Effect of 1mm and 2mm alteration in field size on target dose distribution for head and neck cancers in neutron therapy

Peter Clark du Plessis


Purpose:  To investigate the effect of 1 (one) and 2 (two) millimetre (mm) changes in field sizes on a target volume and normal surrounding tissue’s dose distribution during treatment planning of the planning target volume (PTV) of head and neck cancers.

Methods and materials:  Eleven (n=11) previously treated head and neck (H&N) patients’ plans were modified by 1 and 2mm. These simulated plans were calculated and the dose volume histograms (DVH) were analysed. The target dose and the organs at risk (OAR) maximum doses of the original treatment plans were compared with the newly generated plans. Spearman’s correlation coefficients were calculated and analysed.

Results:  The most prominent change observed was the mean dose difference in the target of -1.4% with a standard deviation of 3.611% (p = 0.227) if the field size was increased by both 1 and 2mm. The organ at risk (OAR) most affected by the field size changes was the left optic nerve with a median dose increase of 0.2% with a quartile range of 0.4% (p < 0.001) if field sizes were increased by 2mm. The organs at risk’s biggest decrease in percentage dose was when the field size was decreased by 2mm, to the spinal cord, left lens, and the left and right optic nerves of 0.1% with a quartile range of 0.4, 0.5, 0.3 and 0.4 and these findings were all statistically significant with p = 0.008, 0.004, respectively and p < 0.0001 for both optic nerves. 

Conclusion:  Changes to the dose distribution were observed with changes in field size. It is therefore important that geometric uncertainties are considered during the treatment planning process. Furthermore protocols to simulate for field size changes in individual patients should be considered as a standard pre-treatment verification procedure for head and neck cancer patients. One should keep in mind that a 1.4 % dose influence to the PTV that was noted is for a neutron dose. Thus in photon terms it is at least 3 times more.


Field size uncertainties, planning simulation

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The South African Radiographer | ISSN 0258 0241

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