Abstract
The presence of air/fluid surrounding implantable devices used for partial breast irradiation may significantly impact dose coverage to at‐risk tissue. Of the 67 total patients retrospectively evaluated for this study, 32 (48%) had greater than 1 cc volume of air/fluid extending outside of the strut‐adjusted volume implant (SAVI) device surface and were selected for comparison of planning approaches. The planning approaches utilized two different definitions of PTV_EVAL. One definition of a PTV_EVAL (PTV_EVALSAVI) was based on expanding 1 cm beyond the SAVI device only while accounting for the air/fluid using the NSABP Protocol B‐39/RTOG Protocol 0413. The second PTV_EVAL definition (PTV_EVALCAV) was based on expanding 1 cm beyond the cavity (SAVI device plus air/fluid volume). The results indicate use of the B‐39 formalism to account for air/fluid displacing the PTV_EVAL may overestimate the dose coverage to the at‐risk tissue, especially for large contiguous volumes of air/fluid. Using the SAVI device to optimize dose covering the PTV_EVALCAV volume surrounding the cavity improves dosimetric coverage to at‐risk tissue by 11.3% and 8.7% for V100 and V90, respectively, while the average V150 and V200 indices for PTV_EVALCAV increased by 9.1 cc and 5.0 cc, respectively, and the average maximum rib and skin doses increased by 11. 1% and 6.1%, respectively. The maximum skin dose, rib dose, V150, and V200 all met the planning objectives despite any increase in these parameters.PACS number: 87.55.kh
Highlights
Whole or partial breast irradiation (PBI) methods are clinically available for breast conservation therapy following lumpectomy, with accelerated partial breast irradiation (APBI) an option for many patients
Multicatheter interstitial therapy dates back several decades and has the most mature clinical data.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] The typical dose of 34 Gy in 10 fractions twice daily (BID) for HDR was based on equivalence of the biological effective dose (BED) of a low-dose rate interstitial delivery of 45 Gy in 4.5 days used in early APBI trials.[1]
Patient selection The strut-adjusted volume implant (SAVI) applicator is available in four sizes to accommodate various cavity volumes: the 6-1, 6-1 mini, 8-1, and 10-1, and their physical characteristics have been described in the literature.[23]. Patients implanted with any size of SAVI device were considered for this work, including patients with a device-to-skin distance less than 5 mm
Summary
Whole or partial breast irradiation (PBI) methods are clinically available for breast conservation therapy following lumpectomy, with accelerated partial breast irradiation (APBI) an option for many patients. Accelerated partial breast irradiation is typically delivered using high-dose-rate (HDR) brachytherapy or using 3D conformal radiotherapy (3D CRT), and delivers a higher dose per fraction over a shorter period of time to the tumor bed plus a 1–2 cm margin.[1] HDRbased approaches include multicatheter interstitial, balloon based systems, hybrid brachytherapy systems, and focused external irradiation.[2]. Multicatheter interstitial therapy dates back several decades and has the most mature clinical data.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] The typical dose of 34 Gy in 10 fractions twice daily (BID) for HDR was based on equivalence of the biological effective dose (BED) of a low-dose rate interstitial delivery of 45 Gy in 4.5 days used in early APBI trials.[1]. A recent study[26] focused on the dosimetric effects of air pocket sizes outside of MammoSite balloons, providing support for the 10% rule commonly used per the NSABP Protocol
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