ESTRO 36 Abstract Book

S426

ESTRO 36 2017

_______________________________________________________________________________________________

dosimetric end-to-end procedures for protons based on

customized anthropomorphic phantoms and different

dosimetric techniques.

Material and Methods

A homogeneous polystyrene phantom and two

anthropomorphic phantoms (pelvis and head phantom)

have been customized to allocate different detectors such

as radiochromic films, ionization chambers and alanine

pellets. During testing, the phantoms were moving

through the workflow as real patients to simulate the

entire clinical procedure. The CT scans were acquired with

pre-defined scan protocols used at MA for cranial and

pelvic treatments. All treatment planning steps were

performed with RayStation v5.0.2 treatment planning

system (TPS). A physical dose of 10 Gy was planned to

clinically shaped target volumes in order to achieve

uniformity better than 0.5% on the dose delivered to the

alanine pellets. In the treatment room the plans were

delivered to the phantoms loaded either with alanine

pellets and radiochromic EBT3 films (figure 1) or two

Farmer chambers. The alanine pellets (5.0 mm diameter

and 2.3 mm thickness) and their read-out were provided

by the National Physical Laboratory (NPL). One of the

challenges of alanine for dosimetry in particle beams is

the known response dependency (quenching) on the

charge, the fluence and the energy of the particles

constituting the mixed radiation field. Corrections for this

were derived by a Monte Carlo dose calculation platform

implemented in a non-clinical version of RayStation.

Results

The measured absolute dose to water obtained with the

Farmer chamber in all delivered plans was within 2% of the

TPS calculated dose. A lateral 2D homogeneity of 3% inside

the treatment field was measured with EBT films. Doses

determined with the alanine pellets after correction for

the quenching effect showed a mean deviation within 3%

and a maximum deviation below 7% in the homogeneous

and anthropomorphic phantoms.

Conclusion

The end-to-end test procedures developed at MedAustron

showed that the entire chain of radiation treatment works

efficiently and with accurate dosimetric results. Our

experience shows that alanine pellets are suitable

detectors for dosimetry audits and developed procedures

can be used to support implementation of scanning beam

delivery technology in clinical practice

PO-0807 Practical advantages of a transmission

chamber in relative dosimetry of Brainlab conical

applicators

B. Tang

, J. Li

, S. Kang

, P. WANG

, L.C. Orlandini

Sichuan Cancer Hospital, Radiation Oncology, Chengdu,

China

Purpose or Objective

The commissioning of a radiosurgery unit requires the

acquisition of specific detectors able to characterize the

geometry and dosimetry of small fields. The acquisition of

the equipment for absolute dosimetry remains the priority

for the Hospitals, considering that relative measurements

can be performed without a reference chamber using long

acquisition time. The commissioning results therefore in a

tedious procedure. In this study, a new transmission

chamber (Stealth Chamber, IBA Dosimetry) was used as a

reference chamber (RC) in relative dosimetry of Brainlab

cone applicators. The timing of the practical procedure

and dosimetry results with and without the reference

chamber, will be analyzed and compared.

Material and Methods

IBA SFD3G diode detector was used to measure the 6MV

photon beam of a Varian Novalis used with Brainlab cone

applicators. Inline and crossline profiles at different

depths and central axis depths doses (PDDs) were

measured with a motorized water phantom (Blue

phantom, IBA Dosimetry) and OmniPro v7.4 software for

every cone. The measurements were acquired with the

transmission reference chamber positioned on the gantry

head in a continuous mode and without RC in a step by

step mode. The details of the acquisition parameters were

reported in Table 1. The total measurements time for each

procedure was registered.

Table 1 The Acquistion parameters with and without

stealth chamber

Acquisition

parameters/method

reference

Stealth

Chamber

Scan mode

step by step continuous

Scan speed

5 mm/s

In-scan positioning speed 5 mm/s

Positioning speed

10 mm/s

Acquisition time

5 s

Stabilzation time

1 s

0.08 s

Results

Profiles at depth 10 cm for 4/15 mm diameter cones and

the depth doses acquired with the two procedures (Figure

1&Figure 2) shown a good agreement. The total

measurement time registered was 490 seconds for the

PDDs acquisition without RC and only 64 seconds when the

scan mode change from “step by step” to “continuous”

after stealth chamber was in place. The overall

measurement time for 4mm diameter was 575 s and 12 s

without and with RC respectively, 735 s and 17 s for the

15 mm diameter cone.

Figure 1

Figure 2