**4. Three-dimensional conformal radiotherapy (3D-CRT)**

In the 3D-CRT technique for the treatment of HNC, usually, three fields are used. This consists of bilateral portals to treat the primary tumor and cervical lymph nodes and an anterior field to treat lower jugular chain and supraclavicular group of lymph nodes. A total dose of 70 Gy in 35 fractions is delivered in two to three phases in the definitive setting and 60–66 GY in 2 Gy per fraction in the adjuvant setting with or without concurrent chemotherapy depending on indications [5, 6]. Due to large hotspots arising in this technique, Field in field technique or field segmentation is generally used to reduce the hot spots. Monoisocentric technique is generally used where the bilateral and anterior fields have a common

*Volumetric and Dosimetric Inconstancy of Parotid Glands and Tumor in Head and Neck Cancer… DOI: http://dx.doi.org/10.5772/intechopen.104745*

isocenter. This removes the problem of beam divergence and field abutment to avoid under dosing of tumor volume and overdosing of critical structures. However, despite the matching of the lower border of bilateral fields and the upper border of the anterior neck field, there is a chance of error in the junction dose due to various dosimetric and physical factors causing inhomogeneity in the dose distribution in that area. The dosimetric factors include field size, beam quality, penumbra etc., while the physical factors include the jaw alignment, isocenter accuracy etc. [7]. Despite, all such efforts, a significant amount of dose is received by OARs (organs at risk) like parotid causing treatment toxicity like xerostomia. This limits the ability of 3D-CRT to spare the OARs when there is a concave-shaped target in the head and neck. Therefore, the 3D-CRT technique is not helpful in parotid sparing and cannot be a replacement for higher precision techniques like IMRT.
