Session Item

Dosimetric evaluation of APBI treatment plans using the HDR SAVI applicator and TRIUMPH-T Trial dosing and planning criteria.

Ten (n=10) patients treated with the 6-1 regular Strut Adjusted Volume implant (SAVI) applicator (Cianna Medicals, Aliso Viejo CA) were selected for this study. Treatment planning was done in Oncentra Brachy (Elekta Brachy, Veenendaal, The Netherlands) using the TRIUMPH-T trial dosing (22.5 Gy in 3 fractions) and dosimetric criteria. The target (PTV-Eval) and organs at risk (OARs) were delineated following NSABP B-39 guidelines with the exception of skin, which was contoured as a 3 mm rind from the external contour, and chest wall (CW) which included ribs and pectoralis muscle, as per institutional preference. Ribs were contoured separately for DVH analysis as recommended in TRIUMPH-T trial. All plans were optimized to achieve treatment planning goals of PTV-Eval coverage of V90% ≥ 90% while limiting Dmax (maximum dose to D0.03cc) to skin and CW less than 100% and 120% of prescription dose, respectively.  V150% and V200% of the PTV-Eval were constrained to below 40 cc and 17 cc, respectively. The ribs Dmax was recorded for documentation. Additionally, a metric, PTV-ratio was defined as a ratio of PTV-Eval to PTV1cm (defined as the volume of breast tissue encompassing 1 cm expansion around the cavity) to study its correlation to achieved planning metrics, where the PTV-ratio, decreases with increasing overlap with OARs.

Result: Cavity+1cm volume averaged at 85.2 ± 3.8 cc which is directly relatable to the of the 6-1 SAVI applicator. PTV-Eval volume measured an average of 58.2 ± 5.1 cc. All plans included in this study met all the clinical treatment planning goals. V90 of PTV-Eval averaged at 96.9+3.2%.  The mean V150% and V200% were 24 ± 2.9 cc and 12.0 ± 1.5 cc respectively. Dmax to skin, CW and ribs were 93.4 ± 4.5 % and 103.0 ± 17 % and 71.4 ± 17.6 %, respectively. Plans with a smaller PTV-ratio volume (indicating closer proximity and overlap with OARs) met constraints marginally. The correlation coefficient between PTV-ratio and PTV-Eval V90, max dose to skin, CW and ribs were 0.87, -0.50, -0.70 and -0.45, respectively.

All plans in the study were able to meet treatment planning constraints outlined in TRIUMPH-T trial. SAVI-plans showed high conformality without compromising coverage (V90%) even with a high degree of proximity to critical structures. It was noted that PTV-ratio shows a strong positive correlation with V90% of PTV-Eval, a strong negative correlation with maximum dose to CW and moderate negative correlation with maximum dose to skin and rib. Use of other predictors such as distance measure coupled with the proposed PTV-ratio can be explored as evaluation criteria to indicate plan complexity and the optimal achievable plan quality, especially coverage.