Dosimetric Comparison of Different Planning Techniques in Left-sided Whole-Breast Irradiation: A Planning Study.

PURPOSE: This planning study compared the various dosimetric parameters of different types of intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques for left-sided breast cancer radiotherapy. MATERIALS AND METHODS: Treatment of 22 left-sided breast cases was planned using two IMRT and VMAT techniques for the prescription of 40 Gy in 15 fractions. For tangential IMRT (Tan_IMRT), five beams were placed as conventional tangential beams. For equally spaced IMRT (Equi_IMRT), six beams were placed equidistantly at 40° interval from 300° to 140°. For tangential VMAT (Tan_VMAT), two arcs were used with the avoidance sector in such a way that the beam covered like tangential fields. For full-arc VMAT (Full_VMAT), similar arcs as Tan_VMAT were used, without avoidance sector. All treatment plans were generated using Eclipse planning system for TrueBeam STx linear accelerator. For planning target volume (PTV), dose parameters including D95%, D99%, V105% homogeneity index (HI), and conformity index (CI) were analyzed. Different dose parameters for the left lung, heart, left anterior descending artery (LAD), right lung, and right breast were also analyzed. In addition, low-dose spillage in the normal tissues and the number of monitor units (MUs) required for the treatment were compared. RESULTS: IMRT technique exhibited superior D95% and D99% for PTV compared with VMAT techniques. VMAT plans provided more V105% (6%) compared with that of IMRT plans (approximately 1%). HI was better in IMRT plans (Tan_IMRT, 0.085 ± 0.015; Equi_IMRT, 0.094 ± 0.011) than in VMAT plans. CI was better in VMAT plans. The mean lung dose (7.7 Gy ± 1.788 Gy) and V5Gy (34.99% ± 6.799%) were better achieved in Tan_IMRT plan than other plans. Right lung, heart, and right breast sparing were better achieved in Tan_IMRT plan. Moreover, low-dose spillage was very less in the Tan_IMRT compared with all other techniques. CONCLUSION: Dosimetric comparison in this study showed that tangential IMRT technique is superior in terms of target coverage, sparing of lung, heart, and right breast, and low-dose spillage control in the left-sided breast-only radiotherapy.

Analyzing the characteristics of 6 MV photon beam at low monitor unit settings.

Analyzing the characteristics of a low monitor unit (MU) setting is essential, particularly for intensity-modulated techniques. Intensity modulation can be achieved through intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT). There is possibility for low MUs in the segments of IMRT and VMAT plans. The minimum MU/segment must be set by the physicist in the treatment planning system at the time of commissioning. In this study, the characteristics such as dose linearity, stability, flatness, and symmetry of 6 MV photon beam of a Synergy linear accelerator at low MU settings were investigated for different dose rates. The measurements were performed for Synergy linear accelerator using a slab phantom with a FC65-G chamber and Profiler 2. The MU linearity was studied for 1-100 MU using a field size of 10 cm ×10 cm. The linearity error for 1 MU was 4.2%. Flatness of the beam was deteriorated in 1 MU condition. The beam stability and symmetry was well within the specification. Using this study, we conclude that the treatment delivered with <3 MU may result in uncertainty in dose delivery. To ensure the correct dose delivery with less uncertainty, it is recommended to use ≥3 MU as the minimum MU per segment in IMRT and VMAT plans.

Analyzing the performance of the planning system by use of AAPM TG 119 test cases.

Our objective in this study was to create AAPM TG 119 test plans for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) in the Monaco planning system. The results were compared with the published studies, and the performance of the Monaco planning system was analyzed. AAPM TG 119 proposed a set of test cases called multi-target, mock prostate, mock head and neck and C-shape to ascertain the overall accuracy of IMRT planning, measurement, and analysis. We used these test cases to investigate the performance of the Monaco planning system for the complex plans. For these test cases, we created IMRT plans with static multi-leaf collimator (MLC) and dynamic MLC by using 7-9 static beams as explained in TG-119. VMAT plans were also created with a 320° arc length and a single or double arc. The planning objectives and dose were set as described in TG 119. The dose prescriptions for multi-target, mock prostate, mock head and neck, and C-shape were taken as 50, 75.6, 50 and 50 Gy, respectively. All plans were compared with the results of TG 119 and the study done by Mynampati et al. Point dose and fluence measurements were done with a CC13 chamber and ArcCHECK phantom, respectively. Gamma analysis was done for the calculated and measured dose. Using the Monaco planning system, we achieved the goals mentioned in AAPM TG-119, and the plans were comparable to those of other studies. A comparison of point dose and fluence showed good results. From these results, we conclude that the performance of the Monaco planning system is good for complex plans.

Influence of increment of gantry angle and number of arcs on esophageal volumetric modulated arc therapy planning in Monaco planning system: A planning study.

The objective of this study was to analyze the influence of the increment of gantry angle and the number of arcs on esophageal volumetric modulated arc therapy plan. All plans were done in Monaco planning system for Elekta Synergy linear accelerator with 80 multileaf collimator (MLC). Volumetric modulated arc therapy (VMAT) plans were done with different increment of gantry angle like 15°, 20°, 30° and 40°. The remaining parameters were similar for all the plans. The results were compared. To compare the plan quality with number of arcs, VMAT plans were done with single and dual arc with increment of gantry angle of 20°. The dose to gross tumor volume (GTV) for 60 Gy and planning target volume (PTV) for 48 Gy was compared. The dosimetric parameters D98%, D95%, D50% and Dmax of GTV were analyzed. The homogeneity index (HI) and conformity index (CI) of GTV were studied and the dose to 98% and 95% of PTV was analyzed. Maximum dose to spinal cord and planning risk volume of cord (PRV cord) was compared. The Volume of lung receiving 10 Gy, 20 Gy and mean dose was analyzed. The volume of heart receiving 30 Gy and 45 Gy was compared. The volume of normal tissue receiving greater than 2 Gy and 5 Gy was compared. The number of monitor units (MU) required to deliver the plans were compared. The plan with larger increment of gantry angle proved to be superior to smaller increment of gantry angle plans in terms of dose coverage, HI, CI and normal tissue sparing. The number of arcs did not make any difference in the quality of the plan.

Comparative analysis of volumetric-modulated arc therapy and intensity-modulated radiotherapy for base of tongue cancer.

The aim of this study was to compare the various dosimetric parameters of dynamic multileaf collimator (MLC) intensity modulated radiation therapy (IMRT) plans with volumetric modulated arc therapy (VMAT) plans for base of tongue cases. All plans were done in Monaco planning system for Elekta synergy linear accelerator with 80 MLC. IMRT plans were planned with nine stationary beams, and VMAT plans were done for 360° arc with single arc or dual arc. The dose to the planning target volumes (PTV) for 70, 63, and 56 Gy was compared. The dose to 95, 98, and 50% volume of PTV were analyzed. The homogeneity index (HI) and the conformity index (CI) of the PTV70 were also analyzed. IMRT and VMAT plan showed similar dose coverage, HI, and CI. Maximum dose and dose to 1-cc volume of spinal cord, planning risk volume (PRV) cord, and brain stem were compared. IMRT plan and VMAT plan showed similar results except for the 1 cc of PRV cord that received slightly higher dose in VMAT plan. Mean dose and dose to 50% volume of right and left parotid glands were analyzed. VMAT plan gave better sparing of parotid glands than IMRT. In normal tissue dose analyses VMAT was better than IMRT. The number of monitor units (MU) required for delivering the good quality of the plan and the time required to deliver the plan for IMRT and VMAT were compared. The number of MUs for VMAT was higher than that of IMRT plans. However, the delivery time was reduced by a factor of two for VMAT compared with IMRT. VMAT plans yielded good quality of the plan compared with IMRT, resulting in reduced treatment time and improved efficiency for base of tongue cases.

Universality in eight-arm star polystyrene and methylcyclohexane mixtures near the critical point.

Measurements of the coexistence curve and turbidity were made on different molecular mass samples of the branched polymer-solvent system eight-arm star polystyrene in methylcyclohexane near its critical point. We confirmed that these systems belong in the Ising universality class. The location of the critical temperature and composition as well as the correlation length, susceptibility, and coexistence curve amplitudes were found to depend on molecular mass and the degree of branching. The coexistence curve diameter had an asymmetry that followed a "complete scaling" approach. All the coexistence curve data could be scaled onto a common curve with one adjustable parameter. We found the coexistence curve amplitude to be about 12% larger for branched than linear polystyrenes of the same molecular mass in either solvent cyclohexane or methylcyclohexane. The two-scale-factor universality ratio R was found to be independent of molecular mass or degree of branching.