Biomedical subjects

Markus Alber

Publications and source records attributed to Markus Alber.

5 recordsLinked to original sources

PubMed2004

Comparison between manual and automatic segment generation in step-and-shoot IMRT of prostate cancer.

PURPOSE: To compare two methods to generate treatment plans for intensity-modulated radiotherapy (IMRT) of prostate cancer, delivered in a step-and-shoot mode. The first method uses fluence optimization (inverse planning) followed by conversion of the fluence weight map into a limited number of segments. In the second method, segments are manually assigned using a class solution (forward planning), followed by computer optimization of the segment weights. METHODS: Treatment plans for IMRT, utilizing a simultaneous integrated boost, were created. Plans comprise a five-field technique to deliver 78 Gy to the prostate plus seminal vesicles. Five patients were evaluated. Optimization objectives of both planning approaches concerned dose coverage of the target volumes and the dose distribution in the rectal wall. The two methods were evaluated by comparing dose distributions, the complexity of the resulting plan and the time expenditure to generate and to deliver the plan. RESULTS: For both planning approaches 99% of the target volumes received 95% of the prescribed dose, which complies with our planning objectives. Inverse planning resulted in more conformal dose distributions than forward planning (conformity index: 1.37 versus 1.51). Inverse planning reduced the dose to the rectal wall compared to a manually designed plan, albeit to a small extent. The theoretical probability of severe rectal proctitis and/or stenosis was reduced on average by 1.9% with inverse planning. Maximal sparing of the rectal wall was achieved with inverse planning for a patient whose target volume was partly wrapped around the rectum. The number of segments generated with inverse planning ranged between 33 and 52, and between 9 and 13 segments for manually created segments. CONCLUSION: Dose coverage of the planning target volumes is adequate for both approaches of planning. Inverse planning results in slightly better dose distributions with respect to the rectal wall compared to manual planning, at the cost of an increase of the number of segments by a factor of 3.

Humans↗

PubMed2003

A revision of the gamma-evaluation concept for the comparison of dose distributions.

A method for the quantitative four-dimensional (4D) evaluation of discrete dose data based on gradient-dependent local acceptance thresholds is presented. The method takes into account the local dose gradients of a reference distribution for critical appraisal of misalignment and collimation errors. These contribute to the maximum tolerable dose error at each evaluation point to which the local dose differences between comparison and reference data are compared. As shown, the presented concept is analogous to the gamma-concept of Low et al (1998a Med. Phys. 25 656-61) if extended to (3+1) dimensions. The pointwise dose comparisons of the reformulated concept are easier to perform and speed up the evaluation process considerably, especially for fine-grid evaluations of 3D dose distributions. The occurrences of false negative indications due to the discrete nature of the data are reduced with the method. The presented method was applied to film-measured, clinical data and compared with gamma-evaluations. 4D and 3D evaluations were performed. Comparisons prove that 4D evaluations have to be given priority, especially if complex treatment situations are verified, e.g., non-coplanar beam configurations.

Algorithms↗

PubMed2003

A comparison of forward and inverse treatment planning for intensity-modulated radiotherapy of head and neck cancer.

BACKGROUND AND PURPOSE: To compare intensity-modulated treatment plans of patients with head and neck cancer generated by forward and inverse planning. MATERIALS AND METHODS: Ten intensity-modulated treatment plans, planned and treated with a step&shoot technique using a forward planning approach, were retrospectively re-planned with an inverse planning algorithm. For this purpose, two strategies were applied. First, inverse planning was performed with the same beam directions as forward planning. In addition, nine equidistant, coplanar incidences were used. The main objective of the optimisation process was the sparing of the parotid glands beside an adequate treatment of the planning target volume (PTV). Inverse planning was performed both with pencil beam and Monte Carlo dose computation to investigate the influence of dose computation on the result of the optimisation. RESULTS: In most cases, both inverse planning strategies managed to improve the treatment plans distinctly due to a better target coverage, a better sparing of the parotid glands or both. A reduction of the mean dose by 3-11Gy for at least one of the parotid glands could be achieved for most of the patients. For three patients, inverse planning allowed to spare a parotid gland that had to be sacrificed by forward planning. Inverse planning increased the number of segments compared to forward planning by a factor of about 3; from 9-15 to 27-46. No significant differences for PTV and parotid glands between both inverse planning approaches were found. Also, the use of Monte Carlo instead of pencil beam dose computation did not influence the results significantly. CONCLUSION: The results demonstrate the potential of inverse planning to improve intensity-modulated treatment plans for head and neck cases compared to forward planning while retaining clinical utility in terms of treatment time and quality assurance.

Head and Neck Neoplasms↗

PubMed2003

[Fluence-modulated radiotherapy with an optimization-integrated sequencer].

On the basis of two clinical cases, we present fluence-modulated radiotherapy with a sequencer integrated into the optimization of our treatment-planning software HYPERION. In each case, we achieved simple relations for the dependence of the total number of segments on the complexity of the sequencing, as well as for the dependence of the dose-distribution quality on the number of segments. For both clinical cases, it was possible to obtain treatment plans that complied with the clinical demands on dose distribution and number of segments. Also, compared to the widespread concept of equidistant steps, our method of sequencing with fluence steps of variable size led to a significant reduction of the number of segments, while maintaining the quality of the dose distribution. Our findings substantiate the value of the integration of the sequencer into the optimization for the clinical efficiency of IMRT.

Humans↗

PubMed2002

[A concept for the optimization of clinical IMRT].

The present paper introduces a concept for the description of treatment objectives of IMRT which emphasizes the assurance of an acceptable dose distribution in risk organs. A number of DVH manipulation tools are available to take into account both the volume effects of normal tissue and the influence of dose fractionation. The optimization of the dose distribution strictly obeys the prescribed risks of complications, as well as the limits of dose homogeneity in the target volume. The application of IMRT is made more efficient by limiting the modulation of the fluence profiles. The use of this algorithm could simplify IMRT in a way that a larger number of patients can profit from this type of treatment.

Humans↗