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Determining the effect of gantry rotation limits on plan quality for head and neck patients with homo-lateral disease

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Determining the effect of gantry rotation limits on plan quality for head and neck patients with homolateral disease.  

The purpose of this study is to determine if there are dosimetric benefits to setting a patient up with a head rotation for a partial arc VMAT technique treating homo-lateral head and neck cancer. If the simulated set-up yields a favourable dosimetry it warrants further investigation into the feasibility of implementing such a technique. This study stems from the variety of approaches for treating homo-lateral disease whilst optimally sparing organs at risk1-5. It is hoped that the current work serves to generate future research on the applicability of a head rotation set-up. 

Thirteen homo-lateral head and neck treatment plans were retrospectively re-planned on Pinnacle treatment planning software to compare three techniques: a 360° arc, a partial arc and a simulated head rotation set-up. Dose to OARs, delivery time, conformity and uniformity were collected. 

The simulated head rotation set-up had the greatest sparing of the mandible (Table 1). The novel approach also had superior sparing of the lips however only tested significant against the partial arc technique (Table 1). The head rotation and partial arc technique had superior sparing of the contralateral parotid gland compared to the 360°arc technique although not statistically significant (Table 1; Figure 3). Figure 2 exemplifies DVHs that represent common configurations found in this study. 

 

The simulated head rotation set-up had comparable sparing of the spinal cord, brain, brain stem, ipsilateral eye and both submandibular glands. While not statistically significant, Figure 3a) demonstrates greater favourability towards the simulated head rotation set-up regarding dose to the brain and brain stem. The simulated head rotation technique had poor sparing of the contralateral eye although not statistically significant. There was a significant reduction in estimated treatment delivery time and MU/cGy using homo-lateral partial arcs.

The novel approach did not significantly differ in uniformity or conformity compared to the 360° arc plans however did differ in conformity favouring the partial arc plan. Figures 4 and 5 outline these results.V95 represents the volume of the 95% isodose line (cc). VoptPTVin represents the volume of the optPTV within 95%.

The mandible and lips represent dose to the oral cavity. It was found that the simulated head rotation set-up had superior sparing of the mandible out of all techniques studied.  The simulated head rotation set-up significantly spared the lips in comparison to the partial arc technique. Additionally, the simulated head rotation set-up and partial arc technique had equally superior sparing of the contralateral parotid gland over the 360° arc. Case 07 is a notable case as it had unfavourable mandible sparing using the simulated head rotation set-up although lip sparing remained superior. This case had the least posterior tumor out of the thirteen cases studied (Figure 6). Figure 7 is an example of the typical size and location of tumors in this study that typically benefitted from the simulated head rotation set-up regarding dose to mandible, lips, and the contralateral parotid gland. 

It is possible that the anterior and posterior location of the tumor can impact the efficacy of a head rotation set-up however further research on a larger sample is required to pursue this theory. The simulated head rotation set-up achieved comparable conformity and uniformity to the optPTV in less time with less MU/cGy which has good clinical benefits.    

It is understood that homo-lateral tumors benefit from superior contralateral OAR sparing with a partial arc technique. A head rotation set-up has implications to improve oral cavity sparing however it is reasonably speculated that the novel approach may be most applicable when considered on a per patient basis as location of tumor may potentially impact OAR sparing. Further investigation is requiredto solidify notable trends in the data.

 

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