Evert Woudstra

Calculation of a pencil beam kernel from measured photon beam data

Usually, pencil beam kernels for photon beam calculations are obtained by Monte Carlo calculations. In this paper, we present a method to derive a pencil beam kernel from measured beam data, i.e. central axis depth doses, phantom scatter factors and off-axis ratios. These data are usually available in a radiotherapy planning system. The differences from other similar works are: (a) the central part of the pencil beam is derived from the measured penumbra of large fields and (b) the dependence of the primary photon fluence on the depth caused by beam hardening in the phantom is taken into account. The calculated pencil beam will evidently be influenced by the methods and instruments used for measurement of the basic data set. This is of particular importance for an accurate prediction of the absorbed dose delivered by small fields. Comparisons with measurements show that the accuracy of the calculated dose distributions fits well in a 2% error interval in the open part of the field, and in a 2 mm isodose shift in the penumbra region.

Calculation models for determining the absorbed dose in water phantoms in off-axis planes of rectangular fields of open and wedged photon beams

Beam models are proposed for the calculation of the dose in off-axis planes of rectangular photon fields, when the data set used in the treatment planning system is based on the simple storage model of Milan and Bentley. For open beams the model separates the off-axis ratio into an envelope profile and two boundary profiles. The envelope profile gives the field intensity of the maximal position of the jaws and has rotational symmetry. The boundary profiles describe the boundaries of the field actually formed by the jaws. In the case of a wedged beam, the model also separates the off-axis ratio into envelope profiles and boundary profiles. To determine these profiles for the non-wedge direction from open beam profiles, the wedge thickness is converted to an equivalent water thickness. In the case of an asymmetric field, the boundary profiles are shifted to the field centre. Results of calculation with these models have been compared with measurements and the simple multiplication of profiles, which has often been used with the Milan-Bentley model. The new models agree within a few per cent with the measurements and are a great improvement compared to the simple multiplication of profiles.

Constrained treatment planning using sequential beam selection

In this paper an algorithm is described for automated treatment plan generation. The algorithm aims at delivery of the prescribed dose to the target volume without violation of constraints for target, organs at risk and the surrounding normal tissue. Pre-calculated dose distributions for all candidate orientations are used as input. Treatment beams are selected in a sequential way. A score function designed for beam selection is used for the simultaneous selection of beam orientations and weights. In order to determine the optimum choice for the orientation and the corresponding weight of each new beam, the score function is first redefined to account for the dose distribution of the previously selected beams. Addition of more beams to the plan is stopped when the target dose is reached or when no additional dose can be delivered without violating a constraint. In the latter case the score function is modified by importance factor changes to enforce better sparing of the organ with the limiting constraint and the algorithm is run again.

Mixing intensity modulated electron and photon beams: combining a steep dose fall-off at depth with sharp and depth-independent penumbras and flat beam profiles

For application in radiotherapy, intensity modulated high-energy electron and photon beams were mixed to create dose distributions that feature: (a) a steep dose fall-off at larger depths, similar to pure electron beams, (b) flat beam profiles and sharp and depth-independent beam penumbras, as in photon beams, and (c) a selectable skin dose that is lower than for pure electron beams. To determine the required electron and photon beam fluence profiles, an inverse treatment planning algorithm was used. Mixed beams were realized at a MM50 racetrack microtron (Scanditronix Medical AB, Sweden), and evaluated by the dose distributions measured in a water phantom. The multileaf collimator of the MM50 was used in a static mode to shape overlapping electron beam segments, and the dynamic multileaf collimation mode was used to realize the intensity modulated photon beam profiles. Examples of mixed beams were generated at electron energies of up to 40 MeV. The intensity modulated electron beam component consists of two overlapping concentric fields with optimized field sizes, yielding broad, fairly depth-independent overall beam penumbras. The matched intensity modulated photon beam component has high fluence peaks at the field edges to sharpen this penumbra. The combination of the electron and the photon beams yields dose distributions with the characteristics (a)-(c) mentioned above.

Automated beam angle and weight selection in radiotherapy treatment planning applied to pancreas tumors

PURPOSE To extend and investigate the clinical value of a recently developed algorithm for automatic beam angle and beam weight selection, for irradiation of pancreas tumors. METHODS AND MATERIALS The algorithm aims at generation of acceptable treatment plans, i.e., delivering the prescribed tumor dose while strictly obeying the imposed hard constraints for organs at risk and target. Extensions were made to minimize the beam number and/or to escalate the tumor dose. For 5 pancreas patients, the clinical value and the potential for beam number reduction and dose escalation were investigated. Comparisons were made with clinical plans and equiangular plans. RESULTS Compared to clinical plans, the generated plans with the same number of beams yielded a substantial reduction in the dose to critical tissues. Using the algorithm, an escalated tumor dose of 58 Gy could be achieved for two cases. Maximum dose escalations required a minimum of 3 to 4 beam orientations. For 13 CT slices and an in-slice resolution of 0.5 cm, the total calculation times were 23-55 min, including precalculation of 180 input dose distributions (15 min). CONCLUSIONS The algorithm yielded acceptable treatment plans with clinically feasible numbers of beams, even for escalated tumor doses. Generated plans were superior to the clinically applied plans and to equiangular setups. Calculation times were clinically acceptable. The algorithm is now increasingly used in clinical routine.

Calculation of absorbed dose distributions from dynamic wedges

In radiotherapy with photon beams, the use of dynamic wedges, which are obtained by the movement of one of the jaws, offers an increasing flexibility relative to the traditional use of metal wedges. But it is a disadvantage for the measurement of absorbed dose distributions, because the absorbed dose at each measurement point can only be obtained after a complete movement of the jaw. Consequently, for radiotherapy planning, an algorithm should be available that does not require measurements for any specific dynamically wedged beam, but is based on only a modest number of measurements. In this paper, an algorithm for the calculation of the dose distribution from dynamic wedges is described. This algorithm uses the convolution of pencil beam kernels with a non-uniform field function. These pencil beam kernels are derived from empirical data resulting from measurements of the open beam only.

Radiation-induced bilateral optic neuropathy in cancer of the nasopharynx. Case failure analysis and a review of the literature.

Case ReportA case history of unanticipated radiation-induced bilateral optic neuropathy, 18 months after induction chemotherapy and radiation therapy for a locally advanced nasopharyngeal carcinoma, is presented. Retrospective reanalysis of the radiation therapy technique, with emphasis on the doses received by the optic pathway structures, was performed. These re-calculations revealed unexpectedly high doses in the range 79 to 82 Gy (cumulative external and brachytherapy dose) at the level of the optic nerves, which explained the observed radiation injury.ConclusionRoutine implementation of computed tomography for 3D dose planning purposes is therefore advocated. Review of the current literature confirms the importance of 3D dose planning in avoiding this complication and highlights the role of MRI in establishing the diagnosis of radiation-induced optic neuropathy.ZusammenfassungKasuistikEin Fallbericht mit unerwarteter strahleninduzierter beidseitiger Optikusneuropathie 18 Monate nach eingeleiteter Chemo und Strahlentherapie wegen eines lokal fortgeschrittenen Nasopharynxkarzinoms wird prÄsentiert. Eine retrospektive Analyse der Radiotherapietechnik mit Berechnung der Dosisbelastung der Sehbahnstrukturen wurde durchgeführt. Aus dieser Berechnung wurde deutlich, da\ die Komplikation durch eine unerwartet hohe Dosis in den Sehnerven (79 bis 82 Gy) verursacht wurde.Schlu\folgerungEine routinemÄ\ige Durchführung einer Computertomographie für die dreidimensionale (Dosis-) Planung wird befürwortet. Ein Rückblick auf die aktuelle Literatur bestätigt die Notwendigkeit einer dreidimensionalen (Dosis-)Planung, um diese Komplikationen zu vermeiden. Die Rolle des MRI beim Nachweis der Diagnose der strahleninduzierten Optikusneuropathie ist hervorzuheben.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system.

PURPOSE Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. METHODS Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. RESULTS Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. CONCLUSIONS Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.

Sharpening the penumbra of high energy electron beams with low weight narrow photon beams

BACKGROUND AND PURPOSE High energy (20-50 MeV) electron beams, available from the MM50 Racetrack Microtron, can be used for the treatment of deep-seated tumors. A disadvantage is the increasing penumbra width as a function of depth. By the addition of a narrow (typically 1 cm wide) photon beam near the field edge, the 50-90% penumbra width of the electron beam is reduced, yielding a significantly increased effective field size. MATERIALS AND METHODS For rectangular electron beams in a water phantom (energies 25 and 40 MeV, field sizes 5 x 5-15 x 15 cm2) a computer program was used to optimize the photon beam parameters (position, weight and width) to obtain a combined beam with the sharpest penumbra at the optimization depth and a beam flatness within certain constraints. The study furthermore included penumbra sharpening of an irregular multileaf collimator-shaped field. RESULTS AND CONCLUSION At optimization depths near R90, photon beam addition reduces the penumbra width by 40-50% (from 15-20 mm to 8-10 mm). Beam flatness at the optimization depth is within +/-5% and hot-spots are < or =120% for all depths. By the addition of narrow photon beams around the rectangular or irregular field, the electron field width can be reduced by 1-3 cm, while the effective field size is maintained.

A comparison of an algorithm for automated sequential beam orientation selection (Cycle) with simulated annealing

Some time ago we developed and published a new deterministic algorithm (called Cycle) for automatic selection of beam orientations in radiotherapy. This algorithm is a plan generation process aiming at the prescribed PTV dose within hard dose and dose-volume constraints. The algorithm allows a large number of input orientations to be used and selects only the most efficient orientations, surviving the selection process. Efficiency is determined by a score function and is more or less equal to the extent of uninhibited access to the PTV for a specific beam during the selection process. In this paper we compare the capabilities of fast-simulated annealing (FSA) and Cycle for cases where local optima are supposed to be present. Five pancreas and five oesophagus cases previously treated in our institute were selected for this comparison. Plans were generated for FSA and Cycle, using the same hard dose and dose-volume constraints, and the largest possible achieved PTV doses as obtained from these algorithms were compared. The largest achieved PTV dose values were generally very similar for the two algorithms. In some cases FSA resulted in a slightly higher PTV dose than Cycle, at the cost of switching on substantially more beam orientations than Cycle. In other cases, when Cycle generated the solution with the highest PTV dose using only a limited number of non-zero weight beams, FSA seemed to have some difficulty in switching off the unfavourable directions. Cycle was faster than FSA, especially for large-dimensional feasible spaces. In conclusion, for the cases studied in this paper, we have found that despite the inherent drawback of sequential search as used by Cycle (where Cycle could probably get trapped in a local optimum), Cycle is nevertheless able to find comparable or sometimes slightly better treatment plans in comparison with FSA (which in theory finds the global optimum) especially in large-dimensional beam weight spaces.