A dosimetric comparison of IMRT vs VMAT optimization in

early stage whole breast cancer

Nader Moshiri

Professional Science Masters in Medical Physics

Physics Department, Florida Atlantic University

Disclosure

There is nothing to disclose.

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Outline

• Purpose

• Introduction

• Method

• Results

• Summary

• Work to be done

• Acknowledgements

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Purpose

To compare the dose to organs at risk (OAR) while using VMAT vs IMRT techniques for early breast cancer.

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Introduction • VMAT (Volumetric Modulated Arc Therapy), formally known

as Intensity Modulated Arc Therapy (IMAT), was first introduced by Dr. Cedric Yu in 1995.

• Elekta PreciseBeam Infinity

• Varian RapidArc

• Philips SmartArc

• RapidArc adjusts not only the treatment aperture, but also the rotational speed of the gantry and the delivery dose rate.

• VMAT usage is growing very fast for various tumors, e.g. prostate, head & neck, pelvis, lungs, GYN as well as breast cancer!

• Nothing on vendor ‘s website about VMAT for breast tumor! 5

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DVHs of IMRT VMAT

R lungs

L lungs

hearts

PTVs

L lungs

R breasts

Method

• Tangent IMRT (forward plan), breath hold (Phase I).

• A retrospective study over 25 whole left breast patients’ plans (7 RapidArc plans & 18 Tangent IMRT).

• Eclipse v 11.0.47 was used to create the alternative plans for each case using the same CT images, contours, energy(6X), Linac and normalization.

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Method Cont’d

• The prescription dose to planning target volume (PTV) was 50 Gy in 25 fractions.

• All plans were normalized such that 100% of the dose covered 95% of PTV.

• All plans were acceptable based on the RTOG1005 constraints.

• Maximum 3 Arcs were applied.

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Null Hypothesis 𝐻0: μ1= μ2 ; the mean doses of the two modalities are equal.

• Two-tail t-test was applied for unequal variances.

• All p-values were calculated for α = 0.05 and confidence interval of %95.

• If p ≤ 0.05 the means are significantly different therefore the null hypothesis is rejected.

• If p > 0.05 the means are not significantly different thus the null hypothesis is valid.

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Results

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Organs Limits IMRT VMAT p-values

RTOG 1005

ideal accepted

Ipsilateral

Lung

V5 22.94 74.37 < 0.0001 <50% <55%

V10 15.67 39.92 < 0.0001 <35% <40%

V20 11.92 14.99 0.0175 <15% <20%

V30 10.11 6.52 0.0018

Dmin(Gy) 0.16 1.15 < 0.0001

Dmax(Gy) 52.19 47.26 < 0.0001

Dmean(Gy) 7.10 11.27 < 0.0001

Contralateral

Lung

V5 0.00 1.77 0.0016 <10% <15%

Dmin(Gy) 0.00 0.55 < 0.0001

Dmax(Gy) 1.28 7.51 < 0.0001

Dmean(Gy) 0.08 2.08 < 0.0001

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IMRTVMAT%

Volume Comparison of ipsilateral lung for four doses

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Results cont'd Organs Limits IMRT VMAT p-values

RTOG 1005

ideal accepted

Heart

V10 4.22 9.15 0.0394 <30% <35%

V25 2.74 0.00 0.0005 <5%

D33% 1.65 5.85 < 0.0001

Dmin(Gy) 0.20 1.75 < 0.0001

Dmax(Gy) 48.66 21.64 < 0.0001

Dmean(Gy) 2.78 7.42 0.0328 <4 <5

Contralateral

Breast

V5 0.01 8.38 < 0.0001

D5% 0.96 5.52 < 0.0001

Dmin(Gy) 0.00 0.53 < 0.0001

Dmax(Gy) 6.15 10.68 0.0217 <3 <3.3

Dmean(Gy) 0.21 2.40 < 0.0001

PTV (L Breast)

Conformity Index 1.30 1.03 0.0003

Planning time 4min 22min 0.0081

MU 403.56 689.28 < 0.0001

Dmin% 65.29 68.44 0.6591

Dmax% 112.33 114.90 0.0787 <115% <120%

Dmean% 106.14 102.00 0.2302

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contralateral lung contralateralbreast

heart ipsilateral lung PTV (left breast)

IMRT

VMATGy

Dose to organs

Summary

• The dose-volumetric results of Tangent IMRT vs RapidArc were significantly different for most of the constraints, although all plans were made within the threshold values recommended by RTOGs.

• Mean doses to left lung, heart, right lung and right breast were significantly different in RapidArc from Tangent IMRT plans, especially for heart!

• Tangent IMRT treatment delivery utilized less total monitor units (MU) than RapidArc (mean of 403.56 vs 689.28 respectively).

• Conformity index was significantly different for the two modalities (1.3 for IMRT vs 1.03 for VMAT).

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Work to be done

• Multi field IMRT (inverse plan) with PTV including axillary nodes (in progress).

• Gain access to a multi-criteria optimization program to compare the doses to OARs and compare with our results.

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Acknowledgements Special thanks to:

• Silvia Pella, PhD, DABR for providing me an easily accessible environment and non-stop support!

• Th. Leventouri, PhD, PSMMP program Director for all her work for our young program.

• Thomas Costantino and David Littlejohn CMDs at SFRO for helping with this research.

• South Florida Radiation Oncology (SFRO) for providing the facilities.

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THANK YOU!

Questions?