ESTRO 2021 Abstract Book

S951

ESTRO 2021

close or positive margins (16 Gy in 8 daily fractions, 2Gy per fraction). All the pts were treated with 3D- conformal RT technique. Skin toxicity and breast oedema were assessed at the end of RT and after 15 days; fibrosis and oedema were assessed at 5 years after RT. Acute and late skin toxicities were evaluated in accordance with the RTOG grading scale. Clinical and dosimetric data were prospectively collected. Univariate analysis evaluated associations between toxicity and dosimetric/anatomical variables (p-value <=0.05). Results Among the patients treated between 2009 and 2015 at our Institution, 425 patients had at least 5 years of follow-up. At RT end, acute skin toxicity ≥G2 and oedema ≥G2 occurred in 88 (20.7%) and 4 (0.9%) patients, respectively. The univariate analysis showed a between skin toxicity and breast volume >1000 cm 3 (p=0.04), boost administration (p<0.01), treated skin area receiving more than 20 Gy (TSA) >400cm 2 (p<0.01) and breast volume receiving 105% of the prescription dose (V105%) (p<0.01). Oedema was related only to breast V105% (p<0.01). At 2 weeks after RT, skin toxicity ≥G2 and oedema ≥G2 occurred in 48 (11.3%) and 4 (0.9%) patients, respectively. Skin toxicity was related only to TSA >400cm 2 (p<0.01) and oedema only to breast V105% (p<0.01). At 5 years after RT, fibrosis ≥G1 occurred in 89 (20.9%) patients and oedema ≥G1 in 36 (8.5%) patients. No association with p<0.05 was found for these toxicities, except for age (p=0.03) and hypertension (p=0.05) and oedema. However, some variables were associated with a p<0.2, i.e. breast volume >1000 cm 3 and breast V105%. Conclusion HRT can be considered a safe treatment for large breast size patients too. A recurrent association was found between the toxicities (at the 3 time points) and the breast volume V105%, which is correlated with the breast size. This may suggest that a more homogenous RT technique may be preferred for patients with larger breast size (volumetric modulated RT techniques usually give a more homogenous dose distribution than the tangential fields technique). A next step would be to deeper investigate the association among the variables with a multivariate analysis. More research is also needed to enlighten patients’ intrinsic characteristics that empower toxicities. PO-1143 One-week ultrahypofractionated RT for whole breast and simultaneous integrated boost in DCIS R. Ciervide 1 , A. Montero 1 , M. Garcia-Aranda 1 , B. Alvarez 1 , A. Prado 2 , X. Chen-Zhaoi 1 , R. Alonso 1 , M. Lopez 1 , O. Hernando 1 , E. Sanchez 1 , J. Valero 1 , M. Nuñez 1 , M. Izquierdo 1 , K. Rossi 1 , C. Cañadillas 1 , J. Marti 1 , D. Zucca 1 , L. Alonso 1 , P. Fernandez-Leton 1 , C. Rubio 1 1 HM Hospitales, Radiation Oncology, Madrid, Spain; 2 HM Hospitales, Radiation Oncology, Madrid, Spain Purpose or Objective Whole breast irradiation (WBI) after breast conserving surgery (BCS) is indicated to improve loco-regional control and survival in ductal carcinoma in situ (DCIS). Former studies showed benefit of tumor bed boost irradiation in local control. Hypofractioned regimens in 3 weeks are considered standard radiation therapy although recent studies have shown the non-inferiority of a treatment regimen of 5 fractions in one week in ductal carcinoma in situ. We present our preliminary results of feasibility and tolerance of a ultra- hypofractionated 5 fractions in one-week WBI schedule with simultaneous integrated boost (SIB) in DCIS. Materials and Methods From April 2020 to January-2021, 29 patients with a median age of 55 years (range 42-76) and histological diagnoses of DCIS were treated according to our institution ultra-hypofractionated radiotherapy schedule after breast conserving surgery. Radiotherapy comprises the whole breast receiving 26Gy of 5.2Gy/fraction and simultaneous integrated boost (SIB) was administered to all patients up to a total dose of 29Gy of 5.8Gy/fraction in 25p (86%) and 30Gy of 6Gy/fraction in 4p (14%) with close/focally affected margins. WBI+SIB was delivered by conformal 3-D technique in 28p (96.5%) and VMAT in 1p (3.5%). Treatment beam arrangement for SIB comprised of coplanar beams: 3 beams in 11p (38.5%), 4 beams in 14p (48%), 5 beams in 3p (10%) and 2 arcs in 1p (3.5%). Dose constraints are detailed in table 1. 26p (90%) received adjuvant hormone therapy Table 1. Dose constraints Organ at risk Ipsilateral lung V12<20% (if only Breast +/- boost) Inhouse constraint Contralateral lung Average Dose <5.6 Gy RTOG 1005 V3.6 <10% RTOG 1005 Heart V12 <5% Inhouse constraint

V7<5%

FAST-FORWARD

V1.5 < 30% (if only breast + boost)

FAST-FORWARD

Contralateral Breast Spinal Canal

V3.6 < 30 %

Max Dose < 27 Gy D0.01 cc< 22.5 Gy

Inhouse constraint

Results With a median follow-up of 3 months (range 1-10), tolerance was acceptable with null o mild toxicity: 17p (59%) developed skin toxicity grade 1. No toxicities ≥grade 2 were reported. A comparative analysis was calculated with Chi-Square test. We found a significant relation between PTV