ESTRO 36 Abstract Book

S944

ESTRO 36 2017

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feasibility of MRI-based atlas-based autosegmentation for

organs-at-risk (OAR) and prostate target volumes, and to

compare the segmentation accuracy with CT-based

autosegmentation.

Material and Methods

Images were retrospectively selected from 6 prostate

patients who received whole field T2 weighted 3D SPACE

MRI and CT in the radiotherapy treatment position (at the

Northern Centre for Cancer Care, Newcastle). Organs at

risk (Bladder, rectum, seminal vesicles, left and right hips,

penile bulb) and the prostate were manually delineated

on the CT and MRI separately. A ‘leave one out’ approach

was used to automatically segment structures onto the

remaining images separately for CT and MRI. Contour

comparison was performed using the DICE index and mean

distance to conformity (MDC) positional metrics. MDC,

DICE and absolute volume were used to assess the

performance of the contouring by comparing the

automatic to the manual contours. A paired t test was

used to determine the statistical significance between MRI

and CT.

Results

The volume analysis (data not presented) showed that

manual and automatic contouring on MRI gave smaller

contours than CT (significantly so for the hips, prostate

and seminal vesicles). The positional analysis results are

shown in table 1. MRI autocontouring was more accurate

than CT for the bladder (MDC significantly so) and the

prostate/penile bulb (although not significantly). There

was little difference in accuracy between CT/MRI

autocontouring for both hips, rectum and seminal vesicles.

Conclusion

Accurate atlas-based automatic segmentation of

structures for prostate radiotherapy is feasible using T1-

MRI; segmentation of the penile bulb and seminal vesicles

was found to be poor. Comparison with CT-based

automatic segmentation suggests that the process is

equally or more accurate using MRI. Although this study

was on a small sample size these results support further

translation of MRI-based segmentation methodology into

clinical practice.

EP-1740 Nationwide audit of multileaf collimators

performance

K. Chelminski

, W. Bulski

The Maria Sklodowska-Curie Memorial Cancer Center,

Medical Physics Department, Warsaw, Poland

Purpose or Objective

The delivery of accurate intensity-modulated radiation

therapy (IMRT) or stereotactic radiotherapy depends on a

multitude of steps in the treatment delivery process. The

proper intensity modulation depends on the proper

functioning of a multileaf collimators (MLC). The aim of

this audit was the control of the proper collimator leafs

positioning.

Material and Methods

The methodology of the audit of small field output

performance was established within the framework of the

CRP E2.40.16 project "Development of Quality Audits for

Radiotherapy Dosimetry for Complex Treatment

Techniques", run by the Health Section of the

International Atomic Energy Agency (IAEA). The

participants of the audit were obliged to irradiate

provided dosimetric films, in a slab phantom, for a specific

leaf arrangement, producing a pattern of five stripes,

commonly called a picket fence. The participants had to

programme such a pattern so that the stripes are 5 mm

wide and are 3 cm distant between themselves. The

Gafchromic EBT2 radiochromic films were placed in a slab

phantom close to maximum dose depth. The irradiation

was 250 MU per stripe.

Results

Thirty two Polish radiotherapy centres took part in the

audit. They were equipped with various accelerator types

and various treatment planning systems. In all cases the 6

MV quality beams were used. The discrepancies between

measured and expected stripe positions were in the range

1.2 mm. For particular participants, the leaf position

discrepancies were in the range -0,5 mm to 0,5 mm. For

particular participants, the mean opening width measured

with films for each pair of leafs was between 6 and 8 mm.

Conclusion

In the audit, the best performance showed the new type

multileaf collimators with 120-160 leafs, The worst

performance showed collimators MLC80 from Elekta. The

results of the audit are very useful for the participants

who should carefully investigate the performance of their

multileaf collimators.

EP-1741 Commissioning of a robotic patient positioning

system equipped with an integrated tracking system

A. Ableitinger

, A. Utz

, A. Zechner

, S. Vatnitsky

, M.

Stock

EBG MedAustron GmbH, Strahlentherapie, Wiener

Neustadt, Austria

Purpose or Objective

Robotic patient positioning systems (PPS) used in clinic

must consider weight-induced couch bending and must

show high reproducibility and stability to achieve the

required positioning accuracy. Extensive commissioning of

these robotic systems is therefore crucial. The aim of the

current work is to determine the positioning accuracy of

the PPS, that is equipped with an integrated optical

tracking system.

Material and Methods

Three different aspects of the PPS were investigated in

this study: the basic characteristics including couch

bending, reproducibility and stability; the relative spatial

deviation in terms of rotation and couch height and the

absolute accuracy of the treatment couch.

The treatment volume of the PPS has a dimension of

115cm x 50cm x 40cm. The robotic system enables couch

rotations of more than 190°, pitch and roll of ±3° and non-

isocentric treatment positions. A photogrammetric

camera tracks the treatment couch of the PPS via

reflecting markers mounted on the bottom side of the

couch (see Fig.1). An iterative position correction loop

aligns the couch to the prescribed position.

The reference instrument was a laser tracker with

reflecting probes. Drilling holes near the indexing

positions (H4-F9) located laterally along the couch every

14cm served as measurement positions for the evaluation

of the basic characteristics and the absolute couch

position. For the relative deviation the drift of one

measurement point on the couch was evaluated.

To determine the bending of the couch it was loaded with

six different weights up to 156kg. The reproducible

positioning for the same couch position (different axis

setting) and the couch stability after 1 hour were

measured with the highest payload.

The evaluation of the absolute spatial deviation was based

on six measurement points being closest to the room

isocenter. They were compared with their expected

coordinates for 1020 different robot positions, poses and

payloads.