Original paperImpact of spot size variations on dose in scanned proton beam therapy
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 , based on the 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 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 ( mm at
Results
In Fig. 1 we display as a function of the spot size deviation for our 8 cases for medium spots and inter-spot spacings of , and . 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 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 , 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.
References (16)
The National Centre for Oncological Hadrontherapy (CNAO): status and perspectives
Phys Med
(2015)- et al.
A case study in proton pencil beam scanning delivery
Int J Radiat Onc Biol Phys
(2010) - et al.
Spot-scanned pancreatic stereotactic body proton therapy: a dosimetric feasibility and robustness study
Phys Med
(2016) - et al.
Dosimetric consequences of pencil beam width variations in scanned beam particle therapy
Phys Med Biol
(2013) - et al.
Technical note: spot characteristic stability for proton pencil beam scanning
Med. Phys
(2016) - et al.
The influence of lateral beam profile modifications in scanned proton and carbon ion therapy: a Monte Carlo study
Phys Med Biol
(2010) - et al.
Impacts of gantry angle dependent scanning beam properties on proton PBS treatment
Phys Med Biol
(2017) - et al.
A pencil beam algorithm for proton dose calculations
Phys Med Biol
(1996)
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