Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

Harari. IJROBP2004:60:S160

The use of intensity modulated radiation therapy (IMRT) in the treatment of head and neck (H&N) cancer is expanding worldwide. As radiation dose to selected normal structures (ie, parotid glands) is reduced, dose to adjacent regions (ie, anterior oral cavity) may be increased. Ultimate H&N cancer outcome and patient quality of life (QOL) is complex and can be influenced by many aspects of the radiation dose profile. This study was carried out to compare the capacity of helical tomotherapy versus linac-based IMRT to achieve target dose conformality and normal tissue sparing in oropharynx cancer patients.

Twenty oropharyngeal cancer patients treated with curative intent using 7 field H&N IMRT between 20012003 were selected for study. Treatment planning and delivery was performed using the PinnacleTM planning system. In general, doses of 6670 Gy at 2.12.2 Gy per fraction were delivered to primary planning target volume (PTV) targets, with 5060 Gy to subclinical PTV targets. Dose constraints were employed for various normal H&N tissue structures such as parotid gland (20 Gy mean dose), spinal cord and anterior oral cavity (35 Gy maximum dose). Comprehensive H&N tomotherapy plans were designed for these 20 patients using identical prescription parameters. For conventional IMRT design, a 7 field arrangement of equally spaced beams was employed, whereas for helical tomotherapy, over 1000 projections per treatment were used (51 projections per rotation with a minimum of 20 rotations). Resultant treatment plans were then rigorously compared and contrasted.

All 40 oropharynx cancer treatment plans analyzed (20 conventional IMRT and 20 helical tomotherapy) met primary treatment objectives regarding dose specification. Specifically, the resultant Equivalent Uniform Dose (EUD) calculations for high and low dose PTV were ≥the prescription dose. In general, helical tomotherapy plans offered favorable dose distributions over that achieved with 7 field IMRT plans as manifested by increased EUD to PTV targets, as well as reduced mean dose to normal tissue avoidance structures. The EUD analysis revealed that tomotherapy plans provided up to 4% increase to high dose PTV over that achieved with 7 field IMRT plans. Simultaneously, mean dose delivered to normal tissue avoidance structures such as contralateral parotid gland, spinal cord and oral cavity was reduced up to 50% with tomotherapy plans. The daily patient treatment time with helical tomotherapy is essentially identical to that with conventional IMRT. However, to ensure comparable delivery times, a jaw opening of 2.5 cm was chosen which yields a 2-fold increase (from 5 to 10 mm) in the conventional 8020% penumbra in the cephalo-caudal dimension.

The overall analysis of H&N treatment plans identifies several potential advantages for helical tomotherapy over that achieved with conventional IMRT. The improved dose distributions are largely explained by the fact that tomotherapy offers a significantly larger number of projections than available with conventional MLC-based IMRT. Nevertheless, the ultimate clinical significance of these dose distribution refinements remains ill defined. Intuitively, it would seem that higher conformality of dose to specified targets, and dose reduction to normal tissue avoidance structures may be of value. To further evaluation of this question, a prospective clinical trial is underway to examine normal tissue functional outcome and QOL in H&N cancer patients receiving IMRT with conventional and tomotherapy techniques. In addition to direct measurement of salivary, auditory, voice and swallow function, validated health outcome surveys are obtained to assess patient perception regarding voice, swallow, hearing and overall H&N function before and following treatment. These data should provide further insight regarding the potential value of refining dose distribution in H&N cancer patients receiving highly conformal treatment using either conventional IMRT or helical tomotherapy