The effect of high density dental materials on dose distributions calculated by Acuros XB in head and neck cancers IMRT cases

Abstract

Acuros XB (AXB) has been implemented and available commercially for clinical use for several years. Different authors have reviewed the algorithm and demonstrated that AXB shows superior performance in terms of dose estimation accuracy. In some cases, patients may be implanted with high density materials, AXB solves the deterministic solution of linear Boltzmann transport equation, in which the physical data, e.g. Hounsfield unit and the type of material must be input to the treatment planning system in order to calculate the dose distribution. However, there are only a few clinical studies to evaluate the effect of high density material on dose distribution by AXB, and clinical data is still lacking. Moreover, most of the patients and their clinicians may not know the material of the dental implants. The universal assignment of material for permanent high density dental implants may contribute an uncertainty to the dose calculation. Thereupon the current study aims to investigate the impact of high density dental materials on dose distributions calculated by AXB in Head and Neck Cancer IMRT cases and hence provide clinical suggestion to unknown dental material assumption for radiotherapy planning.

Three materials were evaluated in this study, namely: titanium alloy, zirconia and stainless steel. 50 patients with high density dental implants and treated with Head & Neck (H&N) IMRT were evenly divided into two groups according to the location of implants. AXB was used to recalculate the dose distribution, originally computed using Analytic Anisotropic Algorithm (AAA). The dosimetric data among material models were compared statistically. In addition, the dose distribution calculated by AXB were verified with measurements of parallel plate ionization chamber and radiochromic films. 6 MV photon beams were delivered to a simple geometric phantom with a strip of material model. The results were presented with graphs and also evaluated by Gamma analysis.

There were no significant differences (P>0.05) among material models in the Planning Target Volume (PTV) coverage and the quality of conformity due to the dental implant for nasopharyngeal cancer (NPC) group (implant outside the PTV ). However, for the Non-NPC group (implant inside the PTV), a large discrepancy was obtained in all PTV parameters. There were statistically significant differences (P<0.05) in PTVmax, PTVmean, Conformation Number and volume covered with 70Gy (V70Gy) among material models. The dose coverage of PTV was highly deteriorated. A large portion of PTV was underdosed. For the stainless steel, the material with highest density, the V70Gy is below 70%, which is 25% poorer when compared with plans calculated by AAA. In the phantom study, ionization chamber and film measurements supported the dose perturbations modeled by AXB. Using a 3% and 3mm criteria Gamma analysis, passing rate was between 95.03% and 99.70% demonstrating that AXB calculation was in agreement with measurements in different material models.

The effect of high density dental material in H&N IMRT cases highly depends on the location of the PTV. For the case with implant outside the PTV, the impact is independent of the type of material and zirconia is recommended for material assumption. However, for the cases with implant inside the PTV, assumption of material should not be made without proper investigation.