The inter- and intrafraction reproducibilities of three common IMRT delivery techniques

PURPOSE

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability.

METHODS

In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files.

RESULTS

Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both.

CONCLUSIONS

The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

Selective nodal irradiation for head and neck cancer using intensity-modulated radiotherapy: application of RTOG consensus guidelines in routine clinical practice

PURPOSE

We have been using intensity-modulated radiotherapy (IMRT) for selective neck irradiation. This article presents an analysis of patterns of failure and their dosimetric correlation.

METHODS AND MATERIALS

Between October 2003 and January 2008, 83 patients with head-and-neck cancer were treated with IMRT. Nodal levels were contoured as per the Radiation Therapy Oncology Group (RTOG) consensus guidelines.

RESULTS

There were 32 relapses with 23 local relapses (21 local relapses alone and 2 local and regional relapses, simultaneously), 9 regional relapses (including 2 simultaneous local and regional relapses), and 5 distant relapses, of which 2 patients had local relapses. At 2 and 3 years, the locoregional relapse-free survival rates were was 68.3% and 60.8%, respectively, while the overall survival rates were 84.1% and 81.7%, respectively. Subgroup analyses revealed significant differences in locoregional relapse-free survival rates for total treatment times of <53 days vs. >53 days, a volume of CTV1PTV (i.e., the volume prescribed 70 Gy) <177 cc vs. >177 cc, a V100 for CTV1PTV of <91% vs. >91%, and a minimum dose to CTV1PTV of <54 Gy vs. >54 Gy. There were no failures in the elective nodal volume, substantiating both the nodal selection criteria and the RTOG consensus guidelines for delineation of neck node levels.

CONCLUSIONS

IMRT for head-neck cancer is feasible, using elective nodal selection criteria along with RTOG consensus guidelines for the radiological boundaries of levels of neck nodes.