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Dosimetric Impact of Setup Tolerance for Breast Radiation Therapy with 3D Surface Imaging

A Havnen-Smith*, Minneapolis Radiation Oncology, BURNSVILLE, MN

SU-E-J-71 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: 3D surface image guidance (3D SIG) has been shown to improve patient positioning accuracy in the treatment of breast cancer, but limited information is available regarding the dosimetric consequences of setup tolerances associated with breast radiation therapy techniques. The purpose of this study was to determine the magnitude of dose-delivery errors associated with setup tolerances of 3 mm/3deg and 5 mm/5deg for set-up and monitoring of whole breast radiation treatment using (3D SIG).

Methods: Five test patients were selected for direct simulation of the maximum acceptable deviation from reference position for tangential beams using field-in-field (FinF) techniques. Dosimetric impact was determined by simulating the maximum allowable patient translations and rotations, recalculating the dose distribution, and propagating the dosimetric error for a full treatment course. For the planned and off-set positions, dosimetric consequences were evaluated by examining the plan maximum dose, the percent coverage of the prescription dose for the lumpectomy bed, the V20Gy and V13Gy percentages for the ipsilateral lung, and the maximum point dose to the heart. The results are shown in Table I below.

Results: For both 3 mm/3deg and 5 mm/5deg off-sets in relatively large-breasted patients (> 500 cc) no significant change is observed. The ipsilateral lung V20Gy and V13Gy had a maximum increase of 5.4% and 5.7%, respectively. Maximum dose to the heart increased by a maximum of about 8% for those patients being treated to the left breast. The patient with the largest dosimetric impact was relatively small-breasted with a lumpectomy cavity in close proximity to the field borders.

Conclusions: As tumor bed coverage and ipsilateral lung dose were heavily impacted for the studied tolerances, it may be beneficial to utilize an appropriately tighter tolerance in patients with breast volume < 500 cc with tumor bed volumes near the periphery of the field apertures.

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