The dosimetric verification of a pencil beam based treatment planning system

Abstract

A new three-dimensional treatment planning system (TPS) based on convolution/superposition algorithms (TMS-Radix from HELAX AB, Uppsala, Sweden) was recently installed at the University Hospital in Lund. The purpose of the present study was to design a quality assurance and acceptance testing programme to meet the specific characteristics of this convolution model. The model is based on parametrization of a non-measurable quantity-the polyenergetic pencil beam. However, the verification of the treatment planning model is still dependent on numerous comparisons of measured depth-doses and dose profiles. The test programme was divided in two basic parts: (i) model implementation and beam data consistency and (ii) model performance and limitations in special situations. The first part was scheduled for all photon beam qualities available before they could be used for clinical treatment planning. The second part was performed for selected energies only. The results indicate clearly that the model is well suited for clinical three-dimensional dose planning and that the TPS handles data as expected. For example, calculated depth-doses for open and wedge beams at depths larger than the depth of dose maximum and profiles for open beams shows a very good agreement with measurements. However, depth-dose deviations at shallow depths, especially for high energies, were found. Monitor units calculated by the system were accurate for most fields except for very large fields, where deviations of several per cent were found.

Dosimetric comparison of pencil beam and Monte Carlo algorithms in conformal lung radiotherapy

“…A patient's body contains different densities, requiring a correction factor for each beam causing beam attenuation 2. The PB algorithm is very fast due to its use of a one‐dimensional density correction, which does not accurately model the distribution of secondary electrons in heterogeneous media,3, 4 while its limitations in a heterogeneous medium are well known. The PB algorithm considers collimated photon beams incident on the patient as clusters of many small, narrow “pencil beams” each of which has central axis ray deposits with certain doses.…”

Verification of dose calculations with a clinical treatment planning system based on a point kernel dose engine

“…These kernels are generated by integrating Monte Carlo (MC) calculated point kernels (energy deposition in an infinite medium around a primary photon interaction point) over depth. A number of previous investigations 5 – 11 studied the accuracy and limitations under various conditions with this particular type of model 00012 implemented in a commercial TPS (Helax‐TMS, MDS Nordion Therapy Systems, Uppsala, Sweden).…”

“…The integration volume for phantom‐scatter calculation is dependent on phantom/patient contour and can, in some cases, give rise to deviations between measured and calculated doses, as has been shown in specific phantom geometries 6 8 Another problem is related to the spatially invariant pencil‐beam kernel not handling off‐axis softening 5 . Incorrect dose calculations in the build up region5 have also been noted.…”

Dosimeter Gel and MR Imaging for Verification of Calculated Dose Distributions in Clinical Radiation Therapy

“…The performance of the specific treatment planning system used in this study have recently been tested for single photon and electron beams (32,33). Using a similar photon beam as in this study, only small differences were found when calculated depth dose curves and dose profiles were compared with diode measurements (32).…”