Motivation

The use of intensity modulated radiation therapy (IMRT) is known to be beneficial in terms of the organ at risk (OAR) dose sparing and as a result reducing once common toxicity. The quality of life after treatment can be increased by preserving critical organ function such as salivation and swallowing.
In fractionated therapy, however, patient positioning uncertainties and interfractional anatomical changes may limit the advantages of IMRT, e.g. if OARs shift into the high-dose region (see  figure below A). As a consequence, the OAR can receive higher doses than originally prescribed.

For that reason safety margins need to be added to the clinical target volumes to keep the tumor control probability high (see figure above B). These margins enlarge the high-dose region and thus worsen the dose distribution from the OARs point of view, since the distance between the avoidance structures and the dose gradients of the high-dose region is decreased. Further interfractional variations in the position of the OAR, which are known to occur in spite of elaborate patient fixation devices [4-6], may also lead to differences in the applied dose distribution.
To overcome these setup problems, image guidance can be used to correct for patient positioning errors prior to irradiation (see figure above C). Since the adaptation is performed with image-based strategies, it is called image guided radiotherapy (IGRT).

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Dosimetric consequences of interfractional motion

Typically, the correction parameter is a shift of the treatment table to re-position the shifted planned target point to the isocenter of the treatment device. The target point correction (TPC) is a wide-spread IGRT strategy and has proven its superiority in treatments of tumor sites with less pronounced deformations. Under deforming anatomy, however, it is still unclear how to deduce an optimal rigid shift correction. Currently, only geometrical position information is consulted prior to the TPC application. But the desired information is its impact on the changes of the dose distribution.
Therefore, we investigate interfractional variations in collectives of patients with treated tumor sites prone to deformations and their effect onto the difference between the prescribed and applied dose [1, 2].
The planned dose in form of DVHs of the spinal coord (blue) and the target volume (red) are plotted in one diagram (see figure below). The planned DVH is subtracted from the DVHs generated for each fraction (colored curves). These differences are plotted in the same diagram. This type of diagram is used to illustrate little dose deviations compared to a set of all DVH curves. The positive curve progressions of Image 2 (Margin-enlarged) imply that more volume of the spinal coord receives the respective dose compared to the correction with IGRT. Regarding the target volume both strategies allow a similar sustainment of the tumor coverage.

The presented study confirms that a correction with IGRT is beneficial in terms of spinal cord sparing in the presence of interfractional motion. This is achieved by a margin reduction of the planning target volume which results in a bigger distance between the cervical cord and the steep dose gradients.

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Publications

[1] Schwarz M, Giske K, Stoll A, Nill S, Huber PE, Desbus J, Bendl R, Stoiber EM: IGRT versus non-IGRT for postoperative head-and-neck cancer patients: dosimetric conseqeuences arising from a PTV margin reduction. Radiat Oncol 7(1):133, 2012

[2] Stoiber EM, Schwarz M, Huber PE, Debus J, Bendl R, Giske K:Comparison of two IGRT correction strategies in postoperative head-and-neck IMRT Patients. Acta Oncol (epub ahead of print), 2012

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