Comparative role of three-dimensional radiotherapy planning and inhomogeneity corrections in carcinoma of the tongue.

The authors have assessed the role of computerized three-dimensional (3-D) and traditional (TD) radiotherapy planning and inhomogeneity corrections in improving target volume coverage and normal tissue sparing in carcinoma of the tongue. Coverage of target volumes in 3-D versus TD plans revealed the following. Volume receiving 95% of dose, clinical target volume (CTV): 1-68% versus 0-24%; gross tumour volume-lymph nodes (GTV-l): 0-80% versus 0-20%; gross tumour volume-primary tumour (GTV-II): 0-65% versus 0-26%. Dose to 95% of target volume CTV 77-92% versus 76-87%; GTV-I: 81-90% versus 61-88%; GTV-II: 82-93% versus 68-87%. Minimum dose to 5% of target volume, CTV: 77-93% versus 74-81%; GTV-I: 81-90% versus 61-88%; GTV-II: 76-93% versus 68-87%. Minimum dose to a volume of no less than 5% of the target volume, CTV: 93-98% versus 88-96%; GTV-I: 87-100% versus 88-97%; GTV-II: 86-98% versus 88-96%. A new parameter (inhomogeneity difference) was devised to study target volume dose homogeneity and was found to be very useful. Dose to two-thirds of the parotid glands in 3-D versus TD plans showed a mean of 46 versus 65% for right parotid glands and 44 versus 56% for left parotid glands in all patients. Better tumour dose homogeneity, increased mean tumour dose, avoidance of geographic misses and better parotid sparing was achieved in 3-D plans as compared to TD plans. We could not demonstrate any role for inhomogeneity corrections using currently available computerized dose algorithms.