Impact of MLC leaf position errors on simple and complex IMRT plans for head and neck cancer

Abstract

The dosimetric impact of random and systematic multi-leaf collimator (MLC) leaf position errors is relatively unknown for head and neck intensity-modulated radiotherapy (IMRT) patients. In this report we studied 17 head and neck IMRT patients, including 12 treated with simple plans (<50 segments) and 5 treated with complex plans (>100 segments). Random errors (-2 to +2 mm) and systematic errors (+/-0.5 mm and +/-1 mm) in MLC leaf positions were introduced into the clinical plans and the resultant dose distributions were analyzed based on defined endpoint doses. The dosimetric effect was insignificant for random MLC leaf position errors up to 2 mm for both simple and complex plans. However, for systematic MLC leaf position errors, we found significant dosimetric differences between the simple and complex IMRT plans. For 1 mm systematic error, the average changes in D(95%) were 4% in simple plans versus 8% in complex plans. The average changes in D(0.1 cc) of the spinal cord and brain stem were 4% in simple plans versus 12% in complex plans. The average changes in parotid glands were 9% in simple plans versus 13% for the complex plans. Overall, simple IMRT plans are less sensitive to leaf position errors than complex IMRT plans.

Delivering RapidArc®: A comprehensive study on accuracy and long term stability

“…2a), data delivery analysis proves that for 95% of gaps smaller than 10 mm, the random DG error is less than 0.7 mm. Based on these findings and data on delivery error simulations [7,10,16,30], we may conclude that MLC delivery errors have reduced effects on the patient dose distributions: in any case, Eclipse plan solutions with a reduced number of small gaps are desirable. This analysis underlines that MLC errors were reproducible over the whole investigation period regardless of the dose rate and gantry speed.…”

On the sensitivity of common gamma‐index evaluation methods to MLC misalignments in Rapidarc quality assurance

“…The shift for the counterclockwise rotation is applied accordingly. For IMRT random leaf errors up to 2 mm were found to have little dosimetric effects 21,22 and initial tests with random leaf errors suggested the same outcome for Rapidarc treatment plans. The modifications representing conceivable worst case scenarios are summarized in Table I.…”

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

“…In a study that proposed a Monte Carlo based IMRT dose verification method, Luo et al 34 found that an average MLC leaf positional error of 0.2 mm can result in a target dose error of about 1.0%. Mu et al 35 studied the impact of random and systematic MLC leaf position errors for head and neck IMRT patients. They found that the dosimetric effect was insignificant for random MLC leaf position errors up to 2 mm, but for a 1 mm systematic error, the average changes in D 95% were 4% in simple plans versus 8% in complex plans with notable differences also seen in spinal cord, brainstem, and parotid glands.…”