ESTRO 36 Abstract Book

S885

ESTRO 36 2017

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The number of patients eligible for inclusion was 18 (3 was

rejected because of missing (2) or faint (1) clips). All

patients received daily IGRT with CBCT using XVI from

Elekta. CBCT’s for the first ten fractions were included

resulting in 180 CBCT. Two matches were performed for

each patient: First, a chest wall-match were performed,

where CBCT images were registered automatically to the

planning CT using a grey value translational match of the

thorax wall. Secondly, a boost specific match was

performed, where the surgical clips were manually

registered to their position on the planning CT.

Results

The systematic (Σ) and random errors (σ) between the

thorax wall and the clips based boost matches are seen in

the table. The CTV-PTV margins were calculated based on

the systematic and random errors using the Van Herk

margin recipe. These numbers are found both using no re-

matching and a re-matching if there were more than 5 mm

differences between the two matches. In the figure the

percentage of rematches needed for a given threshold of

allowed difference between the thorax wall and boost

match is shown in a). The boost CTV to PTV margin

required to account for the difference between chest wall

and boost match for a given threshold is shown in b). The

grey shaded area shows the 90 percent confidence interval

obtained by bootstrapping.

Conclusion

The presented protocol can reduce the required CTV to

PTV margin for the boost region by re-scanning and re-

matching the boost region only for patients where the two

regions differ by more than a set threshold. Results are

presented that can be used for selecting a threshold with

the corresponding required CTV-PTV margin. If e.g. a

threshold of 5 mm is used, the required CTV-PTV margin

can be reduced from approximately 8mm to 5mm and re-

scanning and re-matching will be required in only 5% of

the fractions.

EP-1653 PolymarkTM fiducial markers migration in

Prostate Image Guided Radiation Therapy using CBCT

images

C. Camacho

, I. Valduvieco

, J. Sáez

, A. Herreros

, J.

García-Miguel

, E. Agustí

, C. Castro

Hospital Clinic i Provincial, Radiotherapy, Barcelona,

Spain

Purpose or Objective

Polymer-based fiducial markers, FM (Polymark

) location

was analyzed to test the idea that there is no intraseed

migration within the prostate, which is fundamental for

patient set-up good quality overthe entire course of

radiotherapy treatment (RT).

Material and Methods

Six prostate cancer patients with transperineal placement

of 3mm-long 1mm-diameter polymer-based fiducials

underwent 3mm slice thickness plan CT scan on day 14

after markers implantation, which is consider as a safe

waiting time according to the literature.

All patients were managed with the same IGRT protocol:

before each daily treatment, two planar KV images were

acquired with the OBI 1.4 system (Varian Medical Systems)

at 45º and 315º. A manual marker match between the KV

images and the planning CT DRRs was performed and

automatically transfered to the treatment couch position

to correct the patient position in the three translational

directions (rotations were not taken into account).

Weekly, after patient re-position and just before session

delivery, a CBCT scan is acquired, that is used to assess

rectum and bladder filling (slice thickness between 1mm

and 3mm)

These CBCT images, as being acquired in patient corrected

position, have been used to evaluate the FM locations at

different times during the course of treatment. A total of

37 CBCT images have been analysed to reconstruct the FM

3D coordinates. The displacement of each FM was

calculated relative to its reference position on the

reference planning CT, and also shift of the middlepoint

of each 3 FM set. The distance between markers in each

set at the time of planning CT and during specific

evaluated treatment have also been computed.

Results

The average marker migration observed is 0.68±0.51 mm

(range between 0 – 3.90 mm). This observation seems

independent of the marker position inside the prostate,

but not of the spatial coordinate: the antero-posterior

direction presents the largest FM average displacement.

Although the average migration observed is low, there are

cases among the six patients where the migration

observed an specific day was greater than 2mm. This

observation may be directly related to the degree of

prostate desplacement caused by the influence of the

rectum and bladder, and also with the posible pelvic

rotation in the moment of daily RT (not corrected with the

2D DRR vs KV image comparison).

Changes in distance between pairs of FM in each set have

been, on average, 0.12±0.11 mm (range between 0.02 –

4.38 mm).

Conclusion

The low average FM migration observed is expectable,

according to the waiting time between marker

implantation and the planning CT scan procedure. A futher

investigation should be done in order to reduce this

waiting time.

The fact of having observed cases among all patient with

displacement greater than 2 mm should be taken into

account in the CTV-PTV margins: an adequate expansión

of margins might compensates for this set-up uncertainty.