SU-E-T-181: Clinical Implementation of Task Group 176

Authors


Abstract

Purpose:

The implementation of TG-176 with regards to immobilization devices and couch tops as they effect dosimetric treatment planning.

Methods:

The external devices used clinically were scanned to measure their HU values. Plans were created in the Eclipse treatment planning system (TPS) using these devices, one that accounted for the correct HU value of the each device and another that did not include the device as a structure. A dose subtraction was performed between the two plans to evaluate the dosimetric differences. The metrics used for evaluation included attenuation and surface dose. Plan parameters were varied to evaluate the impact of the devices in different clinical scenarios.

Results:

While the exact HU values of our results are clinic-dependent, the protocol being implemented is widely applicable. We recommend a four step process for implementing this task group. First, physics should scan each treatment device to determine accurate HU values. Second, CT therapists should include in the setup note which table top was used during patient CT simulation and are asked to make immobilization devices as uniform in thickness as possible. Therapists should also index the devices whenever possible so beam will traverse the same area of the device. Third, the dosimetrist should manually correct the HU value for any external device, including the couch. For H&N cases, the rails must be removed from the couch structure. When rails are used during treatments, it is important to make note of their exact position in the setup notes. Finally, physicians should be made aware that there could be changes in surface doses depending on whether or not immobilization devices or couch tops are in the beam path.

Conclusion:

The protocol outlined above was implemented to reduce the errors that arise from ignoring effects of external devices, thus ensuring safer, more accurate patient treatments.

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