Treatment plan dosimetric impact after catheter stretch during multi-catheter interstitial breast brachytherapy

PURPOSE: Nylon 6/6 interstitial brachytherapy catheters may stretch when exposed to moisture, mechanical tension, and body temperature. The purpose of this study is to evaluate the magnitude of catheter stretch during a course of multi-catheter interstitial breast brachytherapy for nylon 6/6 treatment catheters, and to assess the impact this has on treatment plan dosimetry. MATERIAL AND METHODS: Catheters were exposed to water at 37°C for six days and the internal catheter length change (ΔL_W) was measured daily. Additionally, the measured internal catheter length change (ΔL_P), performed prior to each treatment fraction, for seven interstitial breast brachytherapy patients (total of 76 catheters) were retrospectively analyzed. The maximum length change seen in ΔL_P and ΔL_W were introduced as a source positional error to retrospective treatment plans, and treatment plan dosimetry was analyzed. RESULTS: ΔL_W and ΔL_P were on average +4.0% and +1.6% of the catheter lengths in water or tissue after 48 hours. Weak correlation was seen between the average ΔL_P per catheter and both the catheter length within tissue (ρ = 0.36, p = 0.0007), and the mid-catheter depth in tissue (ρ = 0.42, p < 0.0001). The D(90CTV) decreased 1.5% (p < 0.05) and 8.2% (p < 0.05) when the ΔL_P and ΔL_W were introduced to the initial plans. CONCLUSIONS: Nylon 6/6 catheters stretch during a course of multi-catheter interstitial breast brachytherapy treatment. The observed stretch may affect treatment plan dosimetry, if the catheter internal length is only measured immediately after the insertion. Additional catheter length checks are recommended to verify the actual catheter internal length during the treatment.