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Pretreatment quality assurance (QA) is a major concern in complex radiation therapy treatment plans like intensity modulated radiation therapy (IMRT). Present study considers the variations in gamma index for gantry dependent pretreatment verification and commonly practiced zero gantry angle verifications for ten prostate IMRT plans using two commercial medical linear accelerators (Varian 2300 CD, Varian Clinac iX). Two verification plans (the one with all fields at the actual treatment angles and one with all fields merged to 0 degree gantry angles) for all the patients were generated to obtain dose fluence mapping using amorphous silicon electronic portal imaging device (EPID). The gamma index was found depend on gantry angles but the difference between zero and the nonzero treatment angles is in the confidence level for clinical acceptance. The acceptance criteria of gamma method were always satisfied in both cases for two machines and are stable enough to execute the patient specific pretreatment quality assurance at 0 degree gantry angle for prostate IMRTs, where limited number of gantry angles are used.

Modern cancer treatments using radiation therapy is mostly employed with multileaf optimized plans as in intensity modulated radiation therapy. Higher monitor units and continuous motion of multi-leafs during the beam on time need to be strictly monitored for beneficial outcome from IMRT treatments. Complex IMRT plans are widely used in routine clinical practice which requires pretreatment patient specific quality assurance tests [

Pretreatment quality verification is advisable in all IMRT treatment plans to compare the beam fluence maps delivered using continuous motion of multileaf collimators (MLC). Most centers execute the patient specific verification plans generated prior to the first treatment fraction, where all the fields with various gantry angles are merged to zero degree gantry angle and dose fluence generated from TPS is cross-checked using EPID outputs [

Apart from usual zero degree gantry angles, many studies [

This work aims to compare IMRT pretreatment verification for prostate plans, from two commercial medical linac accelerator machines where on-board EPID (PortalVision, Varian Medical Systems, Palo Alto, CA) is available. Dose fluence mapping were performed using EPIDs, to compare with the TPS generated dose maps using gamma index technique (3 mm, 3%, integrated in Eclipse TPS) with two methods: one with actual treatment gantry angles and the other with all gantry angles merged to zero degree gantry angle.

Measurements were taken from two commercial medical linear accelerators—Varian Clinac 2300CD and Varian Clinac iX (Varian Associates, Palo Alto, CA). Clinac 2300 CD is equipped with 80 dynamic MLC and Clinac iX is equipped with Millennium 120 dynamic MLC. Both machines are fully commissioned for treating patients using IMRT technique. Both machines have 6 and 18 MV photon beams and onboard amorphous-silicon EPID (aS500-Varian Medical Systems, Palo Alto, CA). EPID can be positioned at all gantry angles by motorized robotic, three-axis extract arms [

Ten prostate IMRT plans were generated for our study with gantry angles; 0° (anterior), 75° (right anterior oblique), 135° (right posterior oblique), 225° (left posterior oblique), and 285° (left anterior oblique) using 6 MV photon energies as shown in Figure ^{3} PTW 30001) and associated PTW Freiburg electrometer (PTW 10008), using TRS 398 Dw protocol [

Gantry angle directions in 5-field prostate IMRT treatment.

As per the Atomic Energy Regulatory Board of India (AERB) [^{2} for 0°, 60°, 120°, 180°, and 240° gantry angles. These measurements will give the EPID and gantry deviation while gantry on rotation.

The MLC position checks are performed in different ways during the study. Standard shapes (say “

Two sets of verification plans were created, one with the actual gantry angle and the other with all fields merged to zero degree gantry angle. Varian medical system has dedicated portal dosimetry software and is embedded in our Eclipse planning system [

Gamma index pretreatment evaluations were performed for ten patients in two machines separately for actual treatment gantry angles and for the zero degree gantry angles. The criterion of acceptability of the gamma evaluation requires that no more than 5% of the points should have a gamma value larger than one. The percentage of points satisfying the above passing criteria was found by comparing the TPS generated 2D gamma map with EPID provided 2D gamma map in zero and at actual gantry angles. The percentage difference in acceptance criteria for each field (

EPID shifts were found in daily quality assurance checks, for 10 × 10 cm^{2} open square field projections at 0°, 60°, 120°, 180°, and 240° gantry angle positions. Maximum deviations in

Gamma index acceptance criteria were always satisfactory in both linear accelerators for both zero degree gantry and nonzero gantry angle pretreatment quality assurance tests. The difference in two values

Difference in gamma index values (

Difference in gamma index values (

MLC leaf position check from TPS (a) and verification using radiographic film (b) done at Clinac 2300 CD machine.

TPS generated

Shape on radiographic film

Both linacs have independent behavior in the gantry angle dependent pretreatment verifications. Maximum differences in gamma index values were obtained for Clinac iX machine. While for both the machines maximum difference in acceptance criteria was found in 135° and 225° gantry angles, the mean values of the percentage of the points having a gamma less than one for nonzero gantry angles in Clinac 2300 CD machine are

It is always advisable to check the gantry sagging effects for reasonable results from individualized gantry angle pretreatment quality assurance. This improves the IMRT quality and ensures the adequacy of the complex IMRT plans. Periodic mechanical quality assurance for gantry sagging using mechanical pointer attached to collimator must be carried out before IMRT treatments.

Besides the gantry sagging, MLC position accuracy must be studied independently to ensure the MLC position tolerance limits. During this study, collimator leaf position accuracy checks using light field on EPID surface for various gantry angles show negligible deviations. The zero degree gantry angle MLC position was checked for both the machines using EPIDs and radiographic films. Clinac iX and Clinac 2300 CD had a little effect with a mean difference in various leaf positions ranging from about −0.075 to 0.06 mm. The darkening of radiographic film always results in bigger uncertainties on these measurements. Figure

This study shows that the advantages of actual gantry pretreatment quality assurance are totally machine dependent. There is no significant difference obtained once the mechanical quality controls are performed for gantry sagging and MLC leaf positions besides other dosimetric checks. A definite number of gantry angle IMRT plans make these studies possible, while the gantry dependent quality assurance in more complex treatments and ARC therapies is practically time consuming and difficult in fluence mapping.

Patient specific pretreatment QAs should be better done at the treatment angles, if an on-board EPID is available. Pretreatment plan verifications using EPID are preferred to be in gantry angle dependent positions but it was also shown that such requirement is not strictly necessary. If the periodic quality assurances are performed, then the pretreatment IMRT QA could be executed at zero degree gantry angles as well, with considerable time reduction to perform the quality control for each patient. In fewer cases, the zero degree pretreatment analysis is even better than actual degree gantry angles. In a treatment facility where on-board EPID is not available, the time consuming gantry dependent QA using 2D arrays can be restricted to zero gantry angle pretreatment verifications. However, zero degree gantry angle pretreatment plan verifications cannot be employed in continuous arc treatments (i.e., gantry speed stability affects the dose delivery), where gantry dependent quality assurance is difficult.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors sincerely acknowledge the assistance provided by Mr. K. K. Shakir and Mr. A. Siddartha (Medical Physicists, AJ Cancer Institute, Mangalore, India) during the experiments. They are grateful to Professor Dr. Jayaram Shetty (Oncologist, K. S. Hegde Medical Academy, Mangalore, India) for his periodic suggestions.