Session Item

Among the radiation techniques for cutaneous malignancies treatment, the choice of brachytherapy is rapidly rising. Last year, in our department, more than 20 patients were enrolled for skin brachytherapy target. This approach is especially pointed at large targets or irregularly shaped lesions. Although its growing development, the AAPM protocols do not comprehensively cover surface brachytherapy and, currently, no Quality Assurance (QA) guideline are available.  The safety and accuracy of delivery is crucial, since of the lack of well-defined procedures, assuring the real dose to target correspondence. Skin-brachytherapy workflow may be error prone, with a wide liability related to the staff experience, especially with personalized surface mould. Dedicated phantoms for QA are not available, especially for curved surfaces. Further, in the TPS planning phase, an automatic pre-set for catheters reconstruction is not available, a manual approach is required. 

Aim of this work was to present a primary approach for a QA skin-plan. Ten patients (pts) were select for cutaneous brachytherapy with 2÷7 Gy of dose per fraction. The moulds, holding up to 8 catheters, were settled on a pt specific thermoplastic mask, to guarantee treatment reproducibility. For each pt a 3D plan was realized with TPS Oncentra Brachy, based on the TG 43 dose calculation formalism. A manual reconstruction was used in personalized mould planning. Treatments were delivered with Flexitron Unit by ¹⁹²Ir HDR source, driven in the mould. For all pts, two pre-treatment QAplan were replicated, holding the original reconstruction. In QAplan1 three random catheters were selected for the activation of their first dwell position with 15s of transit time. In QAplan2 all the first positions of catheters were switched-on, with 15s each. The EBT3 radiochromic films, covering all the catheter tips, were stuck below the tips of catheters and exposed to each QAplan. Three plans were realized to simulate brachytherpy skin (homogenous, asymmetric, concave) targets on a water-equivalent phantom, aiming to compare planar doses  with the exposed films placed on the top of the phantom. EBT3 films were calibrated with 6MV photons and exposed.

QAplan1 tests the correctness of channel mapping, while QAplan2 shows the match of blurring outline to the 3D plan rendering, together with the shape of catheter tips in the mould. The tests performed confirmed the safety of treatments for all pts and the accuracy of the reconstruction method for all the plans. An agreement of 10% between measured and planned dose distribution was found for all the simulation plans.

Waiting for a higher specific QA procedure committed for skin-brachytherapy, those checks, if performed in a pre-treatment phase, provide a simple, fast and inexpensive method, that can significantly reduce serious errors occurrence. This approach can contribute to improve patient safety and increase the agreement with the required dose distribution.