Adaptive Field Aperture Modification for the Management of Patient Setup Errors

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

To investigate the method and effectiveness of MLC aperture adjustment for compensating patient's set up error.

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Materials/Methods 

In recent years, technological advances have revolutionized radiation therapy planning, beam delivery and on-line patient imaging. However the method of compensating for patient positioning error is still handled in the traditional way: by moving the patient and/or the treatment couch. A number of limitations are associated with this method, including patient disturbance and additional imaging to confirm the couch shifts, which requires additional time and increases the patient's imaging dose. We have investigated the feasibility of an alternate method of compensation for patient set up variation, termed as adaptive field modification (AFM). AFM involves the on-line adjustment of the treatment field aperture. The method of adjusting the MLC apertures has been developed based on patient shift information (ΔX, ΔY, ΔZ) determined by a typical commercial image guidance system. First, two shift vectors are determined: one along the direction of the beam's-eye-view (BEV) and the other orthogonal to the BEV. The MLC aperture is then magnified and/or shifted according to these vectors ensuring that the same volume of tissue is exposed, as planned, while shielding the same volumes of organs-at-risk. Subsequently, the MLC positions are calculated using a polynomial fit function. Planning studies have been performed using Pinnacle 7.6c for conformal as well as IMRT plans to assess the effectiveness of this approach. For each of Prostate, Gynae and Head & Neck sites, 10 patient's plans have been evaluated for 6 randomly selected shifts ranging from 3 mm to 20 mm. The results of the AFM plans, with shifted isocenter position and modified MLC apertures, are compared with the base plans in terms of dose volume histogram, conformity index, dose homogeneity index, maximum dose to organs-at-risk and minimum dose to PTV. In order to put the results in perspective, the change in plan evaluation parameters of the modified plans are compared with those of the usually maximum accepted shift of 3 mm.

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Results 

For conformal Gynae plans, with midline rectal shields generated with segmented field apertures, the agreement between the base plan and AFM plans was found to be excellent: the dose difference in the maximum rectal dose is within 2% and shape of the DVH is virtually identical. Similar results were obtained for the IMRT plans as well. A maximum total dose difference to PTV for prostate IMRT plans due to AFM correction was found to be around 1 Gy for a prescribed total dose of 79.8 Gy. A maximum of 1.0 Gy dose difference was also found in the optical structure when comparing the base and AFM plans for head and neck IMRT with a prescribed dose of 70 Gy. However, these dose differences as well as the values of the plan evaluation parameters due to AFM plans were found to be well within the range of acceptable dose and plan parameter perturbation due to a shift of 3 mm (the usual clinical tolerance for set up error).

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Conclusions 

An alternate approach to couch shift method of compensating for patient set up error has been developed. The method utilizes MLC aperture adjustment and can be used in conjunction with any image guidance system. This method can potentially increase treatment efficiency and accuracy while saving patient disturbances and additional dose.