A free software for the evaluation and comparison of dose response models in clinical radiotherapy (DORES)

Contouring variation affects estimates of normal tissue complication probability for breast fibrosis after radiotherapy

BACKGROUND

Normal tissue complication probability (NTCP) models can be useful to estimate the risk of fibrosis after breast-conserving surgery (BCS) and radiotherapy (RT) to the breast. However, they are subject to uncertainties. We present the impact of contouring variation on the prediction of fibrosis.

MATERIALS AND METHODS

280 breast cancer patients treated BCS-RT were included. Nine Clinical Target Volume (CTV) contours were created for each patient: i) CTV_crop (reference), cropped 5 mm from the skin and ii) CTV_skin, uncropped and including the skin, iii) segmenting the 95% isodose (Iso95%) and iv) 3 different auto-contouring atlases generating uncropped and cropped contours (Atlas_skin/Atlas_crop). To illustrate the impact of contour variation on NTCP estimates, we applied two equations predicting fibrosis grade ≥ 2 at 5 years, based on Lyman-Kutcher-Burman (LKB) and Relative Seriality (RS) models, respectively, to each contour. Differences were evaluated using repeated-measures ANOVA. For completeness, the association between observed fibrosis events and NTCP estimates was also evaluated using logistic regression.

RESULTS

There were minimal differences between contours when the same contouring approach was followed (cropped and uncropped). CTV_skin and Atlas_skin contours had lower NTCP estimates (-3.92%, IQR 4.00, p < 0.05) compared to CTV_crop. No significant difference was observed for Atlas_crop and Iso95% contours compared to CTV_crop. For the whole cohort, NTCP estimates varied between 5.3% and 49.5% (LKB) or 2.2% and 49.6% (RS) depending on the choice of contours. NTCP estimates for individual patients varied by up to a factor of 4. Estimates from "skin" contours showed higher agreement with observed events.

CONCLUSION

Contour variations can lead to significantly different NTCP estimates for breast fibrosis, highlighting the importance of standardising breast contours before developing and/or applying NTCP models.

Risks of lung fibrosis and pneumonitis after postmastectomy electron radiotherapy

Breast cancer patients are treated by a variety of options. Electron beams are utilised in the irradiation of the chest wall postmastectomy due to its dose distribution in the irradiated body. The objectives of this study were to determine the possibility of inducing lung fibrosis and pneumonitis during postmastectomy radiotherapy (PMRT) using electron beams. Electron beams with different energies and gantry angles were used for irradiating the chest wall in PMRT. The normal-tissue-complications-probability of the lung was evaluated. Three computer codes EGSnrc, XTING and DORES were used for simulating the beams and patients, generating dose-volume histograms and evaluating the dose response of the lung. NTCP increases with energy and with gantry angle. Below 15 MeV (which had given very high and unacceptable NTCP values), the largest value of NTCP of fibrosis was 0.036, for 12 MeV, gantry angle 60. The largest value of NTCP of radiation-induced pneumonitis was 0.044, for 12 MeV, gantry angle 60.