Research ArticleClinical Studies
Open Access

The Impact of Dose Calibration Point Differences Between the Treatment Plan and Patient-specific Quality Assurance in Passive Scattering Proton Beam Therapy

YUTARO MORI, TOMONORI ISOBE, HIDEYUKI TAKEI, SATOSHI KAMIZAWA, TETSUYA TOMITA, DAISUKE KOBAYASHI, HIDEYUKI SAKURAI and TAKEJI SAKAE
In Vivo September 2024, 38 (5) 2478-2483; DOI: https://doi.org/10.21873/invivo.13718

Abstract

Background/Aim: Passive scattering proton beam therapy (PSPT) is performed by taking actual measurements of all pre-designated fields in a treatment plan followed by appropriate adjustments to the prescribed dose. For this reason, it is necessary to ensure precision management of the measurements (patient-specific calibration) in the administration of a planned dose. Therefore, this study investigated the impact on dose distribution in treatment planning when the patient calibration point differs from the normalized point in a treatment plan. Patients and Methods: A total of 16 cases were selected, where the patient calibration point and normalized point did not match, and the normalized point used in the treatment plan was changed to the patient calibration point using a treatment planning system (VQA ver. 2.01, HITACHI). At this point, the displacement of the relative dose at the isocenter was estimated as an error owing to the difference compared to the patient calibration point. Results: Overall, the error was within the range of ±1.5%, with the exception of orbit cases. Calibrated points also tended to be lower than the normalized points in the treatment plan. In terms of treatment sites, a greater deviation was observed for head cases. Cases with a large deviation in sites other than the head were attributed to poor flatness within the radiation field owing to a narrower opening of the patient collimator. Conclusion: Dose measurement errors in PSPT due to differing calibration points were generally within ±1.5%, with higher deviations observed in head treatments because of complex structures and narrow collimator openings. A γ analysis for significant deviations showed a 98.7% passing rate, suggesting limited overall impact. It is important to select stable calibration points in dosimetry to ensure high precision in dose administration, particularly in complex treatment areas.

Key Words:

In passive scattering proton beam therapy (PSPT), the pre-patient collimators and compensating filters (boluses) that form the dose distribution are designed according to the field of the treatment plan of the patient (1-3). At the same time, the radiation field forming system, such as the ridge filter and fine degrader, in addition to the depth from the body’s surface to the lesion, vary depending on the conditions of the treatment plan (4-6). In PSPT, all predesignated radiation fields in a treatment plan are measured per field (patient-specific calibration) to adjust the radiation dose (7, 8). For this reason, a high precision of patient calibration is required in order to administer the radiation dose as intended in the treatment plan, in general (9, 10). For patient calibration in our hospital, a point that is assumed to have a flat profile in the lateral direction is determined after the shape of the bolus is verified visually. The measurement depth (MD) is then adjusted such that the direction of depth at this point overlaps with the spread-out Bragg peak (SOBP) center. The dose is measured by this setup, and calibration is performed by comparison with a predicted value based on calculations. The measurement point in this process is the patient calibration point. However, the isocenter is defined as the normalized point of a dose distribution in many cases (11, 12). Therefore, there are cases in which the patient calibration point and the normalized point in a treatment plan are different. An example of this discrepancy is shown in Figure 1. The figure shows that measurement points of the anterior beam (0°), posterior beam (180°), and normalized point all vary. Therefore, there is a concern that the dose may not be administered as defined in the treatment plan in some cases. This study investigated the impact of the difference between the patient calibration point and the normalized point on the dose distribution at the time of treatment planning. This study was conducted after obtaining approval from the Clinical Research Ethics Review Committee (H25-47).

Figure 1.
Figure 1.

Measurement points and normalized point. The figure shows an example of opposed 0° and 180° double-field beams. Measurements were acquired at different points against the isocenter for both 0° and 180° beams.

Patients and Methods

Case selection. The case numbers and corresponding irradiated sites are as follows; Case numbers 1 to 3 are nasal cavity, 4 and 5 are eye, 6 and 7 are spinal canal (C7 to Th7), 8 and 9 are lung, 10 and 11 are esophagus, 12 and 13 are liver, 14 is pancreas, 15 and 16 is prostate. Irradiated sites on the body are broadly grouped as head, chest, esophagus, stomach, and prostate, to incorporate cases from every part of the body where proton beam therapy is applied. In every case, the isocenter is defined as the normalized point in the patient plan, while the patient calibration point varies from the isocenter.

Procedure. Although all patient calibration is performed using a parallel beam at an angle of −90°, the beams in an actual treatment are irradiated from various angles over a range of 360°. Therefore, the position of the patient calibration point in a treatment plan needs to be verified based on the following procedure.

For the patient calibration point, the amount of movement in the lateral direction and MD from the isocenter was verified through visual examination of the beam from −90° (Figure 2). This point is assumed to be (x, y, z). In an actual treatment, a beam is irradiated from a direction by displacement through an angle −90°. In this case, the patient calibration point that moved to the actual treatment coordinate (x′, y′, z′) can be calculated as (ysinθ, ycosθ, z). Using this approach, the position of the patient calibration point irradiated from −90° can be moved to a treatment coordinate irradiated from angle θ.

Figure 2.
Figure 2.

Flow of patient-specific quality assurance. For the y coordinate of the patient calibration point, if the irradiation direction varies between the patient calibration and the treatment plan, the (x, y) coordinates need to be changed from (0, y) to (x’, y’).

Using a treatment planning system (VQA ver. 2.01, HITACHI Corp., Chiyoda-ku, Tokyo, Japan), a normalized point in a treatment plan was changed to the patient calibration point measured from the typically used isocenter. At this point, the magnitude of the displacement of the relative dose at the isocenter after a change of the calibration point was estimated as an error caused by the difference in the patient calibration point.

Results

Figure 3 shows the dose error between the normalized point during treatment planning and patient calibration. The serial number in the horizontal axis corresponds to the case shown in Table I. Overall, the error was generally within ±1.5% with the exception for Cases 4 and 5. The error associated with the calibrated point also tended to be lower than that of the normalized point in the treatment plan. Among the body sites, the deviation tended to be larger for the head (Case 4: −2.2%, Case 5: −2.1%).

Figure 3.
Figure 3.

Dose error between the calibration point defined by the treatment plan and the patient-specific quality assurance calibration point. The case numbers along the horizontal axis are the same as the numbers shown in Case Selection.

View this table:
Table I.

Irradiation conditions for cases 4 and 5.

Discussion

For Figure 3, a significant deviation was observed in the sites other than the head in some cases (Cases 8 and 14) owing to a poor level of flatness within the radiation field, caused by a narrower opening of the patient collimator (approximately 4 cm×4 cm) (13). For cases 4 and 5, where the deviation exceeded 1.5%, a γ analysis was performed using Verisoft (ver. 5.0, PTW-Freiburg, Freiburg, Germany) for the original treatment plan and a plan in which the normalized point was changed to the patient calibration point. Evaluation was performed on an axial surface that included the isocenter, where the dose difference (DD) was defined as 1%, the distance to agreement (DTA) as 1 mm, and the threshold as 30%. The conditions for irradiation are defined in Figure 4 and Table I. Figure 5 shows the result of the γ analysis. Green represents a passing rate of over 90%, yellow represents a passing rate in the range of 80~90%, and red represents a passing rate below 80%. A total of 8,836 points were evaluated and 8,719 points satisfied the conditions (98.7%) while 117 did not (1.3%). Errors occurred in these cases because a complex bone structure of the orbit was included in the target (Figure 4). The dose distribution of the treatment plan causes small fluctuations within the targeted area when the area is uneven, and the treatment is expected to be affected by the movement of the calibration point. However, the passing rate in the γ analysis was 98.7%, which is favorable even in a relatively strict evaluation where DD was 1% and DTA was 1 mm. In particular, changes were observed primarily in the low-dose area, and few disagreements were observed within the target. Of course, if the normalized point and the patient calibration point are the same, the error investigated in this study can be removed. However, there is also a significant advantage in acquiring measurements by offsetting the patient calibration point. In the case of treatment plan as shown in the Figure 6, the error becomes larger if a measurement is taken at the isocenter, which is also the normalized point in the treatment plan. This is because the bolus over the isocenter is subjected to a significant dispersion owing to its complex shape, which compromises the measurement precision (14-16). In such a case, it is useful to change the patient calibration point to a different point where measurements can be acquired in a stable manner to ensure that the precision is maintained.

Figure 4.
Figure 4.

Treatment plan (Case 4 and Case 5). The figure shows an image of a treatment plan for Cases 4 and 5. Both cases target the complex bone structure of the orbit.

Figure 5.
Figure 5.

γ-index of the head case. The figure shows the result of the γ analysis for a change in the dose owing to the difference in the calibration points between Cases 4 and 5. The difference was greater in the low-dose area, while the difference was small in the high-dose area.

Figure 6.
Figure 6.

Advantages of shifting measurement points. When calibrating with isocenter, there is a possibility that accurate measurement cannot be performed due to scattered radiation, not flat profile. By changing the measurement point to a flat region, accurate measurement becomes possible.

Conclusion

The impact of the difference between a normalized point and a patient calibration point was investigated in PSPT. The impact was generally within the range of ±1.5% and was therefore small. A γ analysis was performed as an additional investigation for cases in which the error exceeded 2%. However, the passing rate was 98.7%, which suggests that the overall impact on an irradiated area was limited. An accurate and highly precise dose measurement is difficult if an area with a steep dose gradient is included in the normalized point (isocenter) in a treatment plan, and a method of patient calibration at a point where measurements can be acquired in a stable manner is therefore useful.

Footnotes

  • Authors’ Contributions

    Mori Y and Isobe T: Research conception and design, drafting of the manuscript, and final approval of the version to be published. Takei H and Kamizawa S: Data acquisition, analysis, and interpretation; substantial contribution to the methodology section. Tomita T and Kobayashi D: Statistical analysis and data validation; assisted in the interpretation of results. Mori Y and Takei H: Conducted key experiments and contributed to the experimental design and execution. Mori Y and Kamizawa S: Literature review and background research; contributed to the introduction and discussion sections. Isobe T and Sakae T: Supervised the research project and provided critical revisions to the manuscript. Sakurai H and Sakae T: Coordinated the research activities and ensured the alignment of the project milestones; contributed to the funding acquisition. All Authors have read and approved the final manuscript.

  • Funding

    No financial support for this study was provided.

  • Conflicts of Interest

    The Authors report no conflicts of interest in relation to this study.

  • Received April 30, 2024.
  • Revision received May 25, 2024.
  • Accepted May 27, 2024.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) 4.0 international license (https://creativecommons.org/licenses/by-nc-nd/4.0).

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The Impact of Dose Calibration Point Differences Between the Treatment Plan and Patient-specific Quality Assurance in Passive Scattering Proton Beam Therapy
YUTARO MORI, TOMONORI ISOBE, HIDEYUKI TAKEI, SATOSHI KAMIZAWA, TETSUYA TOMITA, DAISUKE KOBAYASHI, HIDEYUKI SAKURAI, TAKEJI SAKAE
In Vivo Sep 2024, 38 (5) 2478-2483; DOI: 10.21873/invivo.13718
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