Influence of collimator rotation on dose distribution and delivery in intensity modulated radiation therapy for parotid cancer

Seema Sharma, Durai Manigandan, Shikha Goyal, Puja Sahai, Velliyan Subramani, Subhash Chander, Pramod Kumar Julka, Goura Kishor Rath


Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed.

Methods: The computed tomography (CT) datasets of 5 patients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I’mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation.

Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles.

Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT.   


Collimator Rotation; Intensity-Modulated Radiotherapy; Parotid Cancer

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Ezzell GA, Burmeister JW, Dogan N, et al. IMRT commissioning: multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119. Med Phys 2009; 36:5359-73.

Ahnesjo A, Hardemark B, Isacsson U, Montelius A. The IMRT information process-mastering the degrees of freedom in external beam therapy. Phys Med Biol 2006; 51:R381-402.

Intensity Modulated Radiation Therapy Collaborative Working Group. Intensity-modulated radiotherapy: current status and issues of interest. Int J Radiat Oncol Biol Phys 2001; 51:880-914.

Yu CX, Amies CJ, Svatos M. Planning and delivery of intensity-modulated radiation therapy. Med Phys 2008; 35:5233-41.

Rana S, Pokharel S, Zheng Y, et al. Treatment planning study comparing proton therapy, RapidArc and IMRT for a synchronous bilateral lung cancer case. Int J Cancer Ther Oncol 2014; 2:020216.

Yang J, Ma C, Wang L, et al. Effect of collimator and couch angle change on breast IMRT dose distributions. J Appl Clin Med Phys 2009; 10:3058.

Isa M, Rehman J, Afzal M, Chow JC. Dosimetric dependence on the collimator angle in prostate volumetric modulated arc therapy. Int J Cancer Ther Oncol 2014; 2:020419.

Nutting CM, Rowbottom CG, Cosgrove VP, et al. Optimisation of radiotherapy for carcinoma of the parotid gland: a comparison of conventional, three-dimensional conformal, and intensity-modulated techniques. Radiother Oncol 2001; 60:163-72.

Feuvret L, Noël G, Mazeron JJ, Bey P. Conformity Index: a review. Int J Radiat Oncol Biol Phys 2006; 64:333-42.

Ammar H, Eldebawy E, Maarouf E, et al. Evaluation of the peripheral dose and the conformity index for three stereotactic radiotherapy techniques: Arcs, noncoplanar fixed fields and intensity modulation. Int J Cancer Ther Oncol 2014; 2:02042.

Sharma S, Chaudhari P, Biswas A, et al. Impact of head immobilization position on dose distribution in patients of brainstem glioma. Int J Cancer Ther Oncol 2015; 3:030116.

Low DA, Harms WB, Mutic S, Purdy JA. A technique for the quantitative evaluation of dose distributions. Med Phys 1998; 25:656-61.

Deng J, Pawlicki T, Chen Y, et al. The MLC tongue-and-groove effect on IMRT dose distributions. Phys Med Biol 2001; 46:1039-60.

Chapek J, Tobler M, Toy BJ, et al. Optimization of collimator parameters to reduce rectal dose in intensity-modulated prostate treatment planning. Med Dosim 2005; 30:205-12.

Otto K, Clark BG. Enhancement of IMRT delivery through MLC rotation. Phys Med Biol 2002; 47: 3997-4017.

LoSasso T, Chui CS, Ling CC. Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy. Med Phys 1998; 25:1919-27.


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