Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1

accelerator. The influence of the observed deviations on the dose distri-

bution was found to be negligible.

http://dx.doi.org/10.1016/j.ejmp.2016.01.235

A.232

A NATIONAL MULTI-INSTITUTIONAL STUDY TO ASSESS DOSIMETRIC

CONSISTENCY IN TREATMENT PLANNING OF PROSTATE STEREOTACTIC

BODY RADIATION THERAPY (SBRT)

E. Villagg

i a ,

R. Consorti

b ,

C. Marin

o c ,

G. Maggi

d ,

C. Carbonini

e ,

S. Clement

e f ,

M. Esposit

o g ,

M Malisan

h ,

T. Malatest

a i ,

G. Borzì

j ,

C. Olivier

o f ,

S. Russo

g ,

E. Bonanno

b ,

E. Moretti

h ,

P. Mancosu

d .

AUSL Piacenza, Piacenza, Italy

;

Ospedale San Filippo Neri, Rome, Italy

;

Humanitas C.C.O., Catania, Italy

;

IRCCS Humanitas, Milano, Italy

;

A.O. Ospedale Niguarda Cà Granda, Milan,

Italy

;

IRCCS CROB, Rionero in Vulture, Italy

;

Azienda Sanitaria Firenze,

Florence, Italy

;

Azienda Ospedaliera Universitaria, Udine, Italy

;

Isola Tiberina,

Rome, Italy

;

REM, Catania, Italy

Introduction:

The Italian Association of Medical Physics (AIFM) created a

working group to assess planning homogeneity between different tech-

nologies and among different Italian centers to compare and evaluate SBRT

plans for prostate cancer. The purpose was to compare inter-institutional

variations in clinically acceptable dose distributions with variation in the

treatment planning and delivery apparatus, to assess dosimetric consis-

tency among different hospitals.

Materials and methods:

Fourteen institutes participated and received five

CT series with the corresponding RT-DICOM structures of five patients with

prostate cancer. Each participant planning guidelines included prescribed

dose (35 Gy in 5 fractions), specific constraints to rectum, bladder and

femoral heads

[1] .

The DVHs were centrally reviewed. Furthermore, plan-

ners were asked to replan the most critical case by means of additional dose

constraints, aiming to reduce standard deviations of doses and volumes re-

garding both PTV and OARs.

Discussion:

The following dosimetric metrics were quantitatively evalu-

ated: PTV coverage, dose volume parameters of interest for OARs, conformity

index and homogeneity index (HI) for PTV and CTV. Although the a priori

treatment planning constraints weremet in all cases, large inter-institutional

variations inmean doses to OARwere present. Replanning kept results more

consistent, reducing the variability of calculated DVHs for both PTV and OARs

[2] .

Conclusion:

Sharing data from planning on same data CT could improve

homogeneity between centers before multicenter clinical trials. This study

highlights the need for guidelines for treatment planning constraints to

ensure homogeneity in technique and practice. Precise and feasible dosi-

metric parameters can improve dosimetric consistency between different

centers and potentially improve the clinical evidence of multicenter studies.

References

[1]

Marino C, Villaggi E, Maggi G, Esposito M, Strigari L, Bonanno E, et al. Strahlenther Onkol 2015;191(7):573–81.

[2]

King CR, Freeman D, Kaplan I, Fuller D, Bolzicco G, Collins S, et al. Radiother Oncol 2013;109(2):217–21. http://dx.doi.org/10.1016/j.ejmp.2016.01.236

A.233

3D EVALUATION OF PRE-TREATMENT AND IN-VIVO EPID-BASED

DOSIMETRY IN VMAT PROSTATE TREATMENTS: INITIAL EXPERIENCE

WITH A COMMERCIAL SOFTWARE

E. Villaggi

AUSL Piacenza, Piacenza, Italy

Introduction:

The purpose of this study is to investigate the feasibility of

using electronic portal imaging device (EPID) and a commercial software

for 3D in-vivo transit dosimetric verification of volumetric modulated arc

therapy (VMAT) prostate treatments.

Materials and methods:

Twenty RapidArc prostate treatments are planned

by Eclipse with Acuros XB calculation algorithm (Varian Medical System)

and delivered with a Varian Clinac CD accelerator equipped with EPID

aS1000.

DC (dosimetry check, Math Resolutions) is used to reconstruct dose from

gantry-resolved EPID fluences: in air EPID measurements allow pretreat-

ment QA and transit EPID measurements allow in-vivo QA. For each patient,

3D pretreatment patient-specific QA is previously performed by DC. Ab-

solute isocenter doses by ionization chamber and EPID transit measurements

in homogeneous phantom are used to analyze DC performance in recon-

structing in-vivo isodose levels of VMAT plans delivery in a 3D simple

geometry. During the first treatment fraction, EPID transit fluences are ac-

quired to obtain in-vivo patient dose.

3D metrics is used to assess results from 2D and 3D dose distributions and

gamma distributions.

Results:

Absolute differences of isocenter point doses average 3% between

Eclipse planned dose, verification by ionization chamber in homoge-

neous phantom, EPID pre-treat reconstructed dose and EPID transit

reconstructed dose in phantom.

Patient DVH statistics reconstructed from in-vivo EPID acquisitions is com-

pared with planned from TPS. Quantitative comparison is performed in terms

of significant OAR dosimetric parameters and PTV coverage (D50, D2, D98).

Gamma pass rates (3%/3 mm criterion) are at least 88%.

Conclusions:

The possibility of an in-vivo 3D evaluation could have a very

positive impact on improving patient QA. Patient DVH-based metrics allows

VMAT QA evaluation through clinically significant volumes. Further studies

are required to analyze new processes and alert criteria associated with

3D metrics.

http://dx.doi.org/10.1016/j.ejmp.2016.01.237

A.234

EBT3 AND MICRODIAMOND FOR SUPERFICIAL DOSE MEASUREMENTS

UNDER A TOMOTHERAPY H&N IRRADIATION

M. Zani

* , a ,

C. Talamonti

a , b ,

M. Bucciolin

i a , b ,

M. Marinelli

c ,

G. Verona-Rinat

i c ,

P. Bonomo

b ,

L. Liv

i a , b ,

S. Pallotta

a , b .

Dipartimento di Scienze Biomediche

Sperimentali e Cliniche Mario Serio, Università degli Studi di Firenze, Firenze,

Italy;

A.O.U. Careggi, Firenze, Italy;

INFN–Dipatimento di Ingegneria

Industriale, Università di Roma Tor Vergata, Roma, Italy

Introduction:

The aim of this work is to compare the dose assessment ca-

pability of two different dosimeters in the build-up region for a tomotherapy

head and neck plan. Gafchromic EBT3 films and PTW-Frieburg

microDiamond were housed in the neck region of a modified version of

the RANDO phantom and were employed for dose measurements both in

superficial zones and at interfaces with air, in the trachea region.

Materials and methods:

microDiamond (mD) is a synthetic single-

crystal diamond detector, with disk shaped sensitive volume of 0.004 mm

(diameter

2.2 mm, thickness

1 μm). The effective point of measure-

ment is located 1 mm below the detector tip. EBT3 films have an active layer

thickness of about 28 μm and an effective depth of measurement of 100 μm.

Both devices are made of an almost water equivalent material, thus showing

a negligible energy dependence.

The dosimeters were housed inside a dedicated PMMA cylinder, which was

used to replace the neck of the RANDO phantom. Depth dose profiles, start-

ing from the neck surface and reaching a depth of 30 mm, were measured.

Results:

For each point of the measured profiles the deviations found

between mD and EBT3 are within 4%, even in high dose gradient zones.

Moreover, measured data are in agreement with what is predicted by the

treatment planning system (TPS), with a maximum dose difference (DD)

of 3.8% and maximum distance-to-agreement (DTA) of 1 mm for EBT3 and

of 2.5% (DD) and 0.9 mm (DTA) for mD.

Conclusions:

The study allowed to compare mD and EBT3 employing a setup

resembling an in vivo measurement. Dose depth profiles give a complete

insight of the dosimeters behavior and confirm the reliability of the two

dosimeters for measurements under disequilibrium conditions such as in

build-up regions, where high dose gradients are present, and at inter-

faces between air and PMMA.

http://dx.doi.org/10.1016/j.ejmp.2016.01.238

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Abstracts/Physica Medica 32 (2016) e1–e70