Optimized radiotherapy in a locally advanced oropharyngeal carcinoma: Comparison of IMRT and proton beam

Abstract

Curative treatment of oropharyngeal carcinoma with conventional radiation is limited by the tolerance of adjacent normal structures and the severity of acute reactions. Our aim was to compare the isodose distribution and dose volume histograms of two state of the art radiotherapy modalities, IMRT and proton beam radiation therapy in a patient with an advanced oropharyngeal carcinoma. Treatment plans were independently generated for a patient with T4N2c SCC of the right tongue base. Following standard head immobilization, 2.5 mm axial images were obtained with contrast enhancement using a GE CT-simulator. An MRI was also obtained before treatment planning. The contrast-enhanced tumor, entire base of tongue, and involved lymph nodes were contoured as GTV. The rest of the lymphatic system including supraclavicular lymph nodes contoured as CTV. The CTV also included a 1 cm margin around the GTV in the oropharynx. Spinal cord, brain stem, mandible, right and left parotids were delineated as Organs At Risk (OAR). The GTV and CTV were prescribed to 70.2 and 59.4 Gy respectively at 1.8 Gy per fraction, with 5 mm margins. Normal tissue constraints were 44, 54, and 60 Gy for spinal cord, brainstem and mandible respectively. Parotid doses were kept as low as possible. The same volumes, target doses and dose constraints were used for both treatment plans. The most significant finding was the uniformity of dose within the two target volumes and in normal tissues proximal to the CTV. The dose encompassing the GTV and CTV was much more homogeneous and conformal in the proton beam plan. The proton DVH for the GTV was steeper than that of IMRT and resulted in almost all of GTV uniformly receiving the prescribed dose of 70.2 Gy. In the IMRT plan, most of the GTV received a dose in excess of what was prescribed, 50 % of the GTV received a dose of >75 Gy and 25 % >78 Gy. The CTV coverage with protons was also better than with IMRT. Protons had more sparing of non-target tissues. Doses in the IMRT plan were spread out beyond the target volumes producing unwanted radiation at the non-target tissues such as the anterior oral cavity. The DVH for OAR, mandible and spinal cord also demonstrated a better distribution with protons. The median dose to the mandible was less than 10 Gy with protons while 47 Gy with IMRT. Fifty percent of the spinal cord volume received less than 30 Gy with protons, in contrast, 90% received >30 Gy with IMRT (see Figure). Because of the proximity of lymphatic CTV to both parotid glands, there was little difference in parotid doses between IMRT and protons. Protons produced superior dose distributions to the GTV and CTV with less dose to OAR and non-target tissues than IMRT in an advanced oropharyngeal cancer. The inhomogeneity of dose and the location of hot spots within the oropharynx with IMRT is problematic. This can cause an excessive mucositis and compromise the delivery of concurrent chemo radiotherapy and possibly lead to late morbidity. Even though the IMRT plan provided acceptable OAR doses, there is less organ dose with protons and potential risk of late injury.