ESTRO 36 Abstract Book

S967

ESTRO 36

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Conclusion

The ACDS is developing a comprehensive suite of audit

modalities aimed at ensuring patient safety across a range

of clinical practice. The new Level III (end-to-end) audit

joins the integrated range of multi-modality audits

provided by the ACDS including the Level Ib audit (on-site

linac output) covering reference beams, FFF and small

fields and the Level II audit (slab phantom combined with

array) audit covering conformal, IMRT, VMAT and FFF

treatments

plans.

EP-1756 Treatment planning and dosimetric validation

of bone oligomet SABR treatments on TomoTherapy

C. Thomas

, T. Burrows

, R. Lynn

, N. Milesi

, S. Petty

M. Stenson

, K. Blythe

, T. Greener

Guys and St Thomas NHS Foundation Trust, Medical

Physics, London, United Kingdom

Guys and St Thomas NHS Foundation Trust,

Radiotherapy Department, London, United Kingdom

Purpose or Objective

To establish whether the TomoTherapy helical delivery

system can accurately deliver high dose per fraction SABR

treatments to bone oligo-metastases within the NHS

England Commissioning through Evaluation SABR program.

Material and Methods

TomoTherapy Volo treatment planning system was used to

generate example SABR treatment doses of 10Gy and 15Gy

per fraction to a cylindrical PTV within a CT dataset of the

Delta-4 phantom. These treatments were delivered to the

Delta-4 phantom. Treatment plans for clinical oligo-

metastases in bone, with prescription doses of 27Gy/3#

and 30Gy/5# were generated and delivered to Delta-4

phantom, ionisation chamber and Gafchromic film.

Clinical treatment fractions were delivered in 2 half-

fractions in order to allow a mid-fraction imaging scan to

assess intra-fraction motion.

Results

Volo treatment planning system signalled when the

treatment planning objectives were not deliverable and

suggested modified treatment planning parameters. The

test cases measured on Delta-4 passed local gamma

analysis at 3%/3mm with >95% pass rate. Paddick and

CIRTOG conformity indices were improved with the use of

TomoTherapy compared with VMAT solution for the first

clinical case, and dose gradient between target and

critical structures was improved. For the first clinical case

measured on Delta-4, 100% of sampled detectors passed

within 3%/3mm gamma analysis and 98.5% passed within

2%/2mm.

Initial

transverse

EBT2

Gafchromic

measurement of the first clinical case showed satisfactory

qualitative agreement with treatment plan. Subsequent

EBT3 GAFchromic film measurements resulted in 97.5%

gamma passrate at 3%/3mm and mean dose deviation on

representative dose profiles of less than 2.2%. Average

intra-fraction motion between half fractions was 0.68mm

in X, 0.64mm in Y and 0.84mm in Z with standard deviation

of 0.62mm, 0.48mm and 0.79mm respectively.

Conclusion

GSTFT is the first centre with QA approval under the NHS

England CtE programme to treat bone oligometastatic

cases using the TomoTherapy treatment planning and

delivery system. Volo and Hi-Art systems are capable of

generating and accurately delivering homogeneous dose of

up to 15Gy per fraction in phantom studies. Clinically

approved treatment plans for bone oligomet cases

delivering up to 9Gy per fraction have been generated and

accurately delivered to diodes, ionisation chamber and

Gafchromic film. Intra-fraction motion was small and has

permitted the reduction of PTV margin from 4mm to 3mm.

EP-1757 QA of MLC Elekta Agility for Static fields

F. Tato de las Cuevas

Hosp. Univ. de Canarias, Medical Physics Dept., Santa

Cruz de Tenerife, Spain

Purpose or Objective

QA of MLC is one of the main points of any LINAC QA

program. Agility MLC (Elekta) have different properties

than most of the more common MLCs, like less interleaf

transmission. The objective is to perform the Agility MLC

QA in static mode using the electronic portal imaging

device (EPID) and make this process as fast and accurate

as possible.

Material and Methods

The LINAC is an Elekta Synergy with Agility MLC and 6 MV

photons. A software is developed in MATLAB with some

remarkable points:

Elekta iCOMCAT software was employed to

generate and send the strip-test with multiple

segments as a unique treatment, as is much

faster than creating and irradiating a beam for

each segment. With the software of Elekta

iView is difficult to acquire a complete image

of each full segment as this is not fast enough,

so fluency corrections of these segments were

performed, in order to avoid erroneous pixel

values (PV) in the way: a) In a 23x23 open field

is acquired a horizontal profile and measure

the % PV (in the center position of each future

segment), this % is related to the PV of the

position of a reference segment. b) Measure

the mean PV in the center of each strip-test

segment, and obtain the % PV related to the

reference segment PV. c) Rescale the image of

each segment in order to obtain the % PV

(respect the reference segment). Finally make

the sum of all images.

Segments of 2 x 20 cm (cross-plane x in-plane)

to form series of strip-test images with gaps

overlapping from 1.2 to 3 mm are acquired for

taking the MLC reference after calibration. The

strip-test need bigger gap spread than other

MLC in order to detect the gap position

correctly, because of the lower penumbra.

To correct the collimator angle is used the

filtered back projection method, because is very

tricky to use the interleaf leakage, as this MLC

have much lower interleaf transmission than

other MLC, like Millenium (from Varian).

To localize the radiation center (RC) of the EPID

is used a LINAC tray with centered radiopaque

mark. Four 20x20 fields are obtained with this

tray at 4 collimator angles. RC is determined for

gantry 0º detecting the mark position in each

image and obtaining the mean. A vector

displacement is created to obtain RC with one

image at 0º collimator. Tray images for various

gantry angles at 0º collimator are acquired, so

that with just one tray image is enough to detect

RC exactly. This method is faster than using

field edges, where at least 2 images at different

collimator angles must be acquired for each

gantry angle.

Measurements of leaf positions using light projection are

made. Also are obtained strip-test with films and analyzed

with

RIT

software.