntry90

Mean value = 0.525 cm

Correction = 0.5 cm Inf

See page Ref! for Vertical corrections and

couch height setting.

9. Geometry of displacements for non-orthogonal portal images

Where portal image registration is performed for non-orthogonal treatment beams (i.e. the field arrangement contains fields which are not at cardinal angles), any displacements which are measured during registration will not be in absolute lateral, longitudinal and vertical directions and will therefore need transposing in order for them to be used for corrective purposes.

For example, this arrangement of fields at gantry = 0o, gantry = 206o and gantry = 290o

In the example of non-orthogonal portal images, over the initial period of imaging as set out in Figures 1-3, the mean values of the displacement results cannot be translated directly into corrective couch moves as, in the example of the two oblique fields, any displacements would not have absolute values.

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206

0

Right

Inf

Left

Sup

Gntry0

GantryDirection

Ant?Left?

Inf

RightPost?

Sup

Gntry290

GantryDirection

Post?Left?

Inf

Right Ant?

Sup

Gntry206

GantryDirection

290

206

290

0

Positive X value

Positive X value

Positive X value

Negative X value

Negative X value

Negative X value

Using the portal image registration software, PipsPro, all deviations following image registration are given in values of X and Y, as follows:

In the example of the three field arrangement above:

Y Offsets are either Superior or Inferior X Offsets are a combination of Left, Right, Ant & Post (e.g. Left Ant Oblique etc.) X Offsets need to be plotted out to give absolute values so that couch moves can be made

to compensate for any deviations.

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-X

+Y

+X

-Y

Offset

If using graph paper, a ruler and a protractor, the X value deviations can be plotted:

Using the graph paper and ruler, the absolute lateral and vertical deviations can be measured between the planned centre and treated centre.

Alternatively, the Displacement Calculator software can be used to calculate these results (RAW09-01).

0

206

290

Post

Ant

LeftRight

Planned Centre

Treated Centre

26

Select PipsProRegistration

filesClick to

Calculate

Select orientation

9.1 The Displacement Calculator

Data Entry

Results

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10. Geometry of displacements for single, oblique portal images

Where portal image registration is performed for single, oblique treatment beams (i.e. tangential breast fields), any displacements which are measured during registration will be in the longitudinal direction in addition to an anterior oblique or posterior oblique displacement. This oblique displacement, if above the action level (ref!) will need to be transposed into its lateral and vertical components in order to be used for corrective purposes.

For example, a two field tangential beam arrangement to the breast:

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Anterior Oblique displacement

from PipsPro (X)

Posterior Oblique displacement from

PipsPro (X)

Lateral component of the oblique displacement

Vertical component of the oblique displacement

Post Obl

Inf

Ant Obl

Sup

-X

+Y

+X

-Y

Orientation PipsPro displacement

Calculations for Left Breast

Left Medial Oblique field: = (360 – gantry angle)Left Lateral Oblique field: = (180 – gantry angle)

If displacement is Anterior Oblique, this is composed of: X Sin () Anterior and X Cos () LeftIf displacement is Posterior Oblique, this is composed of: X Sin () Posterior and X Cos () Right

Calculations for Right Breast

Right Medial Oblique field: = Gantry angleRight Lateral Oblique field: = (Gantry angle – 180)

If displacement is Anterior Oblique, this is composed of: X Sin () Anterior and X Cos () RightIf displacement is Posterior Oblique, this is composed of: X Sin () Posterior and X Cos () Left

These displacements can then be used directly as absolute couch shifts in the event that corrective action is necessary.

11. Couch height setting

Setting of the couch height is seen to be a more effective method of patient setup reproducibility in the anteroposterior plane (Th van Lin, ENJ. et al (2001), Effectiveness of couch height–based patient set-up and an off-line correction protocol in prostate cancer radiotherapy, International Journal of Radiation Oncology, Biology & Physics, Vol50, Issue 2, p.569-577)

For all supine planned volume treatments, corrections in the anteroposterior plane are performed before the third treatment fraction by setting a fixed couch height, using data from the first and second treatment fractions.

The historical treatment couch height (TCM) for the first two fractions can be retrieved from the RO Treat section of Mosaiq and from this, an ideal couch height (ICH) can be calculated i.e. a couch height which if set on each of the first two days would have resulted in zero displacement anteroposteriorly.

If the displacement (X) is Anterior, ICH (cm) = TCH (cm) + X (cm)

If the displacement (X) is Posterior, ICH (cm) = TCH (cm) - X (cm)

e.g. (1) Treatment couch height = -12.4 cm, displacement from PipsPro is 0.3cm Ant

Ideal couch height (ICH) = -12.4 + 0.3 = -12.1 cm

(2) Treatment couch height = -11.8 cm, displacement from PipsPro is 0.5cm Post

Ideal couch height (ICH) = -11.8 - 0.5 = -12.3 cm

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