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

Background: Post-operative chest wall irradiation improves local control & survival with higher techniques (conformal & intensity modulated) are recommended in left side. These techniques involve mastectomy bed Clinical Target Volume (CTV) delineation which varied clearly among guidelines especially for the lateral margin with subsequent possible effects on dose received by lung & heart (in left sided cases). Aim of work: Comparison between American & European guidelines to a more anatomical (from our point of view) method using wire following anatomical lines regarding post left mastectomy chest wall CTV volume & doses received by the heart & left lung. Methodology: One oncologist had delineated 10 cases referred for radiation therapy of the chest wall & peripheral lymphatics following left modified radical mastectomy. He delineated every case according to Radiation Therapy Oncology Group (RTOG), European Society of Therapeutic Radiology and Oncology (ESTRO) & according to an anatomically set wire during scanning. Plans were set for each delineation by an expert radiation physicist. Results of the 3 plans were compared regarding coverage, homogeneity of CTV & doses to risk organs.
Results: The three CTVs were well covered & homogeneously treated. Statistically significant less left lung V20Gy & V30Gy for anatomical wire based delineation (16.0 +/- 4.1% &12.75 +/- 2% respectively) compared to ESTRO (19.1 +/- 1.73 & 15.2 +/- 5.1 respectively) & RTOG (18.22 + 1.6 & 14.52 + 5.3 respectively), p=0.001 for V20Gy & 0.01 for V30Gy. Cardiac D50% was statistically significantly lower (101.6 + 41.2cGy) vs (141 +/- 81cGy & 132 +/- 93 cGy, p= 0-00001) for ESTRO & RTOG respectively.
Conclusion: Delineation of left mastectomy chest wall CTV based on anatomical landmarks guided by wire set during patient scanning may reduce cardiac & pulmonary doses compared to ESTRO & RTOG. Clinical follow up of cases treated based on this delineation is highly recommended specially to evaluate local recurrence.