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The Effect of Delivery Parameter Errors to the Patient Quality Assurance Result for RapidArc Treatment in Head-And-Neck Patients

J Peng*, W Hu, Z Zhang, Fudan University Shanghai Cancer Center, Shanghai, Shanghai

SU-E-T-360 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To investigate how the delivery parameter errors (DPEs) act on the patient quality assurance (QA) results in RapidArc treatment for head and neck cancer.
Methods: Three DPEs (MLC gap, gantry angle and dose MU error) were scrutinized and quantified by comparing delivery parameters between the planed and actual delivered. The actual delivered parameters were recorded in dynalog-files by computer controller during delivery. Sixteen clinically approved head-and-neck RapidArc plans were checked. Sixty-four error induced plans were reconstructed by applying four types of errors (three DPEs separately and one integrally) to the original 16 plans. The gamma passing rate (GPR) and dose endpoint variations caused by these DPEs was analyzed.
Results: For all patients, the mean error of MLC gap, gantry angle and Dose MU were 0.19±0.06 mm, -0.36±0.03 degree, and 0.5E-4±1.8E-4 MU, respectively. The GPR results were most vulnerable to MLC gap error among three DPEs (ANOVA test, p<0.001). A strong negative correlation (r=-0.538, 2-tailed p<0.05, Pearson correlation analysis) existed between GPR and absolute mean MLC gap error for RapidArc treatment plans in head-and-neck patients. Compared to original plans, the DPEs induced plans showed mean dose variations of 0.6-1.01% on the target of PGTV, PTV and some normal tissues (including spinal cord, brain stem, parotid and larynx). The MLC gap error played the prominent role on affecting the dose differences (ANOVA test, p<0.001) among the three DPEs.
Conclusions: The MLC gap error, Dose MU error and gantry angle error can affect the GPR and differ the actual dose distribution from the planned one. The MLC gap error is the most prominent factor that affects the GPR and clinical dose distribution for head and neck cancer patients. A mean MLC gap error of 0.19mm will increase dose by 1-3% for head and neck cancer patients treated with Rapidarc.

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