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Dosimetric VMAT Comparison using Rapid Arc and Tomotherapy for Parotid Gland Sparing
J. Mercado CMD., RTT., R. Hernandez CMD., RTT.
Department of Radiation Oncology, Miami Cancer Institute, Miami, Florida 33176, USA
Introduction
Technological advances in radiation oncology have demonstrated improved sparing of critical organs using intensity modulated radiation therapy (IMRT). NCCN guidelines indicate this modality is efficient in reducing xerostomia and dysphagia toxicities. This results in improved quality of life for patients with head and neck cancers through salivary gland preservation. Diminished quality of life issues include the inability to chew, taste or sleep (Ajani et al., 2013). Volumetric modulated arc therapy (VMAT Rapid Arc [RA]) utilizes a linear accelerator with dynamic modulated rotational therapy to deliver radiation dose to targeted areas. The use of advanced technology enables multiple planning target volumes (PTVs) to be treated in a single treatment plan known as simultaneous integrated boost while achieving dose sparing to the surrounding critical organs (Stromberger et al., 2015). The purpose of this study is to compare parotid gland sparing achieved through using a helical IMRT Tomotherapy [TT] unit and a linear accelerator. Tomotherapy delivers radiation through coplanar angles whereas the Varian accelerator has the ability to utilize non-coplanar beams.
Results
Data from seven patients were computed and compiled into a table in Appendix A. Comparative analysis was calculated and the median dose for PTV1, PTV2 and PTV3 were found to be 68.8 Gy, 62.8 Gy and 55.7 Gy for RA. Whereas, the results for TT showed 68.8 Gy, 64 Gy and 56.7 Gy. The analysis shows similar PTV coverage for both modalities. Analysis of the spinal cord revealed an average of 27.8 Gy for RA and 33.2 Gy for TT.
Reviewing data for the left parotid gland indicated the median mean dose to be 16.3 Gy for RA and 20.4 Gy for TT. At V30 was 15.2% and V20 was 25.4% for RA. TT was 18.4% and 35.5% respectively. The right parotid mean was 21.4 Gy RA and 27.1 Gy TT. RapidArc data revealed V20 was 23.2% and V30 was 37.8%. Tomotherapy plans showed V20 to be 37.9% and V30 was 47.3%.
Appendix A: Normalized DVH for head and neck patients shows comparable coverage to PTV and OAR’s.
Methods & Materials
This is a retrospective study utilizing seven patients diagnosed with a head and neck cancer with bilateral neck node involvement. All patients were prescribed 70 Gy to PTV1, 63 Gy to PTV2 and 56 Gy to PTV3. The study will be a direct comparison in which the same patient will be planned utilizing both tomotherapy and VMAT modalities. Tomotherapy treatment plans will be delivered utilizing coplanar beams with 6FF. It is a helical slice 6 MV photon beam allowing IMRT delivery through the use of binary MLC with a field width of 5 cm, pitch value of 0.3 and a nominal modulation factor of In comparison, the Rapid Arc plans will consist of three non-coplanar 6MV beams using a TrueBeam accelerator capable of achieving these angles. The first arc will be clockwise (CW), couch angle is 10 degrees and collimator angle is 30 degrees. The second arc is counterclockwise (CCW), couch angle is 0 degrees and the collimator angle is 90 degrees. The third arc is clockwise (CW), couch angle is 350 degrees and collimator angle is 330 degrees. Parotid gland contours and other contours were physician peer approved prior to the implementation of the planning process. All the plans will be normalized such that 95% of all three PTVs received 100% of the prescription dose.
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Figure 1: Non-coplaner beam arrangement for head and neck treatment to spare parotid glands.
Conclusions
This study demonstrated the coverage on PTVs were similar regardless of whether non-coplanar beams were utilized. There was a slight difference with regards to the dose to the spinal cord in which non-coplanar beams using RapidArc resulted in a lower dose. The ability to achieve “additional degrees of freedom with VMAT lead to the better conformity and sparing of OARs in close proximity to the PTVs” (Holt et al., 2013, p. 9). The utilization of non-coplanar beams resulted in a more effective sparing of the parotid organs. This was evidenced by the V20, V30 and the Dmean for both the right and left parotid glands. The study showed a significant decrease in dose to the parotids effectively sparing more of the organ. Thus, improving the overall quality of life for patients with head and neck cancers.
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