Assessing the targeting and dosimetric impact of a novel bioabsorbable marker for external beam APBI R. Sims, J. Gardner, C. Benjamin and T. Milne Auckland Radiation Oncology, 98 Mountain Road, Epsom, Auckland, New Zealand (email:
[email protected]) Track: Physics, treatment planning: applications
Introduction
BZ = 2.92 ± 0.28mm NBZ = 8.79 3.58mm
Studies have demonstrated that both hypo-fractionated radiation therapy and accelerated partial breast irradiation (APBI) is safe and effective for patients with early stage breast cancer [1, and references therein]. Despite the advantages of these regimes relatively large margins are often required to ensure adequate coverage due to the large variation between experts using traditional targeting techniques [2,3]. The novel bio-absorbable marker BioZorb™, developed for breast surgery to indicate the surgical site of tumour removal, has since been shown to be useful in radiation treatment planning and delivery [4]. We have performed a retrospective planning study to investigate the impact this marker would have on target definition and radiation dosimetry at our clinic, compared to the more traditional seroma and clip-based techniques.
BZ = 0.87 ± 0.04 NBZ = 0.48 ± 0.18 BZ = -0.43 ± 1.01cc NBZ = 22.74 ± 15.95cc
BZ = 1.01 ± 0.67mm NBZ = 7.37 ± 4.9mm
BioZorb™ The BioZorb™ marker was developed by Focal Therapeutics (Aliso Viejo, CA) and consists of a semi-rigid, bioabsorbable spiral framework made of polylactic acid (PLA) with a fixed array of six titanium clips [4]. It was originally designed as a scaffold during reconstructive surgery with radio-opaque markers being used for subsequent imaging and follow-up [5].
http://www.focalrx.com/
Figure 1 – BioZorb™ marker and the visualisation on CT (also showing inter-user variation).
Materials and Methods All patients had partial mastectomy for T1 N0 M0 stage breast cancer (ductal carcinoma in-situ or invasive ductal carcinoma). One patient cohort was implanted with the BioZorb™ implant at the time of partial mastectomy (n=23, BZ), while the other had partial mastectomy alone (n=23, NBZ). Two clinical staff members independently contoured the volumes for the surgical bed using either the BZ or standard seroma based methods (NBZ). Radiation treatment plans were then created for each contour set for each patient using the RayStation® treatment planning system (TPS) according to [6] using a VMAT technique, creating a total of 92 treatment plans. Each plan was subject to institutional peer review by an independent planner to ensure the clinical goals were aligned with [6]. The inter-user variation in target delineation was then determined for each patient cohort using contour analysis tools available in ImSimQA® (Oncology Systems Limited, UK). The resulting dosimetric variation within, and between, each patient cohort was analysed using RayStation® and in-house scripting. Statistical significance was performed using an unpaired students t-test and Mann-Whitney test depending on normality of data.
Figure 2 – Box and whisker plots illustrating the inter-user variation within the BZ (blue) and NBZ (red) patient cohort for the dice similarity coefficient (UL), mean distance to agreement (UR), centre of gravity distance (LL) and volume difference (LR). Also shown is the average value for each cohort with ±1SD.
IDL 50% 95% 100%
DSC BZ Cohort 0.98±0.02 0.96±0.02 0.93±0.04
DSC NBZ Cohort 0.80±0.08 0.75±0.10 0.67±0.11
Table 1 – Dice similarity coefficient (DSC) when comparing the 50%, 95% and 100% isodose levels for both experts for both patient cohorts (1SD).
IDL
Vol. Ipsi-Br. BZ
Vol. Ipsi-Br. NBZ
50%
15.4 ± 4.7%
26.6 ± 7.7%
100%
3.9 ± 1.2%
6.3 ± 3.2%
Table 2 – The fractional volume of ipsi-lateral breast covered by the IDL shown for both cohorts (1SD).
Results Inter-user variation for the GTV is shown in Figure 2 for four different volume and distance-based metrics (see caption). The results support the hypothesis that the inter-user variation for the BZ cohort is significantly improved compared to the NBZ cohort (all metrics p
