SU‐GG‐T‐02: Clinical Application of the ENAL Set‐Up Correction Protocol to Compensate for Time Trends in Breast Cancer Treatments

Purpose: Clinical evaluation of the extended NAL (eNAL) set‐up protocol (deBoer2007) for breast cancer patients treated with an integrated boost technique. Method and Materials: For 80 breast cancer patients, two orthogonal planar kilovoltage images and one megavoltage image (for the medio‐lateral beam) were acquired per fraction throughout the treatment course (14 fractions on average). Based on registration of surgical clips in the lumpectomy cavity (4.3 on average) set‐up corrections were derived after the first three fractions and updated once a week thereafter using eNAL. The stability of the clips during the fractionated treatment was derived. Using a t‐test the correlation between clip migration and either the method of surgery or the time elapsed from last surgery was quantified. The impact of the eNAL protocol on the set‐up accuracy for both the tumor bed and the whole breast was evaluated. Results: During the fractionated treatment the mean distance between the clips and their center of mass (COM) reduced by 0.9 ± 1.2 mm (1 SD). The clip migration was not statistically different between patients treated within 100 days after surgery or afterwards (p=0.20). Compared to conventional breast surgery (closing the lumpectomy cavity superficially), clip migration after oncoplastic surgery (suturing the lumpectomy cavity) was slightly smaller, but not significantly different (p=0.13). Throughout the treatment course timetrends in the COM position of the clips >3mm were observed for 61% of the patients. Application of the eNAL protocol on clips resulted in residual systematic errors for the tumor bed of <1 mm in each direction, while the whole breast was treated within about 2 mm accuracy. Conclusion: Surgical clips can safely be used for position verification and correction. By compensating for time trends, the eNAL protocol resulted in better set‐up accuracies for both the tumor bed and the whole breast than the NAL protocol.

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