Re-Evaluating PTV Margin Requirements In Head and Neck Radiotherapy with Daily Image Guidance: Effects on Target Coverage and OAR Dose
Abstract
Purpose
In head-and-neck radiotherapy, relatively large planning target volume (PTV) margins of 5-mm are standardly used to account for setup uncertainty and anatomical variations. However, this comes at the expense of increased dose to nearby structures, resulting in increased toxicity. With the widespread implementation of daily IGRT, it is important to re-evaluate whether smaller PTV margins can maintain acceptable target coverage.
Methods
This study examines the dosimetric impact of reducing PTV margins from 5 mm to 3 mm, with evaluation of target coverage for dose volumes CTV70 and CTV56 and associated OARs over a seven-week treatment course. Eight head-and-neck cancer patients initially planned with 5-mm CTV56 margins were retrospectively replanned using 3-mm uniform PTV expansions. In all cases, 3-mm margins were used for gross disease (e.g. CTV70). Dose was recalculated weekly to account for anatomical changes seen on CBCT imaging, with deformable dose registration used to accumulate dose over the seven-week course. Dose volume histogram metrics were extracted for CTV56 and CTV70, mean doses were collected for the parotid glands and pharyngeal constrictors.
Results
Reducing PTV margins from 5 mm to 3 mm resulted in no statistically significant differences in target coverage, with V99 and V95 decreasing by less than 1% (V95, p = 0.77; V99, p = 0.97). Target coverage to all dose levels remained within clinical tolerances. Corresponding mean doses to the parotid glands and pharyngeal constrictors were modestly reduced by 0.95 Gy (p=0.04) and 0.62 Gy (p=0.01), respectively.
Conclusion
In this study of PTV margins in H&N radiotherapy, weekly evaluation indicated that interfraction anatomical changes did not produce clinically meaningful loss of target coverage when using 3-mm margins for all CTVs. Furthermore, 3-mm CTV56 expansions resulted in minimal differences in target coverage compared to standard 5-mm margins while reducing dose to adjacent OARs, when using daily IGRT.