Purpose: The objective of this study is to quantify the loss of radiobiological effect during a protracted Partial Breast Irradiation treatment using HDR interstitial brachytherapy and the MammoSite balloon applicator. Method and Materials: Given the relatively short half‐life of Ir‐192, a range of dose rates are employed clinically. The position and size of target relative to radiation source differs for MammoSite and an interstitial implant thereby affecting the treatment time. Two treatment plans (one for each modality) were used to simulate the treatment delivery with variable source strengths (3–9 Ci) and treatment times (250–1,000 s). The radiobiological effect was quantified using the Biologically Effective Dose (BED) formalism for each voxel of the target. These values were then aggregated, thereby removing the dose non‐uniformity contribution, into an Equivalent Uniform Dose (EUBED). Two models were employed: BED0 that simply takes into account the total dose, fraction size and α/β ratio, and BED1 which accounts for repair and the delivery time sequence. The PTV for the MammoSite applicator was 86.3 cc, and that of the interstitial implant was 120.5 cc (16 catheters). Results: While the EUBED0 for both modalities, assuming uniform dose distributions is 62.9Gy (α/β=4Gy, fraction size=3.4Gy), the effect of non‐uniform distributions raises the EUBED0 to 74.6Gy and 76.0 Gy for the interstitial case and the MammoSite applicator, respectively. When repair and the delivery time sequence is considered, EUBED1 drops dramatically to 54.5Gy, 39.5Gy and 36.4Gy, for treatment times one, two and three times longer than the actual treatment time. Similar results were noted with the MammoSite treatment. Conclusion: There appears to be a significant loss of radiobiological effect with protracted HDR treatments. We are advocating recording of treatment times along with other dosimetric parameters as these may impact the clinical outcome.

Purpose: This is a proof of concept study with the objective of reconstructing the position of an HDR source in 3D in real‐time using a flat panel detector (FPD). It can potentially become the new standard in Quality Assurance (QA) for treatment delivery. Method and Materials: A matrix of markers (Ball Bearings 4mm in diameter) with precisely known locations was mounted on the cover of a flat panel detector (Acuity, Varian Inc) at variable height. Images acquired with the x‐ray source were used to calibrate the system. A plan with three dwell positions, well defined in 3D was created and delivered. Images were acquired with the FPD during the delivery of ‘treatment’. In house software was created to automatically segment and label the markers' images. A mathematical solution for the ‘near‐intersection’ of two 3D lines was implemented and used to determine the ‘true’ 3D source position. Each line was defined by the 3D positions of each marker and its projection on the FPD. A matrix with N markers will produce N*(N‐1)/2 points of intersection and their mean will result in a more accurate source position. The HDR source was placed on a 5cm solid water to mimic the patient and the FPD was placed at distances varying from 50 to 70cm. Results: The best imaging geometry was determined and images of markers obtained with the HDR source (strength of 6.2Ci) were properly segmented at all distances. During delivery, the source was located at [0,0,50], [0.5,0,50] and [2.0, 0, 50]. The reconstructed positions were [0,0,50.130], [0.497,−0.008,50.106] and [1.984, −0.005,50.053] with a standard deviation of [0.027,0.019,0.115]cm. When intersecting lines in 3D, the mean shortest distance between any two lines was 0.025cm with standard deviation 0.016cm. Conclusion: We proved that the accuracy of source position detection in 3D using a FPD is sub‐millimeter.

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