Abstract
The focus of this work is the dosimetric impact of multileaf collimator (MLC) leaf width on the treatment of prostate cancer with intensity‐modulated radiation therapy (IMRT). Ten patients with prostate cancer were planned for IMRT delivery using two different MLC leaf widths—4 mm and 10 mm— representing the Radionics micro‐multileaf collimator (mMLC) and Siemens MLC, respectively. Treatment planning was performed on the XKnifeRT2 treatment‐planning system (Radionics, Burlington, MA). All beams and optimization parameters were identical for the mMLC and MLC plans. All the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose. The differences in dose distribution between the two different plans were assessed by dose–volume histogram (DVH) analysis of the target and critical organs. We specifically compared the volume of rectum receiving 40 Gy (V40), 50 Gy (V50), 60 Gy (V60), the dose received by 17% and 35% of rectum (D17 and D35), and the maximum dose to 1 cm3 of the rectum for a prescription dose of 74 Gy. For the urinary bladder, the dose received by 25% of bladder (D25), V40, and the maximum dose to 1 cm3 of the organ were recorded. For PTV we compared the maximum dose to the “hottest” 1 cm3 (Dmax1cm3) and the dose to 99% of the PTV (D99). The dose inhomogeneity in the target, defined as the ratio of the difference in Dmax1cm3 and D99 to the prescribed dose, was also compared between the two plans. In all cases studied, significant reductions in the volume of rectum receiving doses less than 65 Gy were seen using the mMLC. The average decrease in the volume of the rectum receiving 40 Gy, 50 Gy, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p<0.0001 for V40 and V50 and p<0.012 for V60. The mean dose reductions for D17 and D35 for the rectum using the mMLC were 20.4% (p<0.0001) and 18.3% (p<0.0002), respectively. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 29%. In the high‐dose range, there was no significant difference in the dose deposited in the “hottest” 1 cm3 of the rectum between the two plans for all cases (p>0.78). In conclusion, the use of the mMLC for IMRT of the prostate resulted in significant improvement in the DVH parameters of the prostate and critical organs, which may improve the therapeutic ratio.PACS number: 87.53.Tf
Highlights
The multileaf collimator (MLC) has been increasingly used in radiotherapy to facilitate delivery of three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT)
It is seen that the ratios for dose to 99% of the PTV (D99) are consistently higher than unity, indicating that the micro-multileaf collimator (mMLC)-based plans consistently resulted in a higher minimum dose (D99)
A typical isodose distribution and dose–volume histogram (DVH) comparison for the planning target volume (PTV) are shown in Fig. 1, where the characteristic difference observed between the mMLC plans and the MLC plans for all of the patients is demonstrated
Summary
The multileaf collimator (MLC) has been increasingly used in radiotherapy to facilitate delivery of three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT). As the primary device used to shape the radiation fields, the leaf width of a MLC device could be of importance. Most commercial MLCs have a leaf width of 1.0 cm defined at the isocenter. With such a leaf width, conformation of the field to an irregularly shaped target may be poor in comparison with mMLC and blocks. The dosimetric effects of the 1.0-cm leaf width multileaf collimators have been described previously.[1,2,3] The limitation of a broad-leaved MLC may be pronounced when employing small fields in stereotactic radiotherapy or treating targets in close proximity to critical organs
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