Variability of Medial and Lateral Borders Delineation in Guidelines for Post-Mastectomy Irradiation Significantly Affects Radiation Dose Received by Left Lung and Heart

Background: Delineation of Clinical Target Volume (CTV) is a basic step in 3 Dimensional Conformal (3DCRT). A notable variation however exists among different guidelines in delineation of chest wall (CW) CTV specially for the lateral margin. Some authors used wire localization for the anatomical insertion of the presumed breast It is expected that lateral margin delineation will affect the standard tangential fields and therefore the ipsilateral lung & heart (in left side) volumes irradiated to high dose. Aim of work: Evaluation of the effect of using various guidelines for chest wall CTV delineation on outcome regarding doses received by heart and left lung (in post left mastectomy irradiation) and compare these outcomes to that of wire based delineation (WBD). Methodology: Ten patients with T3/4 &/or N+ left breast cancer were planned for post mastectomy CW-3DCRT. Delineation of CW by one radiation oncologist followed 2 different guidelines namely RTOG & ESTRO in addition to a 3rd anatomical based wire delineation of chest wall underlying the presumed breast. Three CRT plans for the 3 CTVs were compared regarding coverage, homogenity & toxic dose to heart & lt. lung. Results: CTV was a highly significantly smaller when delineated using WBD vs RTOG or ESTRO guidelines. There was no statistically significant difference between the 3 delineated volumes regarding coverage & homogeneity parameters. A highly statistically significant better (lesser) V20Gy & V30Gy received by lt. lung for plans based on WBD (16.0 +/- 4.1% &12.75 +/- 2% respectively) vs those based on ESTRO (19.1 +/- 1.73 & 15.2 +/- 5.1 respectively) or RTOG guidelines (18.22 ± 1.6 & 14.52 ± 5.3 respectively), p=0.001 for V20Gy & 0.01 for V30Gy. For cardiac dose, a statistically significant lower D50% received by the heart in plans based on WBM delineation (101.6 ± 41.2 Gy) compared to plans based on ESTRO & RTOG guidelines based CTV (141 +/- 81cGy & 132 +/- 93 cGy respectively, p= 0-00001). Conclusion: WBD of post lt. mastectomy chest wall CTV delineation significantly reduced toxic dose received by heart & lt. lung. Larger trial with clinical follow up to test for being not inferior to ESMO &/or ESRTO guidelines based treatment regarding local recurrence.