B6: Comparing Volumetric Arc Therapy (VMAT) and Intensity Modulated Radiotherapy (IMRT) for head and neck cancer throughout treatment: which technique provides better dosimetry in the context of weight loss and tumor shrinkage?

Joanna Mackenzie1,Colleen Schinkel1,Darren Graham1,John Bosch1,Robyn Banerjee1

1Tom Baker Cancer Centre, Calgary,Alberta, Canada

Presenting date: Tuesday 3 November
Presenting time: 13.10-14.00

Background

IMRT and VMAT techniques have been compared at baseline but not throughout treatment when head and neck patients typically develop body contour change from tumour shrinkage and weight loss. The purpose of this study was to compare the two techniques at baseline and at 15 and 30 fractions to identify differences in plan quality.

Method

10 patients undergoing bilateral concurrent chemoradiation for squamous cell carcinoma of the head and neck (6 oropharynx, 3 nasopharynx and 1 larynx cancer) were included. 70Gy was delivered to the high dose PTV (gross disease) and 59.4Gy to the low dose PTV (volume at risk), in 33 fractions. Patients were weighed at initial consult and weekly during treatment. All patients underwent a CT planning scan (#0) and were rescanned at 15 (#15) and 30 fractions (#30). Treatment was delivered using VMAT. 7–field IMRT plans were also created. Both IMRT and VMAT plans were superimposed on repeat CT planning scans at 15 and 30 fractions. Dosimetry variables for target volumes and organs at risk were assessed.

Results

Patients lost on average 8.0% weight. There were no differences in ‘Mean Change’ between IMRT and VMAT delivery for all target volume dosimetry variables (p>0.05). Deterioration in plan quality was observed with both techniques with a reduction in PTV minimum and increase in dose heterogeneity. Increased maximum doses to brainstem and spinal cord were observed with both techniques. The change in mean dose to both parotids between #0 and #30 was greater with VMAT than with IMRT (p <0.05).

Conclusion

Patient weight loss and tumour shrinkage may have contributed to the observed deterioration in plan quality variables. Overall comparison between VMAT and IMRT techniques was similar. Further work aims to determine predictive factors to identify those patients likely to benefit most from replanning during treatment.

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