Frank Van den Heuvel

On the determination of the effective transmission factor for stainless steel ovoid shielding segments and estimation of their shielding efficacy for the clinical situation

Commercially available ovoid tubes for gynecological applications used in conjunction with the microSelectron-HDR (Nucletron International B.V., Waardegelder 1, 3905 TH Veenendaal, The Netherlands) for 192Ir sources, allow for shielding. Publications concerning the transmission properties of these 4.5-mm thick stainless steel (AISI number 303/304) shielding segments are scarce and not compatible for implementation in treatment planning. Therefore the effect of shielding on dose distribution is unknown. The effective transmission factor has been measured and implemented in the planning computations. Screening efficacy was evaluated on 20 actual treatment plans, analyzing dose reduction to critical tissue and comparing dose distribution in planes relevant for this particular application. Due to high transmission (effective transmission factor = 0.85), stainless steel screening segments only provide low, local dose reductions of maximum 15%. A new approach with regard to optimization and source configuration is needed to reduce dose to vulnerable tissue, exploiting the screening segments to a maximum extent. Better shielding, especially at the midline (plane bisecting the ovoids) could be expected by using shielding segments with other geometrical characteristics.

Dynamic radiotherapy: Interactive movement of patient couch for treatment of craniospinal axis

PURPOSE The various techniques that have been described for treatment of the craniospinal axis show the common challenge of edge matching between adjacent orthogonal and parallel photon beams. Such edge matching is needed because the maximum field length provided by modern treatment machines is generally insufficient to treat adults with less than three matching fields. Using the common techniques, field edge matching becomes difficult, if for medical reasons, the patient cannot be treated in the prone position. METHODS AND MATERIALS A scanning couch technique is proposed, with the patient lying in supine position. After treating the cerebral and upper neck regions by two lateral opposed half beam fields defined by asymmetric collimators (split beam), the patient is being moved along the spinal axis through an 8.0 cm wide by 15.0 cm long posterior split beam (allowing edge matching with the lateral fields at the neck region) by means of remote controlled couch movement. Stopping and starting of the scanning field resulted in a linear decrease of dose on both sides of the scan. Two ways of resolving this problem were investigated. RESULTS The administered dose varied less than 8.5% through the craniospinal axis. Flatness of the rectangular scanned field was 0.76%. Apart from dose homogeneity, patient comfort and decreased simulation time are major advantages. CONCLUSIONS The proposed technique represents a suitable alternative using a common linear accelerator, requiring a remote couch controller as an additional component.

Decomposition analysis of differential dose volume histograms: Quantification of DVH's

Dose volume histograms are a common tool to assess the value of a treatment plan for various forms of radiation therapy treatment. The purpose of this work is to introduce, validate, and apply a set of tools to analyze differential dose volume histograms by decomposing them into physically and clinically meaningful normal distributions. A weighted sum of the decomposed normal distributions (e.g., weighted dose) is proposed as a new measure of target dose, rather than the more unstable point dose. The method and its theory are presented and validated using simulated distributions. Additional validation is performed by analyzing simple four field box techniques encompassing a predefined target, using different treatment energies inside a water phantom. Furthermore, two clinical situations are analyzed using this methodology to illustrate practical usefulness. A comparison of a treatment plan for a breast patient using a tangential field setup with wedges is compared to a comparable geometry using dose compensators. Finally, a normal tissue complication probability (NTCP) calculation is refined using this decomposition. The NTCP calculation is performed on a liver as organ at risk in a treatment of a mesothelioma patient with involvement of the right lung. The comparison of the wedged breast treatment versus the compensator technique yields comparable classical dose parameters (e.g., conformity index ≈ 1 and equal dose at the ICRU dose point). The methodology proposed here shows a 4% difference in weighted dose outlining the difference in treatment using a single parameter instead of at least two in a classical analysis (e.g., mean dose, and maximal dose, or total dose variance). NTCP-calculations for the mesothelioma case are generated automatically and show a 3% decrease with respect to the classical calculation. The decrease is slightly dependant on the fractionation and on the α ∕ β -value utilized. In conclusion, this method is able to distinguish clinically important differences between treatment plans using a single parameter. This methodology shows promise as an objective tool for analyzing NTCP and doses in larger studies, as the only information needed is the dose volume histogram.