Analysis of Influence of Errors in Angular Settings of Couch and Collimator on the Dosimetric and Radiobiological Parameters in VMAT Plans.

OBJECTIVE: To evaluate the impact of couch and collimator angular variations on dose volume histogram (DVH), tumour control probability (TCP), and normal tissue complication probability (NTCP) of the volumetric-modulated arc therapy (VMAT) plans. METHODS: Stereotactic radiosurgery and stereotactic body radiation therapy VMAT plans were generated for three different hypothetical planning target volumes (PTVs) that mimic brain metastases, single brain lesion, and single spine lesion. Thirty routine VMAT plans (10 prostate, 10 head and neck, and 10 brain cases) treated in our clinic were also selected for this study. The plans were generated using an Eclipse Treatment Planning System and delivered using a Clinac iX linear accelerator equipped with a Millennium 120 multileaf collimator. All the plans were generated using two complementary full arcs (with gantry angle from 179° to 181° and collimator rotation of 30° and 330°) except the brain tumour cases, which used single full arc with collimator rotation of 30°. In all the cases, the couch angle was zero. Impact of the angular variations in the collimator and couch was studied by varying the collimator and couch angular settings by 1°, 2°, and 3°, and creating six erroneous plans corresponding to the original plans. The variation due to these errors on different DVH and radiobiological parameters (TCP, equivalent uniform dose (EUD), and NTCP) of the PTVs and organs-at-risk (OARs) were observed. The relative percentage of difference in these parameters (ΔD, ΔTCP, ΔEUD, and ΔNTCP) were analysed, and statistical significance was tested. RESULTS: The variation due to collimator misplacement was observed to be larger than the couch misplacement. Furthermore, in both cases, the variation increased as the degree of error increased. Among the DVH parameters, D98%, D95%, and V95Gy were affected more by the errors than D2%, D5%, and D50%, in both hypothetical and clinical PTVs. In the clinical PTVs, the TCP showed the most variation among all parameters. The ΔNTCP of the bladder and brain OARs were zero, whereas for head and neck OARs, it was high. CONCLUSIONS: The couch and collimator angular variation has different effects on different planning situations and different parameters. The outcome produced by the errors is specific to the treatment sites.

Effect of fluence smoothing on the quality of intensity-modulated radiation treatment plans.

A fluence-smoothing function applied for reducing the complexity of a treatment plan is an optional requirement in the inverse planning optimization algorithm of intensity-modulated radiation therapy (IMRT). In this study, we investigated the consequences of fluence smoothing on the quality of highly complex and inhomogeneous plans in a treatment-planning system, Eclipse™. The smoothing function was applied both in the direction of leaf travel (X) and perpendicular to leaf travel (Y). Twenty IMRT plans from patients with cancer of the nasopharynx and lung were selected and re-optimized with use of various smoothing combinations from X = 0, Y = 0 to X = 100, Y = 100. Total monitor units (MUs), dose-volume histograms, and radiobiological estimates were computed for all plans. The study yielded a significant reduction in the average total MUs from 2079 ± 265.4 to 1107 ± 137.4 (nasopharynx) and from 1556 ± 490.3 to 791 ± 176.8 (lung) while increasing smoothing from X, Y = 0 to X, Y = 100. Both the tumor control and normal tissue complication probabilities were found to vary, but not significantly so. No appreciable differences in doses to the target and most of the organs at risk (OARs) were noticed. The doses measured with the I'MRT MatriXX 2-D system indicated improvements in deliverability of the plans with higher smoothing values. Hence, it can be concluded that increased smoothing reduced the total MUs exceptionally well without any considerable changes in OAR doses. The observed progress in plan deliverability in terms of the gamma index strongly supports the recommendation of smoothing levels up to X = 70 and Y = 60, at least for the nasopharynx and lung.

Analysis of Dosimetric Impacts of Cone Beam Computed Tomography-Based Volumetric Modulated Arc Therapy Planning.

OBJECTIVE: To quantify the Hounsfield unit (HU) variations between computed tomography (CT) and cone beam CT (CBCT) and study its impact on volumetric modulated arc therapy (VMAT) plans. METHODS: HU number variations in CT and CBCT images were evaluated using the Catphan-504 phantom, and changes in seven different materials within the phantom (air, polymethylpentene, low-density polyethylene, polystyrene, acrylic, Delrin, and Teflon) were studied. The HU variations in half-fan and full-fan modes of CBCT were evaluated. The effect of variations in the shape of the body cross sections was assessed by reducing the body of the Catphan by 0.5 cm and 1.0 cm. CBCT-based VMAT plans in 27 patients (10 prostate, 10 brain, and 7 head and neck (HN)) were compared with corresponding CT-based plans. The dosimetric variations were assessed referring to different points on the dose volume histogram (D5%, D50%, and D95% for PTVs and D1%, Dmax, and Dmean for organs at risk). The relative percentage of difference (ΔD (%)) between CT- and CBCT-based VMAT plans were examined on these points. To evaluate the dosimetric accuracy, dose distributions were compared using Omnipro-I'mRT software. The VMAT plans were evaluated based on 3 mm-3%, 2 mm-2%, and 1 mm-1% gamma criteria. RESULTS: The HU difference in CT and CBCT was highest for air, Delrin, and Teflon, whereas the difference was less than 20 HU for the other materials. The dose volume histograms of both CT- and CBCT-based plans were in excellent agreement in both phantom and patients, except in HN cases where the difference was 7%. The average 3 mm-3% gamma pass points in brain, prostate, and HN patients were 97 ± 0.2%, 96 ± 0.06%, and 93.3 ± 1.1%, respectively. The gamma pass rates reduced to 88.8 ± 0.06%, 91 ± 0.04%, and 79 ± 6% in 2 mm-2%, and further declined to 76.6 ± 0.09%, 75.2 ± 0.5%, and 60 ± 6% using the stringent 1 mm-1% gamma criteria for brain, prostate, and HN cases, respectively. CONCLUSION: Based on the results of this study, it is our belief that CBCT images can be used as a tool for evaluating the dosimetric variation in patient VMAT plans.