|IMRT for Nasopharyngeal Cancers: Potential
Advantages for Using Helical Tomotherapy
C. Yang, S. IJROBP Volume 66, Issue 3, Pages S671-S672 (1 November 2006)
University of California Davis Cancer Center, Sacramento, CA
Rigorous studies comparing various forms of IMRT such as helical tomotherapy versus conventional MLC IMRT have not been done. As our institution was one of those that participated in the RTOG-0225 protocol, we are in a position to explore the difference between the two modalities in treating nasopharyngeal cancers.
Three patients with locally advanced nasopharyngeal cancers that were entered into the RTOG-0225 study from our institution were selected for this study. For each patient, CTV70, CTV59.4 (divided into upper and lower neck portions) were expanded by 0.5 cm to form correspondent PTVs. The 3 patients were planned on Eclipse TPS (Varian Medical System, Palo Alto, CA) and treated using segmental MLC (SMLC) technique on a Varian Cl2100C. One patientís image and contour data were transferred from Eclipse TPS into the Tomotherapy HI-ART TPS (TomoTherapy Inc., Madison, WI) and planned using 2.5 cm jaw, 0.3 pitch, and 2.5 initial modulation with the same planning goals in RTOG-0225. Dose volume histograms were exported from both planning systems and compared using MatLab/Excel.
All three original plans of the SMLC technique met the protocol criteria with minor deviation in PTV dose coverage. The averaged PTV70 planning results are: volume: 577 cc and one SD of 203 cc; V77 (percentage of PTV70 receiving 77 Gy): 4.1% and 2.3%; V70: 93.8% and 1.8%; V66.5: 99.1% and 0.6%. The averaged dose received by 50% of the contralateral parotid gland is: 27.7 Gy and one SD of 1.4 Gy. In comparison, Tomotherapy achieved a more homogeneous dose coverage of the PTVs while reducing dose to the spinal cord and contralateral parotid (Figure 1). Mandible receives a mean dose of 24.1 Gy (Tomo) versus 52.8 Gy (SMLC) and larynx: 22.3 Gy (Tomo) versus 40.5 Gy (SMLC). All other critical normal structures have a better or comparable DVH in Tomo plan than SMLC one. Additionally, Tomo optimization process takes significantly shorter time than the SMLC one (8 hours versus 16 hours in this example) to satisfy the protocol requirement.
Helical tomotherapy shows real advantages over SMLC based IMRT in normal tissue sparing and homogeneous target coverage in nasopharyngeal cancers. While a plan generated on Tomotherapy TPS can meet the most stringent requirement specified in RTOG-0225, several features are still needed including basic planning tools such as being able to add new structures (contours) and Roomís-Eye-View 3D dose cloud plan review. Most importantly, a DICOM data export compliant with the Advanced Technology QA Consortium DICOM conformance statement is still needed.