ESTRO 36 Abstract Book

S418

ESTRO 36 2017

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Results

The measured absolute dose rates agreed with the values

quoted in the calibration certificates of the plaques within

the experimental uncertainty, with typical differences

below 5%. The relative standard uncertainties obtained

were of 3.8% for dose distributions measured at planes

perpendicular to the symmetry axis at 5 mm from the

surface of the plaque, and of 7.4% for planes containing

the symmetry axis. These values are comparable to those

reported by other authors using plastic phantoms, but

avoiding the uncertainties associated to the conversion

from dose–to–plastic to dose–to–water. A good agreement

was obtained between measurements and simulations,

improving upon published data (see figures for data of

depth-dose curves, and lateral profiles at 5 mm from the

surface of the plaque, for the CCX plaques).

Conclusion

We developed a practical experimental method to

measure with the EBT3 radiochromic film the dose

distributions in water produced by

106

Ru/

106

Rh ophthalmic

plaques. The obtained results were of similar or better

quality than those obtained using solid phantoms. These

setups may ease the quality assurance procedures to the

users of these plaques.

PO-0794 Comprehensive quality assurance test for high

precision teletherapy

S. Wegener

, A. Spiering

, O.A. Sauer

University Hospital, Radiation Oncology, Würzburg,

Germany

Purpose or Objective

Modern radiation therapy aims to minimize negative side

effects on healthy tissue by tailoring the dose distribution

as accurately as possible to each individual tumor. This

leads to a progressively increasing complexity of the

treatment plans and demands a very high precision of all

involved components. Even small errors can significantly

compromise treatment techniques which require such an

extensive precision as stereotactic radiation therapy. A

suitable quality management for such techniques should

include a regular end-to-end test that closely mimics the

entire procedure of the actual patient treatment while

being able to reliably detect a variety of possible errors

e.g. in calculation, positioning and movement, spatial

precision and absolute dose application. We present a test

that was introduced into the clinical workflow and

evaluated its sensitivity to those errors.

Material and Methods

Prior to the irradiation, a custom-built phantom insert for

the ArcCHECK (Sun Nuclear, USA) allowed for automatic

registration of the cone beam CT to reference data. A 12-

field plan including gantry and table rotations targeting a

spherical volume of approx. 2 cm diameter was measured

weekly using a Synergy accelerator with an Agility MLC

(Elekta, Sweden). Signals were obtained from all diodes

along the cylinder surface of the ArcCHECK and additional

dose was measured with an ionization chamber in the

phantom center. For each measurement the plan was

compared to the calculation of the treatment planning

system via gamma evaluation and every diode reading was

compared to the averaged diode readings from previous

weeks. Additionally, errors were induced to test the

sensitivity for phantom malposition, machine geometry

problems and MLC positional inaccuracies.

Results

Due to the phantom set up according to the cone beam CT

registration, the measurements were very reproducible

without any observable user-to-user differences. The

typical dose map for the diode cylinder is shown in fig. 1.

For all diodes, mean values with small standard deviations

were obtained from many consecutive measurements. Any

diode deviation observed for the correct application of the

test plan never exceeded three standard deviations, while

much larger discrepancies could be detected for all

induced errors (example: fig. 2).