Is there a single spot size and grid for intensity modulated proton therapy? Simulation of head and neck, prostate and mesothelioma cases

Our plain-English summary of Is there a single spot size and grid for intensity modulated proton therapy? Simulation of head and neck, prostate and mesothelioma cases, by L. Widesott, A.J. Lomax, M. Schwarz (Medical Physics, 2012).

The study

• Authors: L. Widesott, A.J. Lomax, M. Schwarz
• Journal: Medical Physics
• Published: 2012;39(3):1298-308
• Cited by: 35 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Purpose: To assess the quality of dose distributions in real clinical cases for different dimensions of scanned proton pencil beams. The distance between spots (i.e., the grid of delivery) is optimized for each dimension of the pencil beam.

Methods: The authors vary the σ of the initial Gaussian size of the spot, from σ(x) = σ(y) = 3 mm to σ(x) = σ(y) = 8 mm, to evaluate the impact of the proton beam size on the quality of intensity modulated proton therapy (IMPT) plans. The distance between spots, Δx and Δy, is optimized on the spot plane, ranging from 4 to 12 mm (i.e., each spot size is coupled with the best spot grid resolution). In our Hyperion treatment planning system (TPS), constrained optimization is applied with respect to the organs at risk (OARs), i.e., the optimization tries to satisfy the dose objectives in the planning target volume (PTV) as long as all planning objectives for the OARs are met. Three-field plans for a nasopharynx case, two-field plans for a prostate case, and two-field plans for a malignant pleural mesothelioma case are considered in our analysis.

Results: For the head and neck tumor, the best grids (i.e., distance between spots) are 5, 4, 6, 6, and 8 mm for σ = 3, 4, 5, 6, and 8 mm, respectively. σ ≤ 5 mm is required for tumor volumes with low dose and σ ≤ 4 mm for tumor volumes with high dose. For the prostate patient, the best grid is 4, 4, 5, 5, and 5 mm for σ = 3, 4, 5, 6, and 8 mm, respectively. Beams with σ > 3 mm did not satisfy our first clinical requirement that 95% of the prescribed dose is delivered to more than 95% of prostate and proximal seminal vesicles PTV. Our second clinical requirement, to cover the distal seminal vesicles PTV, is satisfied for beams as wide as σ = 6 mm. For the mesothelioma case, the low dose PTV prescription is well respected for all values of σ, while there is loss of high dose PTV coverage for σ > 5 mm. The best grids have a spacing of 6, 7, 8, 9, and 12 mm for σ = 3, 4, 5, 6, and 8 mm, respectively.

Conclusions: The maximum acceptable proton pencil beam σ depends on the volume treated, the protocol of delivery, and optimization of the plan. For the clinical cases, protocol and optimization used in this analysis, acceptable σs are ≤ 4 mm for the head and neck tumor, ≤ 3 mm for the prostate tumor and ≤ 6 mm for the malignant pleural mesothelioma. One can apply the same procedure used in this analysis when given a "class" of patients, a σ and a clinical protocol to determine the optimal grid spacing.

In plain English

This was a computer treatment-planning study in radiation physics. It did not study asbestos exposure. The authors simulated intensity modulated proton therapy (IMPT), which delivers many small scanned proton beams ("spots"). They planned one nasopharynx (head and neck) cancer case, one prostate cancer case and one malignant pleural mesothelioma case. They varied the beam width (sigma) from 3 to 8 mm and the spacing between spots from 4 to 12 mm. They then checked target coverage against each case's dose goals; organ dose limits were always met. For the mesothelioma case, the lower-dose target met its goal at every beam width, but there was some loss of coverage of the higher-dose boost target above 5 mm. The best spacings there were 6, 7, 8, 9 and 12 mm for widths of 3, 4, 5, 6 and 8 mm. The authors concluded that the maximum acceptable beam width depends on the volume treated, the delivery protocol and plan optimization. They found acceptable widths of 4 mm or less for head and neck, 3 mm or less for prostate and 6 mm or less for mesothelioma. They assumed the target was static.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Radiotherapy treatment-planning simulation (computer dose-calculation study) comparing proton beam sizes and spot spacings
• Who was studied: Three clinical planning cases: one nasopharynx cancer, one prostate cancer and one malignant pleural mesothelioma. The article does not give patient numbers beyond one case each, or locations or dates. For the mesothelioma case, the treated target included the hemithorax cavity, diaphragm and pericardial reconstruction, mediastinal fatty tissue and drain sites.
• Exposure: No asbestos exposure was studied. The variable studied was the width of the scanned proton beam, from 3 to 8 mm, with spot spacing ranging from 4 to 12 mm. The mesothelioma plans used two beams. They delivered 50 Gy in 25 fractions to the main target and a simultaneous 60 Gy boost to areas at increased risk of recurrence.
• Main results:
  • Mesothelioma case: the prescription for the main (lower-dose) target volume was met for all beam widths. There was some loss of coverage of the higher-dose boost volume for beam widths above 5 mm.
  • Mesothelioma case: the best spot spacings were 6, 7, 8, 9 and 12 mm for beam widths of 3, 4, 5, 6 and 8 mm respectively.
  • Mesothelioma case: organ-at-risk dose limits were met for all beam widths considered.
  • Head and neck case: beam width had to be 4 mm or smaller to cover 95% of the high-dose target volume with more than 64.7 Gy(RBE). It had to be 5 mm or smaller for the lower-dose target volume.
  • Prostate case: only beams of 3 mm width met the first coverage requirement. The distal seminal vesicles target was well covered with beams as wide as 6 mm.
• The authors' conclusion: The authors concluded that the maximum acceptable proton beam width depends on the volume treated, the delivery protocol and the plan optimization. For their cases, they gave acceptable widths of 4 mm or less (head and neck), 3 mm or less (prostate) and 6 mm or less (malignant pleural mesothelioma).
• Limitations the authors state:
  • The target was assumed to be static.
  • The authors did not assess elliptical spots or non-regular spot spacing.
  • The dose protocols came from TomoTherapy. The authors said conclusions could differ with a protocol from another photon system, such as VMAT or RapidArc.
  • The use of a range shifter for low-energy spots likely influenced the head and neck results.
  • The authors said the effect of beam size on plan robustness needs to be verified more systematically.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Proton Therapy, Mesothelioma, Pleural Mesothelioma.

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