ESTRO 2024 - Abstract Book

S4113

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

2821

Digital Poster

Dosimetric comparison between whole breast IMRT plans calculated on planning-CT and on daily CBCT.

Lucia Zirone 1 , Elisa Bonanno 1 , Giuseppina Rita Borzì 1 , Nina Cavalli 1 , Andrea Girlando 2 , Martina Pace 1 , Carmelo Marino 1

1 Humanitas - Istituto Clinico Catanese, Medical Physics, Catania, Italy. 2 Humanitas - Istituto Clinico Catanese, Radiotherapy, Catania, Italy

Purpose/Objective:

The purpose of this work is to develop a method of dosimetric comparison between the original treatment plan and the delivered one during the first four fractions using CBCT images.

Material/Methods:

A dataset of 20 patients was used for the purposes of this study. Treatment plans were delivered by a Varian TrueBeam 2.7, and performed at Humanitas Istituto Clinico Catanese (HICC). These plans were characterized by an hypofractionation of 2.67 Gy × 15 fractions, two fields (Field 1 and Field 2), fixed jaws, energy beam of 6 MV, and dose rate equal to 600 monitor units (MU) per minute. Eclipse v. 16.1 Treatment Planning System (TPS) by Varian Medical System, Acuros 16.1.2 algorithm, and calculation grid size equal to 0.15 cm were used for the plans calculation. Treatment setup was verified with Align RT (By Vision RT) as SGRT system and daily CBCTs. For each patient, daily CBCTs acquired during the first four fractions were used to perform a dosimetric comparison with the original treatment plan. Planning CTs (p-CTs) and CBCTs were co-registered online in the treatment unit and then processed using the TPS contouring module.

On CBCTs, the ipsilateral lung and body structures were created. Both for p-CTs and CBCTs, to eliminate differences in Hounsfield Units (HU), -700 HU were assigned to the lung structure, while 0 HU was assigned to the body structure.

Treatment plans were recalculated on the p-CTs and on the four CBCTs (CBCTfr1, CBCTfr2, CBCTfr3, CBCTfr4) using the same plan and MUs as the original plan.

The following dose-volume parameters were evaluated: V95% for PTV and V20Gy for ipsilateral lung. Finally, the relative percentage discrepancies between the dose-volume parameters of the p-CTs and CBCTs were calculated according to: ΔI=((I p-CT -I CBCT )/I p-CT )x100, where I indicate the dose-volume parameter.

Results:

Figures show the calculated relative percentage discrepancies for each dose-volume parameter, while the table shows the minimum and maximum deviations.