2. A Treatment planning System
The Eclipse TPS Version 15.6.05, was used for all calculations in this study. The dose calculation algorithms employed by the TPS were Anisotropic Analytical Algorithm (AAA) and Acuros XB algorithm (AXB) for photon beam and the Electron Monte Carlo (eMC) for electron beam. Dose calculations were performed using a virtual water tank (a cube of water created in the Eclipse TPS) for most basic validation tests. For heterogeneity correction tests, plans were made in a heterogonous phantom in TPS, and a number of points were added to the plan representing the location of the ionization chamber. For IMRT/VMAT dose validation tests, clinical reference plans were re-calculated in ArcCHECK’s virtual phantom and exported to the machine for delivery. All plans were generated in the TPS for the MPPG 5.a validation tests in a single test patient. For basic validation tests, each plan contained beams with the same field size but different energies. All dose calculations were performed using a grid size of a 2 mm and exported separately for each beam for comparisons.
2. B Measurement equipment and technique
Beam data and point doses were acquired following the MPPG 5.a’s recommendations using various detectors in a scanning water tank, solid water phantoms, or a 3D detector array device. Table 1 summarises the MPPG 5.a tests with an associated description alongside the equipment used. The radiation source was a Varian iX linear accelerator (Linac) equipped with a Millennium 120 multi-leaf collimator (MLC). The Linac delivers photon beams of 6 and 18 MV (with flattening filter), 6 FFF (flattening filter-free), and electron beams of 6, 9, 12, 16 and 20 MeV. A PTW 3D MP3-M water tank (PTW, Freiburg, Germany) was used with the following PTW detectors; Semiflex ionisation chamber (active volume of 0.125 cm3), a 3D Pinpoint ionization chamber (active volume of 0.016 cm3), and an Electron diode (active volume of 0.03 cm3) for dosimetric measurements with the acquired data processed using PTW’s MEPHYSTO software version 3.4 (PTW, Freiburg, Germany). Following the TRS-398 protocol for reference dosimetry [12], a PTW Farmer and Roos Parallel Plate chambers were used with a PTW UNIDOS electrometer. For heterogeneity measurements, custom made phantoms were designed from solid water slabs (Gammex-RMI, Middleton, WI, USA) and Styrofoam to mimic lung tissue as shown in Fig 2. A schematic of phantoms dimensions, all with 10 cm backscatter after the Styrofoam is illustrated Fig 3; (a) photon heterogeneity dose measurements, (b) 6 MeV electron dose measurements; (c) 9 MeV electron dose measurements; and (d) for 12-20 MeV electron dose measurements. Furthermore, a Gammex electron density CT phantom (Gammex RMI, Middelton, WI, USA) was also used to establish the CT-to-ED curve, and the Sun Nuclear 3D ArcCHECK diode array (Sun Nuclear, Melbourne, FL, USA) was used for IMRT/VMAT verification. All phantoms were scanned on a Phillips Big Bore computed tomography (CT) scanner (Philips Healthcare, Amsterdam, Netherlands) with a 2- mm slice thickness, and then CT datasets were exported to the TPS for dose calculations
2. C The MPPG 5.a validation tests
2. C. 1 Basic beam validation
The basic photon and electron beam algorithm validation tests 5.1 - 5.3 are essential tests to verify the agreement of the dose distribution in the planning module with the commissioning data, reference clinical calibration conditions, beam profiles and output factors for large and small field sizes. In test 5.1, dose distributions of a 40 x 40 cm2 field incident on a large water phantom at an SSD of 100 cm were compared to the corresponding commissioning profiles. Test 5.2; photon and electron absolute dose calculation, under reference conditions, were also performed; a field size of 10 x10 cm2 at an SSD of 100 cm, at reference depth were compared to the measured dose per MU at the same reference conditions. Test 5.3, measured percentage depth doses (PDD), beam profiles, and output factors for square field sizes of 2, 3, 4, 5, 7, 10, 20, and 40 cm2 were compared to calculated data.
Furthermore, the MPPG 5.a recommends six additional basic validation tests for photon beams, five with static MLC field shapes with the jaws placed at clinically relevant positions (5.4-5.8) and one with a non-physical wedge (5.9). For electron beams, two validation tests, 8.1 and 8.2, were performed in a homogenous water phantom. Test 8.1 used a custom cut-out shape in a 15 x 15 applicator. This field represents a clinically relevant shaped field. Test 8.2 aims to verify the surface irregularities obliquity using a standard 10 x 10 applicator at an oblique beam incidence of 20°. Fig 1 illustrates the Beam’s Eye View of the MLC fields created in the TPS. Test 5.4, a small MLC-shape, was intended to mimic a small MLC defined treatment field. A large MLC-shaped field with extensive blocking (e.g., mantle shape) was used in test 5.5. Test 5.6 is an off-axis field with the maximum allowed leaf over-travel in the X-direction with the Y direction defined by the jaws. Test 5.7 is an irregular MLC field shape. Test 5.8 is an MLC defined field at an oblique incident angle of 20° from the vertical. Test 5.9 verifies a large field with an enhanced dynamic wedge (EDW) measured using an ArcCHECK diode array. A 20 × 20 cm2 EDW field with wedge angles of 15 º, 30 º, 45 º and 60º were measured for 6 and 18 MV photon beams. For this test, two sets of measurements were performed, one with Y1‐IN wedge orientation and the other with Y2‐OUT wedge orientation.
All beam profile and PDD measurements for basic validation tests (5.4-5.9, 8.1-8.2) were obtained in a 3D scanning water tank. For photons, one PDD and two beam profiles at dmax, 10 cm or 25 cm depth were measured in the inplane and crossplane directions. For electrons, one PDD and one inplane and one crossplane profiles were measured at a depth of the 90% isodose. All scanning beam data were compared to the calculated data from Eclipse TPS using an open-source scanning data analysis tool ScanDoseMatch (http://www.qxrayconsulting.com/sdm/) [7, 13]. Gamma analysis was performed using a gamma criterion of a 2% / 2 mm following the MPPG 5.a recommendations. Besides the scanned data, additional verification point doses were also acquired at 1.5 cm and 10 cm depths along the central axis (CAX) for tests 5.4, 5.5, 5.7 and 5.8. The absolute dose at each point of the measured profile was acquired using a calibrated Pinpoint chamber.
2. C. 2 Heterogeneity correction validation
The minimum recommended validation tests include confirming the CT-to-ED curve (test 6.1) and verifying TPS calculations beyond heterogeneities (tests 6.2 & 8.3). The CT-to-ED curve was verified in the TPS by scanning an electron density phantom and confirming the CT values of various inserts were reported correctly in the TPS. For dose verification beyond the heterogeneous media for photon beams (test 6.2), measurements were taken at two points located along the CAX; one upper and one lower placed at 4 and 22 cm (4 cm behind the Styrofoam slab), respectively. For electron beams (test 8.3), one measurement was collected close to dmax in heterogeneity phantom, and at least two measurements were acquired along the CAX in the dose fall-off region. Measurements near dmax were taken at 2.5 cm and 3.5 cm for 6 and 9 MeV and at 6.5 cm for 12, 16 and 20 MeV. Measurements along the dose fall-off region were taken at 3 and 3.5 cm for 6 MeV, 4 and 4.5 cm for 9 MeV, 9 and10 cm for 12 MeV, 7 and 10 cm for 16 MeV, and 7 and 11 cm for 20 MeV. All measurements were compared to the Eclipse TPS points representing the location of the ionization chamber.
2. C. 3 IMRT/VMAT dose validation
Several validation tests are recommended for IMRT/VMAT delivery modality. Tests 7.1 and 7.2 are for a very small MLC defined shape field. The PDD and output factor for a 2 x 2 cm2 MLC-shape field were measured in a scanning water tank with a PTW E Diode. For tests 7.3 and 7.4, twelve cases; two IMRT and ten VMAT plans for prostate, endometrium, pelvis, head and neck, brain, and rectum treatment sites, were used. All plans were re-calculated onto the ArcCHECK verification phantom, delivered and evaluated using gamma criteria of 3% /3 mm and 2%/2 mm with absolute dose, global normalization, and a low dose threshold of 10%. Point dose measurements were also performed using a Pinpoint ionization chamber inserted into the ArcCHECK. Finally, for test 7.5, the MPPG 5.a recommends an external review as a complete end-to-end test for IMRT/VMAT delivery validation. Therefore, to verify the accuracy of the TPS, an end-to-end test was performed using the Australian Clinical Dosimetry Service (ACDS) Audit phantoms [14-16] The ACDS audit plans were created, delivered and analyzed following the ACDS Audit specifications.