ESTRO 2021 Abstract Book

S940

ESTRO 2021

shows that one of five pts presents another tumor during her life, but two thirds are registered previous or concomitantly to the (primary)BCA. A randomized treatment keeping into account genetic variability would not be possible, but maybe genetics will give the final response regarding the radiotherapy carcinogenicity.

PO-1132 Incidental radiation to internal mammary nodes in breast cancer- comparison of conformal techniques A. Mehta 1 , P. Kumar 1 , S. N.S 2 , A. S.K. 1 1 Shri Ram Murti Institute of Medical Sciences, Radiation Oncology, Bareilly, India; 2 Shri Ram Murti Institute of Medical sciences, Radiation Oncology, Bareilly , India Purpose or Objective Irradiation of internal mammary nodes (IMN) in breast cancer is controversial. It is postulated that the low risk of recurrence in IMN may be due to the incidental dose delivered. The impact of evolving conformal techniques on its incidental dose needs evaluation. Materials and Methods Twenty post Modified Radical Mastectomy patients who received adjuvant radiotherapy were selected. For each patient, 3DCRT and IMRT plans were generated for this study (40 plans). The clinical target volume (CTV) included the chest wall along with axillary and supraclavicular nodes as indicated while the IMN was not included in any patient. An expansion of 5mm was taken for Planning Target Volume (PTV) (PTV_planning). In the 3DCRT technique, two tangential beams were used for the chest wall along with a single supraclavicular field for nodal irradiation if indicated with monoisocentric technique. In the IMRT technique, inverse planning was done and five to seven tangential fields were used. The dose prescribed was 50 Gy in 25 fractions. The constraints for Organs at Risk were Ipsilateral Lung < V20-35%; Heart- Mean dose < 15 Gy, V25 < 10%; Contralateral lung and Breast- Mean < 5 Gy; and Planning risk Volume Spinal Cord- Maximum dose < 50 Gy. Retrospectively, IMN was delineated in the first three intercostal spaces. A PTV margin of 5 mm was taken (PTV_IMN). The dosimetry of PTV_planning and PTV_IMN were compared amongst the two techniques. Statistical analysis was done by paired t-test or Wilcoxon Signed-Rank test. Results An average volume of 65.03% (SD - 12.45, Range- 46.04 - 87.61) of the PTV_IMN was overlapping with PTV_planning.

3DCRT Mean (SD) or Median (IQR)"

IMRT Mean (SD) or Median (IQR)"

Parameter

p value

PTV_planning Dmean

49.60 (0.64)

49.99 (0.97)

0.14

D50

50.11 (0.69)

50.04 (0.42)

0.68

D90

45.83 (0.77)

48.04 (1.30)

<0.0001

D95

44.08 (1.45)

46.83 (3.32)

<0.0001

Homogeneity Index 0.28 (0.10)

0.14 (0.07)

<0.0001

Conformity Index 1.28 (0.18)

1.16 (0.18)

0.03

PTV_IMN Dmean

42.55 (5.65)

42.92 (4.02)

0.67

D95

21.59 (6.85,38.28)"

22.85 (17.09,30.64)"

0.41

D90

29.86 (12.10,42.72)"

27.33 (21.27, 34.55)"

0.76

V95

45.46 (20.62)

47.80 (18.55)

0.63

V30

86.82 (13.34)

87.02 (10.44)

0.92

PTV coverage was comparable amongst both techniques in terms of Dmean and D50. D90 and D95 were significantly higher in the IMRT technique but a minimum of 90% of the volume received 90% of the prescribed dose with both techniques fulfilling the prespecified objective. The dose homogeneity and conformity were significantly better in the IMRT technique. PTV_IMN coverage was comparable amongst both techniques. Less than half the volume received a dose of 45 Gy. Considerably a large volume was exposed to a lower dose of 30 Gy. Conclusion The incidental IMN dose was not impacted by better conformity of the IMRT technique. But the IMN dose was not sufficient for subclinical control by either technique. Considering the lower incidence of regional failures for IMN nodes, further studies are needed to determine whether a lower incidental dose can achieve subclinical control. PO-1133 Deep inspiration breath hold: Dose comparison and anatomical factors analysis using 3D slicer C. Kuo 1 , C. Chang 1 , H. Cheng 1 , J. Lin 1,2 , J. Tsai 1,2 1 Shuang Ho Hospital, Taipei Medical University, Radiation Oncology, Taipei, Taiwan; 2 College of Medicine, Taipei Medical University, Department of Radiology, School of Medicine, Taipei, Taiwan Purpose or Objective The aim of the study was to evaluate the organ-sparing potential of deep inspiration breath hold (DIBH) technique for left-sided breast radiotherapy compared with different respiratory phases, middle of lung expansion (MLE) and start of lung expansion (SLE). In addition, the correlations between anatomical factors