Comparison of Measured and Calculated Halcyon 3.0 Beam Data

Introduction

Methods

Measurements
The measurements were made using 6 MV-FFF photon beams by utilizing the Varian Halcyon v3.0 system. The output of the machine was calibrated to deliver 1 cGy/MU under reference conditions; source to skin distance (SSD)=100 cm, field size = 10×10 cm², and at depth=d_max (1.3 cm). Before measuring the beam data, the output of the machine was checked using RW3 water- equivalent slab phantom and a 0.6 cc farmer-type ionization chamber according to the TRS-398.[] The setup was made with the following steps as there is no light field, optical distance indicator, and isocenter lasers in the Halcyon. The alignment of the phantom was performed with the bore lasers and then loaded to the beam center. The chamber position was verified with two orthogonal MV images. TPR_20,10 at 10×10 cm² was also measured for 6MV-FFF.

A PTW MP3-M water tank (PTW-Freiburg, Germany) and a semiflex 0.125 cm³ ionization chamber (type 31010, PTW-Freiburg, Germany) were used for all beam data measurements. The PDD and lateral dose profiles were obtained for open fields with sizes from 4×4 cm² to 28×28 cm² determined by MLC settings. The SSD was set to 100 cm. The lateral dose profiles were measured at a depth of 10 cm under the same setup conditions. All data were smoothed and normalized to maximum value for each PDD curve. The normalization was conducted to the central-axis value for each profile. The beam data analysis was performed with Mephysto mc2 software.

Calculations
Vendor provides RBD set and configures dose calculation models using these data in Eclipse TPS. In this study, the virtual water phantom with size of 40×40×40 cm³ was created in Varian Eclipse v17.1 TPS (Varian Medical Systems, Palo Alto, CA). The PDD and profiles were calculated with the AAA v17.1 algorithm for the same field sizes as described in ion chamber measurements. The dose for each calculation was set at 100 MU. The calculation grid size was 2.5 mm. The PDDs were normalized to their maximum values for each field size. The profiles were normalized to the corresponding central axis value for each field size.

Analysis
The calculated dose distribution with AAA algorithm for PDD and profiles were compared with our measurements. The PDD value at 10 cm (PDD₁₀), which is beam quality specifier according to TG-51, and depth of dose maximum (d_max) were recorded.

The field width and penumbra were analyzed under profiles. Usually, the radiation field size of FF photon beams is specified at 50% of the isodose level of profile. This definition cannot be implemented for the field size of FFF photon beams because the 50% isodose level occurs at the high dose gradient part of the profile. The determination of the field size and penumbra was carried out according to the Atomic Energy Regulatory Board of India Task Group (AERB-TG) recommendations.[]

The inflection point (IP) is defined as the midpoint on either side of the high gradient region (sharply de scending part) of the beam profile. The starting point (S) and ending point (E) of the high gradient region of the beam profile are identified. The vertical separation between S and E is the height (h) of the high gradient region of the beam profile. The point at h/2 is considered as the point of inflection. The lateral separation between left and right IP (IP_L and IP_R) along the central axis is taken as field width. For determining penumbra, dose value at IP was taken as reference dose value (RDV). Points Pa and Pb are located at 1.6 and 0.4 times of RDV, respectively. Lateral separation between Pa and Pb on either side of the profile is measured as penumbra.[] Figure 1 shows all definitions. The field size and penumbra were calculated using graphical manual calculations method for all profiles. The lateral distance from the central axis at 90%, 75%, and 60% dose points on either side of the beam as the degree of unflatness was also analyzed, based on the protocol in AERB-TG (Fig. 2).

Fig. 1: Schematic diagram for determining inflection points, field size, and penumbra.
RDV: Reference dose value

Fig. 2: Schematic diagram for determining lateral distances at 90%, 75%, and 60% dose points on the beam profile.

Results

Fig. 3: Measured and calculated PDDs for each field size.
PDD: Percentage depth dose

Table 1: The d_max and PDD₁₀ of the PDDs between measured and calculated

The normalized central axis lateral dose profiles are measured and calculated for each field size at depth of 10 cm are shown in Figure 4. The comparison of radiation field size and geometric penumbra values of measured and calculated central axis dose profiles at 10 cm depth for SSD=90 are presented in Table 2. The measured field size and penumbra values indicated an excellent agreement with calculated values with a maximal discrepancy of 0.17 cm and 0.5 mm for all field sizes, respectively. The lateral distance from the central axis at 90%, 75%, and 60% dose points on either side of all measured and calculated profiles for SSD=90 is summarized in Table 3. The differences between calculated and measured lateral distances for all field sizes were within 0.20 mm.

Fig. 4: Measured and calculated lateral dose profiles for each field size.

Table 2: Field size and penumbra analysis

Table 3: Lateral distance from the central axis at 90%, 75%, and 60% dose points on either side for measured and calculated profiles

Discussion

Conclusion

Peer-review: Externally peer-reviewed.

Conflict of Interest: All authors declared no conflict of interest.

Ethics Committee Approval: The study was approved by the Academic Coordination Community of Istanbul University Institute of Oncology (no: 2065990, date: 24/08/2023).

Financial Support: None declared.

References