Session Item

Sunday
November 29
08:45 - 10:00
Physics Stream 2
Application of machine learning to CTV definition
2130
Symposium
Physics
14:39 - 14:47
Bilateral breast treatment comparative: mono and bi-isocentric VMAT treatment plan.
PH-0166

Abstract

Bilateral breast treatment comparative: mono and bi-isocentric VMAT treatment plan.
Authors: CORAL|, Anna(1)*[annacoral_91@hotmail.com];Espinosa López|, Núria(2);Bermejo Martinez|, Sonia(3);Gomez De Segura Melcon |, Guillermo(2);Nolla Nieto|, Xavier(2);Mancera Soto|, Míriam(2);Lizondo Gisbert|, Maria(4);
(1)Hospital de la Santa Creu i Sant Pau, Department of Medical Physics- Dosimetrist, Barceloana, Spain;(2)Hospital de la Santa Creu i Sant Pau, Department of Medical Physics- Dosimetrist, Barcelona, Spain;(3)Hospital de la Santa Creu i Sant Pau, Department of Radiation Oncology- Technician, Barcelona, Spain;(4)Hospital de la Santa Creu i Sant Pau, Department of Medical Physics- Medical Physicist, Barcelona, Spain;
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Purpose or Objective

To compare mono and bi-isocentric VMAT treatment plan for synchronous bilateral breast irradiation. We considered the main benefit in minimising the dose received by organs at risk (OAR) (lung, heart, oesophagus, spinal cord and liver) while keeping the same coverage in the planning treatment volumes (PTVs). 

Material and Methods

Nine bilateral breast carcinoma patients were selected with a breast PTV prescription of 50Gy (2GY/fraction) without lymph nodes affectation. The treatment was retrospectively replanned doing two different VMAT techniques: one with one isocenter and the other with two isocenters, always respecting the same longitudinal and vertical coordinates. VMAT treatment plans were optimized using the photon optimizer algorithm (PO v13.5.35). Dose was calculated with AAA_13.5.3 (Eclipse). Always the same optimization objectives were used:

StructureLimitVol [%][Gy]Priority
PTVp_L_5000opthigher050.490
PTVp_L_5000optlower10050.2100
PTVp_R_5000 opthigher050.490
PTVp_R_5000 optlower10050.2100
Liverhigher5520
Esophagushigher10510
Hearthigher5265
Lung_Rhigher10165
Lung_Lhigher10165
Spinal Cordhigher0520

The VMAT planning caractheristics were the standard ones:

- 1 isocenter: 3 arcs, the gantry angulation running from about 120º to about 230º approximately, with collimator angles were set to 90º.

-2 isocenters: 2 arcs for each breast. In left breast the gantry angulation running from about and in right breast from 50º to 210º. Collimator angles were set to 5º and 355º.

In both cases a “Pseudo Skin-Flash” procedure was applied.

The DVHs from the more relevant OARs in this pathology (heart, lung, esophagus, liver and spinal cord) and PTVs (right and left breast) were extracted. We analysed if there were differences between the planning group with one isocenter and the planning group with two isocenters. The statistical test used was Kolmogorov-Smirnov test. 

Results

In the figure 1 we can observe the mean HDVs with confidence intervals for OARs and PTVs for one and two isocentric VMAT plans. In all OARs there were significant dose reductions when planning with two isocenters (p<0,001) being the lungs, esophagus and spinal cord the ones with most dose reduction. PTV DVHs were also significantly different to the one isocenter, but being the difference small and no clinically relevant as dose objectives were always fulfilled.

In the figure 2 shows the dose distribution of one of the bilateral-patients replanned in this study. Fig.2a shows doses distribution in a planning done with one isocenter, and Fig.2b shows the dose distribution in a planning done with two isocenters. It can be seen the dose sparing at OARs when VMAT bi-isocentric plan is used.




Conclusion

The VMAT bi-isocentric treatment planning for bilateral breasts seems to reduce the doses received for the lungs, esophagus and spinal cord. Thus, it could be convenient to use this approach when using VMAT planning for bilateral breast cancer patients.