Session Item

Our aging population means the number of patients with bilateral hip prostheses is increasing. Such high-density implants present multiple challenges when planning a radiotherapy treatment. The majority of commercial dose calculation algorithms are unable to correctly model electron scatter at the implant-tissue interface. The aim of this work is to quantify these errors for Eclipse’s analytic anisotropic algorithm (AAA) and to use the results to inform a safe and efficient approach to treatment planning.

Following a literature search, we could not find data comparing the AAA with measurements downstream of a metallic inhomogeneity for a 6MV beam. We irradiated a hip prosthesis of mean density 3.5g/cc inside a water tank using a 10cmx10cm beam and took measurements using a Farmer chamber and gafchromic film. The water tank containing the prosthesis was scanned using CT, so that the measurements could be compared to doses calculated using the AAA.

The doses calculated using AAA were consistently below the Farmer chamber measurements, with an error of -3.1% 1.3cm downstream of the prosthesis, reducing to -0.1% 9.3cm downstream of the prosthesis. The measurements and calculated doses approach each other as the distance from the prosthesis increases. Similar behaviour is seen for the film measurements. This is in contrast to previously published data for 18MV, where the AAA was compared to film measurements when irradiating a metal slab in water [1]. The different behaviour could be due to the different geometries, particularly if the slab had a cross sectional area larger than the field size. Our film measurements at the upstream prosthesis-water interface indicate a large error of -15.6% in line with the existing literature.

The large backscatter peak at the upstream tissue-prosthesis interface not modelled by the AAA should be borne in mind when planning treatments involving nodes, for which the PTV may directly abut the prosthesis. In our department we have decided to keep 105% hotspots at least 1cm from the prosthesis. Furthermore the relatively low errors several cm downstream of the prosthesis suggest it is not necessary to completely prevent the beam from entering the PTV through the prosthesis. We use a VMAT planning approach with strongly weighted low dose objectives on the hip PRVs to minimise the proportion of the beam that enters the PTV through the prosthesis. This minimises the errors associated with the AAA calculation whilst covering the target. Allowing a small fraction of the dose to enter the PTV through the prosthesis has allowed us to achieve V95%=99.4% (average over 5 test plans) compared to V95%=96.2% for our previous fixed field IMRT approach, where no dose was permitted to enter through the prosthesis.

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