High Precision Boosting of the Nasopharynx—Dosimetric Comparison of Conventional and Conformal Radiotherapy Techniques

FJ Lagerwaard, PC Levendag, A van Nimwegen, C de Pan, PJCM Nowak

Hong Kong J Radiol 2001;4:137-42

Objective: We report a 5-year local relapse-free survival of 94% for T1 to 2 nasopharyngeal carcinoma using endocavitary brachytherapy as a boost to deliver cumulative doses of 95 Gy to the nasopharynx. Brachytherapy may be less than adequate for extension of disease into the parapharynx. The aim of the current planning study was to evaluate target coverage and sparing of critical normal structures when boosting the nasopharynx with 3-D conformal external beam radiotherapy, intensity modulated radiotherapy, and fractionated stereotactic radiotherapy in comparison to conventional treatment, i.e. parallel-opposed fields.

Materials and Methods: In 17 consecutive patients (12 with limited residual nasopharyngeal carcinoma after external beam radiotherapy, 5 with extensive residual nasopharyngeal carcinoma), computed tomography scans were obtained with the nasopharyngeal brachytherapy applicator in situ. The target volume and normal structures were contoured. Dose distributions were computed for class solutions using parallel-opposed, 3-D conformal external beam radiotherapy and intensity modulated radiotherapy techniques or stereotactic radiotherapy.

Results: Adequate target coverage was achieved by all treatment techniques; however, there was an unacceptably high dose to the parotid glands with parallel-opposed fields. Mean dose to the parotid glands was lowest with stereotactic radiotherapy (8.8% ± 3.2%), followed by intensity modulated radiotherapy (12.0% ± 3.2%) and 3-D conformal external beam radiotherapy (14.8% ± 5.0%). The combination of intensity modulated radiotherapy and stereotactic radiotherapy enabled superior parotid gland sparing (6.5% ± 2.8%). The same pattern was found for the 5 patients with extensive residual nasopharyngeal carcinoma.

Conclusion: Highly conformal dose distributions with considerable sparing of major salivary glands can be obtained with stereotactic radiotherapy or intensity modulated radiotherapy. As a potential future route, at present we are investigating intensity modulated stereotactic radiotherapy for boosting extensive residual nasopharyngeal carcinoma.