Intensity-modulated radiation therapy (IMRT) beams traverse nontarget normal structures
not irradiated during three-dimensional conformal RT (3D-CRT) for head and neck cancer
(HNC). This study estimates the doses and toxicities to nontarget structures during
IMRT.
Oropharyngeal cancer IMRT and 3D-CRT cases were reviewed. Dose-volume histograms (DVH)
were used to evaluate radiation dose to the lip, cochlea, brainstem, occipital scalp,
and segments of the mandible. Toxicity rates were compared for 3D-CRT, IMRT alone,
or IMRT with concurrent cisplatin. Descriptive statistics and exploratory recursive
partitioning analysis were used to estimate dose "breakpoints" associated with observed
toxicities.
A total of 160 patients were evaluated for toxicity; 60 had detailed DVH evaluation
and 15 had 3D-CRT plan comparison. Comparing IMRT with 3D-CRT, there was significant
(p </= 0.002) nonparametric differential dose to all clinically significant structures
of interest. Thirty percent of IMRT patients had headaches and 40% had occipital scalp
alopecia. A total of 76% and 38% of patients treated with IMRT alone had nausea and
vomiting, compared with 99% and 68%, respectively, of those with concurrent cisplatin.
IMRT had a markedly distinct toxicity profile than 3D-CRT. In recursive partitioning
analysis, National Cancer Institute's Common Toxicity Criteria adverse effects 3.0
nausea and vomiting, scalp alopecia and anterior mucositis were associated with reconstructed
mean brainstem dose >36 Gy, occipital scalp dose >30 Gy, and anterior mandible dose
>34 Gy, respectively.
Dose reduction to specified structures during IMRT implies an increased beam path
dose to alternate nontarget structures that may result in clinical toxicities that
were uncommon with previous, less conformal approaches. These findings have implications
for IMRT treatment planning and research, toxicity assessment, and multidisciplinary
patient management.