A topology-based method to mitigate the dosimetric uncertainty caused by the positional variation of the boost volume in breast conservative radiotherapy.
BACKGROUND
To improve local control rate in patients with breast cancer receiving adjuvant radiotherapy after breast conservative surgery, additional boost dose to the tumor bed could be delivered simultaneously via the simultaneous integrated boost (SIB) modulated technique. However, the position of tumor bed kept changing during the treatment course as the treatment position was aligned to bony anatomy. This study aimed to analyze the positional uncertainties between bony anatomy and tumor bed, and a topology-based approach was derived to stratify patients with high variation in tumor bed localization.


METHODS
Sixty patients with early-stage breast cancer or ductal carcinoma in situ were enrolled. All received adjuvant whole breast radiotherapy with or without local boost via SIB technique. The delineation of tumor bed was defined by incorporating the anatomy of seroma, adjacent surgical clips, and any architectural distortion on computed tomography simulation. A total of 1740 on-board images were retrospectively analyzed. Positional uncertainty of tumor bed was assessed by four components: namely systematic error (SE), and random error (RE), through anterior-posterior (AP), cranial-caudal (CC), left-right (LR) directions and couch rotation (CR). Age, tumor location, and body-mass factors including volume of breast, volume of tumor bed, breast thickness, and body mass index (BMI) were analyzed for their predictive role. The appropriate margin to accommodate the positional uncertainty of the boost volume was assessed, and the new plans with this margin for the tumor bed was designed as the high risk planning target volume (PTV-H) were created retrospectively to evaluate the impact on organs at risk.


RESULTS
In univariate analysis, a larger breast thickness, larger breast volume, higher BMI, and different tumor locations correlated with a greater positional uncertainty of tumor bed. However, BMI was the only factor associated with displacements of surgical clips in the multivariate analysis and patients with higher BMI were stratified as high variation group. When image guidance was aligned to bony structures, the SE and RE of clip displacement were consistently larger in the high variation group. The corresponding PTV-H margins for the high- and low-variation groups were 7, 10, 10 mm and 4, 9, 6 mm in AP, CC, LR directions, respectively. The heart dose between the two plans was not significantly different, whereas the dosimetric parameters for the ipsilateral lung were generally higher in the new plans.


CONCLUSIONS
In patients with breast cancer receiving adjuvant radiotherapy, a higher BMI is associated with a greater positional uncertainty of the boost tumor volume. More generous margin should be considered and it can be safely applied through proper design of beam arrangement with advanced treatment techniques.
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.
Peng-Yi
PY
Lee
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.
Chih-Yuan
CY
Lin
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.|Department of Medicine, China Medical University, Taichung, Taiwan.|Department of Medicine, Taipei Medical University, Taipei, Taiwan.
Shang-Wen
SW
Chen
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.|Department of Medicine, China Medical University, Taichung, Taiwan.
Chun-Ru
CR
Chien
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.
Chun-Nan
CN
Chu
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.
Hsiu-Ting
HT
Hsu
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.|Department of Medicine, China Medical University, Taichung, Taiwan.
Ji-An
JA
Liang
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan.
Ying-Jun
YJ
Lin
Department of Radiation Oncology, China Medical University Hospital, 2nd Yu-De Road, North District, Taichung City, Taiwan. shiau158@ms22.hinet.net.|Department of Medicine, China Medical University, Taichung, Taiwan. shiau158@ms22.hinet.net.|Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan. shiau158@ms22.hinet.net.
An-Cheng
AC
Shiau
D000328:Adult; D000368:Aged; D015992:Body Mass Index; D001943:Breast Neoplasms / Q000532:radiotherapy*; D002285:Carcinoma, Intraductal, Noninfiltrating / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008875:Middle Aged; D011874:Radiometry; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012189:Retrospective Studies; D013525:Surgical Instruments; D035501:Uncertainty
Adjuvant radiotherapy; Breast cancer; Simultaneous integrated boost; Tumor bed uncertainty