Evaluation of two tomotherapy-based techniques for the delivery of whole-breast intensity-modulated radiation therapy
Gonzalez VJ, Buchholz DJ, Langen KM, Olivera GH, Chauhan B, Meeks SL, Ruchala KJ, Haimerl J, Lu W, Kupelian PA
International Journal of Radiation Oncology, Biology, Physics
01 May 2006 (Vol. 65, Issue 1, Pages 284-290)

Purpose: To evaluate two different techniques for whole-breast treatments delivered using the Hi-ART II tomotherapy device.

Historically, whole-breast radiotherapy as part of breast conservation therapy has been performed mainly with tangential fields. Because of the simple geometry of tangential fields, the lack of need for dose escalation in breast conservation, and the perceived relatively low complication rates, there has been little impetus for change in the technical delivery of breast radiotherapy. Tangential fields provide adequate coverage of the target tissue (i.e., the breast). However, pulmonary complications, cardiac complications, and fibrotic changes in the irradiated soft tissues are well documented consequences of whole-breast irradiation. It is not clear how modern conformal techniques, including intensity-modulated radiotherapy (IMRT), will impact clinical outcomes. However, IMRT techniques have been investigated for whole-breast irradiation in an effort to increase dose homogeneity and/or decrease normal-structure doses. In addition to compensators and their use, multileaf collimator (MLC) based techniques have been investigated. A common approach is to modulate the intensity of the two tangential fields; i.e., the gantry angles used for IMRT are identical to those used for standard tangential radiation therapy.

The availability of helical tomotherapy units is increasing, and the evaluation of this device for breast cancer treatments is of interest. In the current study, the use of helical tomotherapy units for the treatment of whole-breast patients is tested. Two different irradiation techniques are evaluated. Both techniques use the same hardware, but in one technique the gantry rotates during delivery, whereas in the second technique gantry positions are stationary.

In helical tomotherapy, the gantry continuously rotates around the patient, who is translated through the beam delivery plane. This technique allows beam delivery from any gantry angle. In comparison with whole-breast treatments with standard tangential radiation therapy, the use of all gantry angles could result in a delivery of low doses to areas in the body that would normally receive only scatter dose. The organs of particular concern are the contralateral breast and lung. This situation can be mitigated by constraining delivery through certain structures or angles. To prevent dose delivery to a structure of interest, the structure can be designated as a blocked during the tomotherapy planning process. This inhibits the use of any beamlet that passes through this structure, therefore limiting the dose to just scatter dose. It is also possible to directionally block a structure. This allows beamlets only to exit from a structure, but not to enter the structure on its path to the target. By using such methods, the treatment delivery is constrained to a smaller range of directions and a smaller set of beamlets. However, because the gantry speed is constant, as the number of treatment directions decreases, the treatment delivery efficiency decreases. This is not a significant problem for most delivery types, but can be a larger consideration for cases such as breast when the desired treatment is constrained to a very small number of directions. To avoid this inefficiency, an obvious extension of helical tomotherapy delivery is therefore the use of static gantry positions, combined with simultaneous couch translation and MLC modulation. This option, called topotherapy, seems particularly well suited for the treatment of the whole breast. If the static gantry angles are identical to the tangential beam angles, this technique is similar to intensity-modulated tangential fields.

It is the purpose of this work to evaluate and compare treatment plans that are based on helical and static treatment modes. To establish a common framework for comparison, the quality of the helical tomotherapy plans was restricted by enforcing delivery times comparable to simple 2 tangential beam directions (on the order of 6 to 9 min, depending on the extension of the target inferior-superior). A longer treatment time would allow a higher degree of beam modulation and would potentially allow the design of better plans.

Methods and Materials: Tomotherapy uses the standard rotational helical delivery. Topotherapy uses a stationary gantry while delivering intensity-modulated treatments. CT scans from 5 breast cancer patients were used. The prescription dose was 50.4 Gy.

Results: On average, 99% of the target volume received 95% of prescribed dose with either technique. If treatment times are restricted to less than 9 min, the average percentage ipsilateral lung receiving ≥20 Gy was 22% for tomotherapy vs. 10% for topotherapy. The ipsilateral lung receiving ≥50.4 Gy was 4 cc for tomotherapy vs. 27 cc for topotherapy. The percentage of left ventricle receiving ≥30 Gy was 14% with tomotherapy vs. 4% for topotherapy. The average doses to the contralateral breast and lung were 0.6 and 0.8 Gy, respectively, for tomotherapy vs. 0.4 and 0.3 Gy for topotherapy.

Conclusions: Tomotherapy provides improved target dose homogeneity and conformality over topotherapy. If delivery times are restricted, topotherapy reduces the amount of heart and ipsilateral lung volumes receiving low doses. For whole-breast treatments, topotherapy is an efficient technique that achieves adequate target uniformity while maintaining low doses to sensitive structures.