ESTRO 2020 Abstract Book

S807 ESTRO 2020

without compromising target coverage, with a shorter irradiation time, compared to the SF technique.

Minimum isodose coverage (Gy)

Moderate Risk (MR)

Dose (Gy)

Protocol

PTV

Prostate + seminal vesicle (SV) + 10 mm Prostate + SV + 6mm

CHHiP

PTV1

48

45.6

CHHiP IGRT PTV1

48

45.6

CHHiP

PTV2 Prostate + 10 mm/5 post

57.6 54.6

CHHiP IGRT PTV2 Prostate + 6 mm /3 post

57.6 54.6

CHHiP

PTV3 Prostate + 5 mm /0 post

60

57

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%

PROFIT

PTV60

60

PIVOTALboost PTV60

60

D98 ≥ 57Gy

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.

PO-1499 Incidental dose to obturator nodes in prostate radiotherapy trials CHHiP, PROFIT and PIVOTALboost I. Gleeson 1 1 Addenbrooke's Hospital - Oncology Centre, Medical Physics, Cambridge, United Kingdom Purpose or Objective Elective nodal radiotherapy remains a controversial topic in high risk patients. Previous works suggest less conformal techniques result in higher incidental doses to pelvic nodes leading to increased failures among those receiving more conformal techniques. Incidental dose to the pelvic nodes differs between techniques with the obturator nodes been highlighted as a region of highest dose differences. This work investigates the incidental dose received by the obturator nodes with moderately hypofractionated prostate only radiotherapy techniques used in the trials CHHiP, CHHiP IGRT sub-study, PROFIT and PIVOTALboost arm A. Material and Methods 5 patients were planned by the same planner using 6 MV single arc VMAT following trial protocols CHHiP, CHHiP IGRT sub study, PROFIT and PIVOTALboost. Target volumes were grown as per their respective trial protocol as shown in the Table and one set of OAR were used for all plans which was outlined as per CHHiP. Plans were prescribed 60 Gy in 20 fractions. Satisfactory plans were produced in line with their protocols and obturator pelvic nodes were retrospectively outlined as per PIVOTALboost guidelines and incidental doses were assessed.

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) for left breast cancer irradiation using voluntary breath hold. Radiation dose for heart, lungs and contralateral