ESTRO 2020 Abstract Book

S80 ESTRO 2020

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 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:

Structure

Limit Vol [%] [Gy] Priority

PTVp_L_5000opt higher 0 PTVp_L_5000opt lower 100 PTVp_R_5000 opt higher 0 PTVp_R_5000 opt lower 100

50.4 90 50.2 100 50.4 90 50.2 100

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.

Liver

higher 5 higher 10 higher 5 higher 10 higher 10 higher 0

5 5 2 1 1 5

20 10 65 65 65 20

Esophagus

Heart

Lung_R Lung_L

Spinal Cord

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.

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