The use of computed tomography in radiotherapy treatment planning for breast cancer. How does conventional radiotherapy planning compare with virtual?

PURPOSE: To compare, in terms of ballistics and dosimetry, a conventional and a virtual simulation in 14 patients without changing the set-up.

PATIENTS AND METHODS: 14 women with breast cancer were treated with postoperative radiotherapy from October 2003 to November 2004. Whole breast irradiation alone was indicated (50 Gy at International Committee on Radiation Units [ICRU] point in 25 fractions over 5 weeks) with, in some cases, an additional boost of 16 Gy to the tumor bed--that was not taken into account in this study. After CT scan, tangential fields were conventionally simulated using a Mecaserto Phebus-type simulator-CT scan. The planning target volume (PTV) was the clinical target volume (CTV) expanded with an additional margin of 1 cm in all directions but towards the skin. Both the lungs and the heart were delineated as organs at risk. Dosimetries were computed for the two beams arrangements i.e. 2D conventional and 3D virtual.

RESULTS: The mean age of 14 women was 51.4 years (range 26-65). Laterality was the left breast for 6 patients and the right for 8. Few differences were noticeable in terms of gantry angles. The 3D medial fields were more medial with a mean of 8 mm (range 0-15). The 3D lateral fields were more posterior with a mean difference of 8 mm (range 0-25). The dosimetry analysis showed that, with regard to conventional simulations, at least 95% of the CTV received in all cases > 95% of the prescribed dose. However, in 8 out of 14 patients (57%), 15% of the PTV received < 95% of the prescribed dose. The ICRU 50 quality criterion that at least 95% of the PTV (PTV(95%)) should receive at least 95% of the prescribed dose was never met with conventional simulation. In the case of virtual simulation, the ballistics of the treatment were designed to meet the ICRU quality criterion and thus the PTV95% was higher here than with the conventional simulation by a mean of 17.6% +/- 9.7%. The percentage of CTV receiving a dose higher than 107% of the prescribed dose was 21.3% +/- 12% with conventional and 24% +/- 11% with virtual simulation.

CONCLUSION: The high incidence of breast cancer, the essential role of radiotherapy in its treatment and the potential ensuing toxicity explain why so many studies are devoted to the improvements brought to the techniques of this treatment. The virtual planning of the treatment, however, comes up against many difficulties. The countering of the CTV is complex and necessitates a combination of clinical examination and imagery. The choice of margins around the CTV has not been standardised and is largely dependent both on the equipment used and the quality control methods.