ESTRO 2020 Abstract Book

S414 ESTRO 2020

becomes clear what dose rate and dose rate distributions were used.

However, re-irradiation of the whole breast with significant dose is considered inappropriate because of the high risk of serious late side effects. Due to the ability of focusing radiation dose by interstitial multicatheter brachytherapy (iBT) to a limited volume (while sparing surrounding normal tissues), iBT is a promising method to retreat the tumour bed with an effective dose after previous BCS and RT. Results of iBT as re-irradiation after 2nd. BCS is summarized in Table 1 .

SP-0732 Towards clinical implementation of FLASH radiotherapy M. Vozenin 1 1 Centre Hospitalier Universitaire Vaudois, Department of Radiation Oncology, Lausanne Vaud, Switzerland Abstract text Radiation therapy is a cornerstone of cancer treatment and is used in over 50% of cancer patients, however its efficacy remains suboptimal in many radiation-resistant tumors. Although recent development in radiotherapy allowed higher precision and efficacy, the collateral damages to healthy tissue remain a limitation and being able to deliver high curative radiation doses to tumors depends on the ability to spare normal tissues from harmful effects of radiation. Today this goal is now at hand using FLASH-radiotherapy. FLASH-RT consists of dose delivery within an extremely short irradiation time, i.e. the dose is delivered in micro-seconds whereas with radiotherapy at conventional dose rate the dose is delivered over minutes. The main interest of FLASH-RT is its biological effect namely impressive normal tissue sparing at doses enabling remarkable tumor control, this biological effect has been called the FLASH effect by our group (review in Vozenin et al 2019). Experimental and clinical work is ongoing to understand the parameters relevant for the generation of the FLASH effect and its safe translation to the clinic. In addition, recently many groups have developed FLASH-RT program and discrepancies in findings have been reported. Several reasons can account for this discrepancy with prior results, and we would like to highlight here the critical aspects necessary for the characterization of the FLASH effect. Such considerations will hasten the development of further experimentation to move the field forward in a productive and efficient manner.

Table 1. Results of brachytherapy as re-irradiation after 2nd. BCS

Excel lent/ good cosm esis %

RT scheme Dose (Gy) x fr.

FUP (yea rs)

2nd. LR % (n) 15.9 (11 of 69) 6.7 (1 of 15) 26.7 (4 of 15) 0 (0 of 9) 5.1 (2 of 39) 2.8 (1 of 36) 3.8 (1 of 26) 25.8 (16 of 62) 10 (4 of 39)

5- year OS %

Techniqu e

Author

Hannoun- Levy

30; 45- 50

LDR

4.2

91.8 NR

Chadha LDR

30; 45 3

100 100

Maulard LDR

30

3.3

61 16

Resch

PDR

40-50

5

100 55

Kauer- Dorner

PDR

50.1

4.75

87 37

Guix

HDR

2.5x12 7.4

96.7 94

45- 50/3.4x1 0

Trombett a

LDR/HDR

3.2

NR 92

Houvenae ghel

LDR

45-56

6.1

80 NR

Smanyko HDR

4.4x5

5

81 70

Symposium: Dose accumulation and re-irradiation for thoracic malignancies

30- 55/50/3. 4-4.4x5- 10

GEC- ESTRO

LDR/PDR /HDR

4.1 (9 of 217)

4

88.7 85

SP-0733 Challenges and solutions for thoracic re- irradiation G. Van Tienhoven Academic Medical Center, Amsterdam, The Netherlands

9.3 (49 of 527)

All patients

3- 7.4

61- 100

16- 100

The rate of 2 nd. IBTR after re-irradiation with iBT is similar to that of reported after sMT (e.g. 10%). The multicentric propensity-score matched cohort analysis of the GEC- ESTRO Breast WG comparing the results of 2 nd. BCS + iBT and sMT proved similar 5-year local tumor control (93.6% vs 91.6%), DFS (82.1% vs 77.5%), and OS (85.9% vs 87.3%) without significant increase of the risk of late radiation side effects after iBT. Therefore, 2 nd. BCS + re-irradiation using multicatheter iBT is a clinically validated option for selected patients with IBTR after previous BCS + RT.

Abstract not received

SP-0734 Possibilities for second breast conserving treatment after a local recurrence C. Polgár 1 1 National Institute of Oncology, Center of Radiotherapy, Budapest, Hungary Abstract text The standard treatment of early-stage breast cancer is breast-conserving surgery (BCS) and radiotherapy (RT) of the conserved breast. In spite of adequate BCS and RT, the rate of ipsilateral breast tumour recurrence (IBTR) is approximately 5-10%. Therefore, there is a need for safe and effective local salvage treatment strategies. The standard treatment for IBTR is salvage mastectomy (sMT). Other options are 2 nd. BCS with or without re-irradiation. However, the rate of 2 nd. IBTR without re-irradiation has been reported in the range of 20-30%. Thus, re-irradiation after 2 nd. BCS may decrease the chance of 2 nd. IBTR.

SP-0735 Thoracic normal tissue tolerance to re- irradiation

Presentation cancelled

SP-0736 Treatment planning challenges for re- irradiation in the thoracic region

Presentation cancelled