Consistency of ESTRO and RTOG contouring guidelines for target volume delineation in early stage breast cancer

Introduction: Adjuvant radiotherapy (RT), including regional lymph node (RLN) irradiation is an important treatment in early stage breast cancer patients after breast-conserving surgery both the Radiation Therapy Oncology Group (RTOG) and European Society for Radiotherapy and Oncology (ESTRO). Our primary aim was to quantitatively compare target volumes delineated by ROs, to assess if either guideline has superior contouring reproducibility. Methods: Three ROs contoured breast clinical target volumes (CTVs), axillary lymph node levels 1-3, supraclavicular and internal mammary nodal (CTVn_IMN) volumes for 8 post-operative patients using the ESTRO and RTOG guidelines for each patient. The inter-observer variability in contouring was measured by the generalized Dice Similarity Coefficient (DSC). Mean DSC values were calculated to compare the common volume accuracy between observers.ss Results: The breast CTV contours showed the highest level of agreement between the ROs within each guideline (ESTRO DSC mean 0.92±0.04 and RTOG DSC mean 0.90±0.03). Due to the small standard deviation in DSC values, this difference of 0.02 was found to be statistically significant (p=0.031). The mean DSC for CTVn_IMN was 0.63 (±0.10) for ESTRO and 0.62 (±0.20) for RTOG guidelines, showing the least level of agreement for both guidelines. Within the axillary nodal volumes, level 1 showed the greatest agreement among the ROs for both ESTRO and RTOG guidelines with a mean DSC of 0.81 (±0.08) & 0.82 (±0.06) respectively. The range of mean DSC values for ESTRO and RTOG guidelines was 0.66-0.92 and 0.62-0.90 respectively. Conclusion: Overall, neither guideline showed a notable improvement in reproducibility between ROs.