Intraosseous High Dose Rate Brachytherapy (HDR) with Intramedullary Nailing for Long Bone Metastases – A Feasibility Study

Abstract

Post-operative two-dimensional external beam radiotherapy (2D EBRT) is standardly given after intramedullary nailing of impending long bone fractures caused by metastases to reduce the risk of tumor progression and subsequent fracture/fixation loss. In this setting, 2D EBRT is delivered to the length of implanted hardware to treat all surgically contaminated bone that may harbor micrometastases. Consequently, a large volume of uninvolved soft tissue and initially uninvolved bone receives the prescription dose. We assess the feasibility of intraosseous HDR in association with surgical fixation prior to intramedullary nailing to deliver an ablative dose to a limited volume within and immediately surrounding tumor. We compare modeled HDR dose to conventional, post-operative 2D EBRT and pre-operative Stereotactic Ablative Radiotherapy (SAbR) for the same lesion. A human cadaver was used to assess the feasibility of implanting a single channel, 4.7 Fr metal-tipped HDR catheter in a femoral shaft tumor modeled on pre-operative CT. Under fluoroscopy, a 4 mm cannulated drill was used to enter the bone and insert the catheter via the greater trochanter into the marrow cavity. The catheter tip was advanced 10 mm beyond the distal extent of modeled tumor. The prescription dose was prescribed to the planning target volume (PTV) defined as gross tumor + bone marrow 4 mm proximal and 14 mm distal to the tumor, accounting for possible tumor spread during catheter insertion. Comparative post-operative 2D EBRT and pre-operative SAbR plans were created. For 2D EBRT, dose was delivered with AP/PA beams to a PTV defined as tumor and any femur containing surgical hardware + 5 mm margin. For SAbR, dose was delivered to the tumor + 3 mm with volumetric modulated arc therapy. Plans for all scenarios were created with EclipseTM (Varian Medical Systems). Parameters including soft tissue dose, conformity index (volume receiving ≥100% of the prescription dose / PTV), gradient index (volume receiving ≥50% of the prescription dose / PTV) and VX (absolute volume receiving ≥ dose X) were compared. Volumes (PTV) for 2D EBRT, SAbR, and HDR were 1010.6, 9.5, and 6.2 cc, respectively. With all PTVs covered by ≥95% of the prescription dose, the conformity indices for the 2D EBRT, SAbR, and HDR plans were 3.08, 1.1, and 2.43, respectively, while the gradient indices were 1.15, 0.78, and 0.64, respectively. Dose to soft tissue for 2D EBRT was highest with V100% (outside of the PTV), V50%, and V20% of 2465.7, 3784.7, and 4549.6 cc, respectively, compared to SAbR 0, 14.4, and 143.9 cc, respectively, and HDR 0, 7.7, and 111.7 cc, respectively. Intraosseous HDR prior to intramedullary nailing is feasible based on cadaveric simulation. Compact prescription dose can be delivered with steep dose falloff and accordingly low soft tissue dose. In vivo cases are forthcoming and will further assess feasibility and safety.