ESTRO 2020 Abstract book

S857 ESTRO 2020

for left breast cancer irradiation using voluntary breath hold. Radiation dose for heart, lungs and contralateral breast were compared for VMAT and HYBRID planning techniques. The accuracy of dose delivery was estimated for both treatment methods. Material and Methods For this study 16 patients with left breast cancer were selected retrospectively. All of them had got hypofractionated radiotherapy (2.66Gy daily, 16 fractions). For each patient 3D,VMAT and HYBRID plans were calculated using Eclipse 13.7 TPS (Varian) for TrueBeam STxlinear accelerator. Pretreatment verification was performed using Octavius 4D phantom (729detector array). Respiratory Gating system (Varian) was used for voluntary breath hold.Dose evaluation was performed according to RTOG 1005, QUANTEC (ipsilateral lung:mean dose ≤ 13.5 Gy, V20 ≤ 30%, V15 ≤35%, V10 ≤40%, V5 ≤55%; heart: mean dose≤5 Gy,V10≤25Gy). Results The proportion of 3D and VMAT parts in HYBRID plan is an option which allows to find an optimum between decreasing mean dose and low dose for heart, lungs, breast and increasing V20 (volume of lung that receives radiation dose ≥20 Gy) for ipsilateral lung. For this study the proportion of 3D part had never exceeded 50% of all monitor unites per HYBRID plan.

and incidental doses were assessed.

Minimum isodose coverage (Gy)

Moderate Risk (MR)

Dose (Gy)

PTV

Protocol

Prostate + seminal vesicle (SV) + 10 mm

CHHiP

PTV1

48

45.6

CHHiP IGRT PTV1 Prostate + SV + 6mm 48

45.6

PTV2 Prostate + 10 mm/5 post

57.6 54.6

CHHiP

57.6 54.6

CHHiP IGRT PTV2 Prostate + 6 mm /3 post

PTV3 Prostate + 5 mm /0 post

60

57

CHHiP

CHHiP IGRT PTV3 Prostate + 3 mm /0 post 60

57

Prostate + proximal 1 cm SV + 10 mm/7 post + proximal 1 cm SV + 3 mm Prostate

CTV V60 ≥ 99%PTV V57 ≥ 99%

PTV60

60

PROFIT

60

D98 ≥ 57Gy

PIVOTALboost PTV60

D98

≥

PIVOTALboost PTV47 Prostate + SV + 6 mm 47

44.65Gy

Results CHHiP plans gave significantly higher incidental doses than the other techniques. The CHHiP IGRT, PROFIT and PIVOTALboost plans had about a 50% reduction in doses (V38 – V44 Gy). Mean doses did not differ as much.

Figure 1. Comparison of dose distribution for VMAT and HYBRID The HYBRID technique allowed us to reduce the mean dose for heart up to 45%, for contralateral lung up to 64% in comparison to VMAT plans. As for ipsilateral lung, the mentioned proportion optimum of 3D and VMAT was applied for all calculated plans and mean dose for HYBRID plans never exceeded mean dose in VMAT plans more than 6%. On the contrary, in some cases mean dose for ipsilateral lung in HYBRID plans was smaller, even up to 35%.

Conclusion The large reduction of incidental obturator nodal dose in CHHiP IGRT, PROFIT and PIVOTALboost plans should be noted for high risk patients where unintentional increases in failures may occur making elective nodal radiotherapy ever more relevant and results from PIVOTALboost greatly anticipated. PO-1500 HYBRID VMAT as a flexible planning method for breast cancer radiotherapy with voluntary breath- hold A. KAPRUSYNKA 1 , M. Mayorava 1 , E. Titovich 1 , A. Nikiporchik 1 1 N. N. Alexandrov National Cancer Centre of Belarus, Department of Radiation Therapy Engineering, Minsk, Belarus Purpose or Objective The aim of this study was to determine the possibility of using the HYBRID planning technique (mix of VMAT and 3D)