Potential of intensity-modulated radiotherapy to escalate doses to head-and-neck cancers: what is the maximal dose?

PURPOSE

To investigate the potential of intensity-modulated radiotherapy (IMRT) to escalate doses to head-and-neck cancer and find the maximal dose that could be prescribed to the target volume with IMRT while doses to critical organs were maintained at their currently acceptable levels. The secondary goal was to search for limits in current IMRT technology.

METHODS AND MATERIALS

For a group of 12 head-and-neck cancer patients with different tumor locations and shapes, we performed IMRT planning using a simultaneous integrated boost strategy, that is, the gross tumor volume (GTV), clinical target volume (CTV), and electively treated nodes were treated simultaneously at different dose levels. The critical structures involved in the treatment field that needed to be spared included the brainstem, spinal cord, and parotid glands, depending on the disease site. Nine coplanar 6-MV photon beams were used for planning with the IMRT system developed at our institution, and dose-volume criteria were used for optimization. By varying the optimization parameters, we gradually increased the dose to the GTV while keeping the dose to the critical structures at less than the acceptable tolerance level. The criteria for accepting the plan included the following: (1) the prescription dose to the GTV had to cover 99% of the volume, and the dose homogeneity of the GTV needed to be <10%; (2) the prescription to the CTV (which was set either at 60 Gy or 10 Gy less than that of the GTV) had to cover 95% of the volume, and the same amount of normal tissue outside the CTV received the CTV prescription dose as in the current acceptable plan; (3) the prescription to the electively treated lymph nodes needed to cover 90% of the volume; and (4) the maximal dose to the brainstem and spinal cord had to be <55 Gy and 45 Gy, respectively. For parotid glands, the dose needed to be as low as possible without compromising the target doses. The deliverable plans as determined by the actual multileaf collimator leaf sequences were used for the final evaluation. To verify that the acceptable plans were deliverable, the experimental measurements of planar dose distribution were performed in phantom with film.

RESULTS

The maximal dose to the GTV varied from 86 to 176 Gy if the CTV dose increased with the GTV dose. It was reduced to 76-82 Gy if the CTV dose was kept at 60 Gy. The competing criteria usually are the requirements of the tolerance doses to the critical organs and target dose homogeneity, not the target prescription dose. Using more beams only increased the dose marginally. The results could change significantly if a different set of criteria for the plan evaluation were used. Dosimetric measurements confirmed that such a high dose and dose gradient could be delivered accurately with dynamic multileaf collimators. Statistical analyses showed no significant correlations between the maximal doses and the number of GTVs and volume of GTVs and CTVs.

CONCLUSION

Doses to head-and-neck cancers with simultaneous integrated boost IMRT can be escalated to a greater level than currently prescribed clinically. The limit of IMRT in head-and-neck cancer has not been reached at the current prescription level of 70 Gy. Such high total and fractionated doses should be carefully evaluated before being prescribed clinically.