ConPas: a 3-D Conformal parotid gland-sparing irradiation technique for bilateral neck treatment as an alternative to IMRT

Background and Purpose: Intensity-modulated radiotherapy (IMRT) is used in most reported techniques for bilateral neck irradiation that aim at parotid gland sparing. A relatively simple conformal porotid-sparing technique (ConPas) was developed that uses no beam-intensity modulation. The purpose of this paper is to demonstrate, in patients with Larynx or hypopharynx carcinoma, that ConPas enables adequate coverage of the primary tumor and the bilateral neck nodes, while keeping the mean parotid dose (MPD) < 26 Gy. Patients and Methods: Treatment plans using ConPas and the conventional technique (using one anteroposterior supraclavicular and two Lateral beams) were computed for ten consecutive patients with T1-4 N0-1 larynx or hypopharynx carcinoma (not T1 glottic). A dose of 46 Gy was prescribed to the primary tumor and the bilateral neck nodes, planned either with the conventional technique or ConPas, followed by a boost up to 70 Gy with a simple two-field technique. The target coverage of both techniques was compared using the V-95 the percentage of the planning target volume (PTV) of the primary tumor and nodal regions receiving at least 95% of the prescribed elective dose. The MPDs and the normal-tissue complication probabilities (NTCPs) of the parotid glands were compared using either technique including the boost up to 70 Gy. Dosimetric verification of the technique has been carried out, using ionization chamber measurements and film dosimetry. Results: The mean V-95 was 85.2% and 91.2% (p = 0.08), the mean MPD 38.7 Gy and 25.4 Gy (p < 0.001), and the mean NTCP for the parotid glands 0.87 and 0.22 (p < 0.001) for the conventional technique and ConPas, respectively. The dosimetric verification shows a good agreement between dose calculation and measurement. Conclusion: ConPas enables adequate target coverage and clinically relevant parotid sparing in bilateral neck irradiation without beam-intensity modulation.