ESTRO 2020 Abstract book

S458 ESTRO 2020

previous BCS and RT. Results of iBT as re-irradiation after 2nd. BCS is summarized in Table 1 . Table 1. Results of brachytherapy as re-irradiation after 2nd. BCS

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. SP-0735 Thoracic normal tissue tolerance to re- irradiation A. Van der Kogel 1 1 University of Wisconsin Medical Radiation Research Center, Department of Human Oncology, Madison- WI, USA Abstract text Re-irradiation of thoracic malignancies may be limited by the tolerance of various critical structures, including the heart, esophagus, main bronchi, lung and spinal cord. Extensive data exist for spinal cord but hardly any for other tissues. Some very limited data have been published for lung, but the results are based on whole thorax irradiation of mice, and the relation with partial lung irradiation with very high doses and steep gradients is questionable. With solid data often lacking even for de-novo irradiated tissues NTCP calculations have mostly relied on LQ-based modeling with assumptions of generic a/b values. In this presentation I will briefly review the knowledge of retreatment tolerance of some thoracic normal tissues, and will address the question of the validity (and usefulness) of the LQ model in the current era of hypofractionation and SBRT. An innovative approach to NTCP modeling integrating clinical, biological and physical knowledge is badly needed. SP-0736 Treatment planning challenges for re- irradiation in the thoracic region E. Adams 1 1 St. Luke's Cancer Centre Royal Surrey County Hosp, Radiotherapy Physics, Guildford, United Kingdom Abstract text As patient survival after radiotherapy improves, there is an increasing need for re-irradiation of patients for local relapse, treatment of metastatic disease or treatment of a second primary. When performing treatment planning for such cases, it is necessary to take account of both dose distributions in order to ensure that normal tissue toxicity risks are accounted for. However, this presents a number of challenges, particularly with the complexity of modern techniques which employ multiple IMRT or VMAT fields with highly-conformal high dose volumes and a large low dose bath. In the thorax, re-irradiation is frequently carried out with stereotactic ablative radiation therapy (SABR) in order to optimally treat small targets such as local recurrences and oligometastases. This further complicates the assessment of combined doses as the radiobiological models are not well-defined at the high dose per fraction employed in SABR, and there is little conclusive data on the appropriate normal tissue tolerances when re-irradiating with this technique. An additional issue in the thorax is that there can be large variations in patient anatomy due to differences in patient position or management of breathing motion. Image registration for dose summation therefore requires non-rigid techniques to account for what may be large anatomical deformations. This presentation will discuss these challenges, their implications and how they may be addressed.

RT scheme Dose (Gy) x fr.

2n d. LR % (n) 15. 9 (11 of 69) 6.7 (1 of 15) 26. 7 (4 of 15) 5.1 (2 of 39) 2.8 (1 of 36) 3.8 (1 of 26) 25. 8 (16 of 62) 10 (4 of 39) 4.1 (9 of 21 7) 9.3 (49 of 52 7) 0 (0 of 9)

5- ye ar OS %

FUP (yea rs)

Excellent/ good cosmesis %

Author Technique

Hannoun- Levy

30; 45- 50

91. 8 NR

4.2

LDR

10 0 100

Chadha LDR

30; 45 3

30

3.3

61 16

Maulard LDR

10 0

55

Resch

PDR

40-50 5

Kauer- Dorner

PDR

50.1

4.75

87 37

96. 7 94

Guix

HDR

2.5x12 7.4

45- 50/3.4x 10

Trombett a

3.2

NR 92

LDR/HDR

Houvenae ghel

LDR

45-56 6.1

80 NR

Smanyko HDR

4.4x5 5

81 70

30- 55/50/ 3.4- 4.4x5- 10

GEC- ESTRO

LDR/PDR/ HDR

88. 7 85

4

61 - 10 0

3- 7.4

All patients

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%)