ESTRO 2021 Abstract Book

S1665

ESTRO 2021

plans and PTV min

plans passed the gamma analysis with a success rate higher than 95%. The critical organ doses in treatment

plans prepared using PTV min

were lower than in those using PTV original

. The results were statistically significant (p<0.05).

Conclusion Our study investigated the effect of reducing tumor margins in the treatment of GBM on critical organ doses and toxicity by using custom phantoms produced with 3D printer technology. The findings show that the reduction of PTV margins in IMRT and VMAT treatments is important in terms of radiation-induced toxicity, and personalized 3D printing is a method that can be used for quality control in radiotherapy. PO-1954 DIBH radiotherapy in left breast cancer: Positional accuracy and assessment of dose reduction to OAR H. Hayakawa 1 , T. Kurita 1 , T. Nagano 1 , S. Okada 1 , C. Makita 2 , Y. Kajiura 1 , M. Matsuo 2 1 Gifu General Medical Center, Radiation Oncology, Gifu, Japan; 2 Gifu University, Radiation Oncology, Gifu, Japan Purpose or Objective Irradiation of the left breast or chest wall was associated with an increased cardiac mortality and morbidity. The relative risk for ischemic heart disease increased by 4-7 % for each 1 Gy increase in mean heart dose. Deep inspiration breath hold (DIBH) for left-sided breast cancer has been shown to reduce heart and lung dose. Respiration gating using the Varian Real- Time Position Management (RPM) system enables pushing the heart away from the tangential field during deep inspiration and thus optimizing the treatment plan and target coverage. The aim of this study was to assess the positional accuracy and investigate the reduction of organs at risk (OAR) doses with DIBH compared to free breathing (FB) in patients receiving left sided radiotherapy with three dimensional conformal techniques. Materials and Methods From 2019 to 2020, 5 left sided breast cancer patients underwent DIBH radiotherapy. For each patients, two CT scan were acquired with and without breath hold (FB_CT and DIBH_CT) and virtual simulation was performed for conventional tangential fields, utilizing 4MV photon fields. For each patient a FB and a DIBH treatment plans were calculated and dose- volume histograms were compared. Doses to heart, left anterior descending (LAD) coronary artery, and lungs were assessed. The clinically defined breast volume was covered by 95% of the prescription dose. Adjuvant radiotherapy was prescribed with a total dose of 42.56 Gy in 16 fractions or 50 Gy in 25 fractions. The RPM system was used to acquire patient’s breathing trace during DIBH_CT acquisition and treatment delivery. The electronic portal imaging device (EPID) was used in cinematographic (CINE) mode to capture a sequence of images during beam delivery as quality assessment for intra-fractional position verification. The intra-beam motion was assessed and the distance from beam central axis (CA) to the internal chest wall (ICW) was measured on each CINE image. There were compared to the planned distance on digitally reconstructed radiograph (DRR). Results For all patients a reduction of at least 10% in lung mean dose and at least 40% in heart mean dose was observed when DIBH was applied. LAD mean and maximum doses were decreased by more than 72% and 78% with DIBH, respectively. The maximun intra-beam motion for any one patient measurement was 0.17 cm (SD 0.12). The mean difference between the distance from CA to ICW in DRR was 0.14 cm (SD 0.32) and the equivalent distance on cine imaging was 0.15cm (SD 0.39). Conclusion DIBH for left breast irradiation could reduce the dose to the surrounding OAR, particularly LAD. RPM system was sufficient for the required level of accuracy in DIBH treatment. PO-1955 OAR dose constraints consensus in hypofractionated breast cancer radiotherapy is urgently needed! R. Ben Amor 1 , M. Bohli 1 , D. Aissaoui 1 , J. Yahiaoui 1 , A. Hamdoun 1 , L. Kochbati 1 1 Abderrahmen Mami Hospital, Radiation Oncology, Ariana, Tunisia Purpose or Objective Hypofractionated radiotherapy is widely used and has even become a standard in several countries, but consensus on dose constraints for OARs still has not been established. Our aim was to verify the reproducibility of the dose constraints adopted by FAST-Forward trial investigators in the 40 Gy/15fr arm. Materials and Methods We retrospectively analyzed dosimetric plans of 81 breast cancer patients, treated with 3D-hypofractionated monoisocentric radiotherapy from December 2017 to February 2020. The prescribed dose was 40.05 Gy in 15 fractions to the whole breast. Dose volume histograms (DVH) were evaluated and we recorded heart mean dose (MHD), cardiac volume receiving 2 Gy (V2Gy) and 10 Gy (V10Gy), Lung mean dose (MLD) and volume of ispsilateral lung receiving 12 Gy (V12Gy). These variables were compared to the dose constraints used in the Fast-Forward trial namely: volume of ipsilateral lung receiving 12 Gy less than 15%, volume of heart receiving 2 Gy less than 30% and that receiving 10 Gy less than 5% in their 40 Gy/15fr group. Results The mean MHD, V2Gy and V10Gy were respectively 1.78 Gy, 16.14 % and 2.56 %. V2Gy was less than 30% in 86.4% of cases (n=70). V10Gy was less than 5 % in 75.3% of patients (n=61).

The mean MLD and V12Gy were respectively 5.58 Gy and 36 %. V12Gy was less than 15% in 63% of patients (n=51) (table 1).

OAR

Variables

Percentage

Concordance(%) with FAST-Forward trial

-Mean V2 Gy (%) -V2 Gy < 30% -Mean V10 Gy (%) -V10 Gy < 5% -Mean V12 Gy (%) -V12 Gy < 15 %

Heart

16.14%[0.05-45.22]

86.4% of cases (n=70)

2.56%[0-9.71]

75.3% of cases (n=61)

Lung

36 % [0.1-29.4]

63% of cases (n=51)

Table 1: Comparison of doses received to organs at risk