Abstract
Utilization of high energy photons (>10 MV) with an optimal weight using a mixed energy technique is a practical way to generate a homogenous dose distribution while maintaining adequate target coverage in intact breast radiotherapy. This study represents a model for estimation of this optimal weight for day to day clinical usage. For this purpose, treatment planning computed tomography scans of thirty-three consecutive early stage breast cancer patients following breast conservation surgery were analyzed. After delineation of the breast clinical target volume (CTV) and placing opposed wedge paired isocenteric tangential portals, dosimeteric calculations were conducted and dose volume histograms (DVHs) were generated, first with pure 6 MV photons and then these calculations were repeated ten times with incorporating 18 MV photons (ten percent increase in weight per step) in each individual patient. For each calculation two indexes including maximum dose in the breast CTV (Dmax) and the volume of CTV which covered with 95% Isodose line (VCTV, 95%IDL) were measured according to the DVH data and then normalized values were plotted in a graph. The optimal weight of 18 MV photons was defined as the intersection point of Dmax and VCTV, 95%IDL graphs. For creating a model to predict this optimal weight multiple linear regression analysis was used based on some of the breast and tangential field parameters. The best fitting model for prediction of 18 MV photons optimal weight in breast radiotherapy using mixed energy technique, incorporated chest wall separation plus central lung distance (Adjusted R2=0.776). In conclusion, this study represents a model for the estimation of optimal beam weighting in breast radiotherapy using mixed photon energy technique for routine day to day clinical usage.
Highlights
Breast conservation surgery (BCS) followed by whole breast irradiation (WBI) is the standard of care for early stage breast cancer
For each calculation two indexes including maximum dose in the breast clinical target volume (CTV) (Dmax) and the volume of CTV which covered with 95% Isodose line (VCTV, ) 95%IDL were measured according to the dose volume histograms (DVHs) data and normalized values were plotted in a graph
With respect to the model we described above patients with higher SEP1 and central lung distance (CLD) in contrast to SEP2, have hotter areas in the breast and need more high energy photons for a more homogenous treatment plan
Summary
Breast conservation surgery (BCS) followed by whole breast irradiation (WBI) is the standard of care for early stage breast cancer. Numerous large scale well designed randomized clinical trials indicate that this treatment is comparable to mastectomy in terms of local tumor control and overall survival while it maintains patients cosmesis. The objectives are covering the breast with a therapeutic homogenous dose distribution while protecting healthy tissues from excessive irradiation and toxicity It is well known that acute and late complications of WBI such as erythema, edema, desquamation, pain, and telangiectasia and breast hardness are related to heterogeneous dose distribution in target volume and limiting areas of breast from receiving excessive dose (radiation hot spots) is of particular importance for achieving an acceptable long term cosmesis (Taylor et al, 1995; Das et al, 1997; Carruthers et al, 1999; Stillie et al, 2011)
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