Elsevier

Physica Medica

Volume 57, January 2019, Pages 58-64
Physica Medica

Original paper
Impact of spot size variations on dose in scanned proton beam therapy

https://doi.org/10.1016/j.ejmp.2018.12.011Get rights and content

Highlights

  • Robustness of IMPT treatment plans to spot size variations depends on spot size.

  • Inter-spot spacing doesn’t influence robustness to spot size variations.

  • Tolerance limits on spot size variations in proton therapy are proposed.

  • Gamma analysis and DVH parameters may lead to different tolerance limits.

Abstract

Background

In scanned proton beam therapy systematic deviations in spot size at iso-center can occur as a result of changes in the beam-line optics. There is currently no general guideline of the spot size accuracy required clinically. In this work we quantify treatment plan robustness to systematic spot size variations as a function of spot size and spot spacing, and we suggest guidelines for tolerance levels for spot size variations.

Methods

Through perturbation of spot size in treatment plans for 7 patients and a phantom, we evaluated the dose impact of systematic spot size variations of 5% up to 50%. We investigated the dependence on nominal spot size by studying scenarios with small, medium and large spot sizes for various inter-spot spacings. To come to tolerance levels, we used the Γ passing rate and dose-volume-histograms.

Results

Limits on spot size accuracy were extracted for 8 sites, 3 different spot sizes and 3 different inter-spot spacings. While the allowable spot size variation strongly depends on the spot size, the inter-spot spacing turned out to be only of limited influence.

Conclusions

Plan robustness to spot size variations strongly depend on spot size, with small spot plans being much more robust than larger spots plans. Inter-spot spacing did not influence plan robustness. Combining our results with existing literature, we propose limits of ±25%, ±20% and ±10% of the spot width σ, for spots with σ of 2.5, 5.0 and 10 mm in proton therapy spot scanning facilities, respectively.

Introduction

In Intensity Modulated Proton Therapy (IMPT), dose is delivered to the patient by combining the dose from numerous small proton beams (spots) with a certain lateral size, energy, position, and number of protons. To ensure that the planned and delivered dose correspond, the spot characteristics must be stable. The lateral size of the spots is a parameter for which it is challenging to guarantee perfect stability over time [1], [2], [3], [4], [5]. Beam size changes could occur as a result of variations in the proton accelerator (e.g. beam energy, divergence, offset) and in the beam transport (magnet currents, gantry). If beam size modifications persist over many fractions, dose modifications in the patient can occur, with the risk of compromised target coverage and/or overdosage in critical structures [1], [2], [3], [4].

Although the importance of spot size stability is known, literature is scarse and there are no general guidelines available on recommended values of this parameter for existing and future proton therapy spot scanning facilities. Chanrion et al.[1] report that dose modifications can occur for beam size changes 25%, based on dose parameters for prostate and skull-base patients. Parodi et al. [3] suggest ±50% as tolerance limit, based on target coverage for a spherical phantom. Finally Lin et al. [4] report ±10%, based on the Γ analysis of 28 patients.

None of these studies systematically studied the dependence on beam width and inter-spot distance, and moreover none of these studies reported both dose parameters and the Γ analysis together. The latter is useful to understand the full impact of spot size inaccuracies, both in view of machine commissioning as well as patient safety.

The goal of this work is twofold. First, we intend to quantify the clinical influence of spot size changes as a function of spot size and inter-spot distance. This will be done by performing a robustness analysis for 7 patients and a phantom. Second, we combine our results with the existing literature to extract tolerance levels for spot size changes.

Section snippets

Methods and materials

Our patient group (Table 1) consists of 7 patients (pelvis, chest-wall, rectum, chordoma, cardiac, retro-peritoneal, spinal sarcoma) and 1 phantom. For each case we created treatment plans with the Astroid treatment planning system [6], [7]. For optimizing the target and organ-at-risk dose, multi-criteria optimization is used, based on the computation of a set of Pareto optimal plans [8]. Plans were made with 3 different spot widths (values at iso-center in air): small spots (σ=2.5 mm at

Results

In Fig. 1 we display Γ as a function of the spot size deviation for our 8 cases for medium spots and inter-spot spacings of 1σ, 1.5σ and 1.75σ. Results for small and large spots are given in the Supplemental Material. By comparing the black, green and black lines in Fig. 1, we notice that the inter-spot distance had generally only a very small impact on the impact of spot size deviations (see Section 4).

For each patient, we indicate at which spot size deviation the Γ crosses the 90% value,

Discussion

The above study demonstrated the possible clinical impact of spot size inaccuracies for different beam widths and different inter-spot distances for a new patient group. Our study has revealed several new issues and complements previous work about spot size variations.

This work is the first study where the impact of spot size and inter-spot distance on plan robustness to spot size changes has been studied in a systematic way. We found that spot size has a strong influence on the dose impact and

Conclusion

The impact of spot size variations is patient and spot width dependent. Small spot plans are much more robust to spot size changes than large spot plans. Inter-spot distance did not play a major role in the robustness of plans to spot size changes. As rough relative tolerance levels for proton beam width changes, we propose ±25%, ±20% and ±10% for spots with σ2.5, 5, and 10 mm, respectively. Such rough guidelines can be used for instance during development, planning and construction of new

Disclosure of conflicts of interest

The authors have no conflicts of interest to disclose.

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