Dosimetric effects near implanted vascular access ports: an examination of external photon beam calculation.

Vascular access ports are used widely in the administering of drugs for radiation oncology patients. Their dosimetric effect on radiation therapy delivery in photon beams has not been rigorously established. In this work the effects on external beam fields when any of a variety of vascular access ports is included in the path of a high energy beam are studied. This medical physics study specifically identifies side-scatter and back-scatter consequences as well as attenuation effects. The study was divided into two parts: Firstly, a total of 18 ports underwent extended HU range CT scanning followed by 3-D computer treatment planning, where independent homogeneity and heterogeneity plans were created for photon beams of energy 6 MV and 18 MV using a Pencil Beam Convolution (PBC) algorithm. Dose points were analyzed at locations all around each device. A total of 1,440 points were reviewed in this section of the study. Secondly, a mock-up of the largest vascular access port was created in the treatment planning workspace for further investigation with alternative treatment planning algorithms. Plans were generated identically to the above and compared to the results of dose computation between the Pencil Beam Convolution algorithm, the Analytical Anisotropic Algorithm (AAA), and the EGSnrc Monte Carlo algorithm with user code DOSRZnrc (MC). A total of 300 points were reviewed in this part of the study. It was conclusive that ports with more bulky construction and those with partial metal composition create the largest changes. Similar effects are seen for similar port configurations. Considerable differences between the PBC and AAA in comparison to MC are noted and discussed. By thorough examination of planning system results, the presented vascular access ports may now be ranked according to the greatest amount of change exhibited within a treatment planning system. Effects of backscatter, lateral scatter and attenuation are up to 5.0%, 3.4% and 16.8% for 6 MV and 7.0%, 7.7% and 7.2% for 18 MV respectively.