Elsevier

Radiotherapy and Oncology

Volume 86, Issue 2, February 2008, Pages 276-284
Radiotherapy and Oncology

Treatment planning
Clinical implications of the anisotropic analytical algorithm for IMRT treatment planning and verification

https://doi.org/10.1016/j.radonc.2008.01.011Get rights and content

Abstract

Purpose

To determine the implications of the use of the Anisotropic Analytical Algorithm (AAA) for the production and dosimetric verification of IMRT plans for treatments of the prostate, parotid, nasopharynx and lung.

Methods

72 IMRT treatment plans produced using the Pencil Beam Convolution (PBC) algorithm were recalculated using the AAA and the dose distributions compared. Twenty-four of the plans were delivered to inhomogeneous phantoms and verification measurements made using a pinpoint ionisation chamber. The agreement between the AAA and measurement was determined.

Results

Small differences were seen in the prostate plans, with the AAA predicting slightly lower minimum PTV doses. In the parotid plans, there were small increases in the lens and contralateral parotid doses while the nasopharyngeal plans revealed a reduction in the volume of the PTV covered by the 95% isodose (the V95%) when the AAA was used. Large changes were seen in the lung plans, the AAA predicting reductions in the minimum PTV dose and large reductions in the V95%. The AAA also predicted small increases in the mean dose to the normal lung and the V20. In the verification measurements, all AAA calculations were within 3% or 3.5 mm distance to agreement of the measured doses.

Conclusions

The AAA should be used in preference to the PBC algorithm for treatments involving low density tissue but this may necessitate re-evaluation of plan acceptability criteria. Improvements to the Multi-Resolution Dose Calculation algorithm used in the inverse planning are required to reduce the convergence error in the presence of lung tissue. There was excellent agreement between the AAA and verification measurements for all sites.

Section snippets

Treatment planning

Twenty prostate treatment plans were produced, using a fixed beam arrangement with fields at gantry angles 180°, 100°, 45°, 315° and 260° (IEC1217). The prescribed dose was 60 Gy in 20 fractions. The organs at risk (OARs) considered in the inverse planning process were the rectum, the bladder and the femoral heads. The tolerance doses were for no more than 50% of the bladder or rectum to receive 57 Gy and no more than 2.0 cm3 of the femoral heads to receive 55 Gy.

Eighteen head and neck plans were

Treatment planning

Example DVHs for a prostate plan, a nasopharynx plan and a lung plan are shown in Fig. 1. The results of the comparison of the IMRT treatment plans as calculated by the two algorithms are shown in Table 1.

In the prostate plans, the AAA predicted lower doses than the PBC algorithm, including slightly lower coverage by the 95% isodose. However, of those differences that were statistically significant, only the reduction of 0.6% in the minimum PTV dose was of any clinical significance. A similar

Discussion

If the AAA is taken to provide a more accurate representation of the dose distribution from a given treatment, the comparisons between the AAA and PBC dose distributions in Table 1 provide an indication of the difference between the dose predicted by the PBC algorithm and that actually delivered.

In the prostate plans, the distributions were found to be very similar, with no differences between the OAR doses that were considered clinically significant. In all but three patients, the AAA

Conclusions

The AAA was, in general, not found to significantly alter the quality of IMRT treatment plans for the prostate, parotid or nasopharynx compared to the PBC algorithm. However, some small differences were seen and the importance of evaluating treatment plans on an individual basis cannot be overemphasised. The PBC algorithm overestimated the PTV coverage in IMRT plans for NSCLC, the AAA’s more accurate modelling of lateral electron transport demonstrating significant increases in the volume of

References (28)

  • B.H. Shahine et al.

    Experimental evaluation of interface doses in the presence of air cavities compared with treatment planning algorithms

    Med Phys

    (1999)

  • Tissue inhomogeneity corrections for megavoltage photon beams, AAPM Report 85

    (2004)
  • A.O. Jones et al.

    Comparison of inhomogeneity correction algorithms in small photon fields

    Med Phys

    (2005)
  • P. Sandilos et al.

    Technical note: evaluation of dosimetric performance in a commercial 3D treatment planning system

    Br J Radiol

    (2005)

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