Adjuvant intensity-modulated proton therapy in malignant pleural mesothelioma. A comparison with intensity-modulated radiotherapy and a spot size variation assessment

Our plain-English summary of Adjuvant intensity-modulated proton therapy in malignant pleural mesothelioma. A comparison with intensity-modulated radiotherapy and a spot size variation assessment, by S. Lorentini, M. Amichetti, L. Spiazzi, et al (Strahlentherapie und Onkologie, 2012).

The study

• Authors: S. Lorentini, M. Amichetti, L. Spiazzi, S. Tonoli, S.M. Magrini, F. Fellin, M. Schwarz
• Journal: Strahlentherapie und Onkologie
• Published: 2012;188(3):216-25
• Cited by: 22 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Purpose: Intensity-modulated radiation therapy (IMRT) is the state-of-the-art treatment for patients with malignant pleural mesothelioma (MPM). The goal of this work was to assess whether intensity-modulated proton therapy (IMPT) could further improve the dosimetric results allowed by IMRT.

Patients And Methods: We re-planned 7 MPM cases using both photons and protons, by carrying out IMRT and IMPT plans. For both techniques, conventional dose comparisons and normal tissue complication probability (NTCP) analysis were performed. In 3 cases, additional IMPT plans were generated with different beam dimensions.

Results: IMPT allowed a slight improvement in target coverage and clear advantages in dose conformity (p < 0.001) and dose homogeneity (p = 0.01). Better organ at risk (OAR) sparing was obtained with IMPT, in particular for the liver (D(mean) reduction of 9.5 Gy, p = 0.001) and ipsilateral kidney (V(20) reduction of 58%, p = 0.001), together with a very large reduction of mean dose for the contralateral lung (0.2 Gy vs 6.1 Gy, p = 0.0001). NTCP values for the liver showed a systematic superiority of IMPT with respect to IMRT for both the esophagus (average NTCP 14% vs. 30.5%) and the ipsilateral kidney (p = 0.001). Concerning plans obtained with different spot dimensions, a slight loss of target coverage was observed along with sigma increase, while maintaining OAR irradiation always under planning constraints.

Conclusion: Results suggest that IMPT allows better OAR sparing with respect to IMRT, mainly for the liver, ipsilateral kidney, and contralateral lung. The use of a spot dimension larger than 3 × 3 mm (up to 9 × 9 mm) does not compromise dosimetric results and allows a shorter delivery time.

In plain English

This was a radiotherapy treatment-planning study. It did not study asbestos exposure. The authors used the CT scans of 7 patients with malignant pleural mesothelioma who had been treated at Brescia University Hospital in Italy with chemotherapy, surgery and photon radiotherapy (IMRT). For each patient they designed new computer plans with both IMRT and intensity-modulated proton therapy (IMPT), then compared the predicted doses. No patient received protons. In these plans, IMPT gave a more even and better-shaped dose to the target and lower doses to nearby organs. The average mean dose to the opposite lung was 0.2 Gy with IMPT and 6.1 Gy with IMRT. Model-estimated complication risks were also lower with IMPT, for example 1.1% versus 82% for the kidney on the treated side. In 3 patients, larger proton spot sizes slightly reduced target coverage but shortened the estimated delivery time. The authors concluded that IMPT allowed better sparing of the liver, the same-side kidney and the opposite lung. They noted that no clinical study using protons had ever been performed.

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: Treatment-planning (dosimetric) comparison: new IMRT and IMPT computer plans were made for previously treated patients and compared. Plans with different proton spot sizes were also compared.
• Who was studied: CT datasets of 7 patients with malignant pleural mesothelioma treated with surgery, chemotherapy and radiotherapy at Brescia University Hospital, Italy (out of 45 treated this way from 2005 to 2010). Six of the 7 had right-sided disease. Additional spot-size plans were made for 3 of the patients.
• Exposure: Planned radiotherapy after surgery: 50 Gy in 25 daily fractions of 2 Gy to the planning target volume, with a 60 Gy boost in 3 of 7 cases. Photon IMRT plans were compared with proton IMPT plans, and proton spot sizes of 3, 6 and 9 mm were compared. Asbestos exposure was not studied.
• Main results:
  • Target coverage was similar. For the 50 Gy target, the dose reached by 99% of the volume was on average 1 Gy higher with IMPT, a difference that was not statistically significant (p = 0.2).
  • For the 50 Gy target, IMPT plans were more homogeneous (homogeneity index 0.16 for IMRT, 0.08 for IMPT, p less than 0.0001) and more conformal (conformity index 1.14 for IMRT, 1.02 for IMPT, p less than 0.01).
  • Opposite (contralateral) lung: average mean dose was 6.1 Gy with IMRT and 0.2 Gy with IMPT (p less than 0.001, Table 3).
  • Average mean doses were lower with IMPT for the same-side kidney (29.5 Gy with IMRT vs 14.3 Gy with IMPT, a 15.2 Gy reduction) and the liver (24.5 Gy vs 14.2 Gy, a 10.3 Gy reduction), according to Table 3.
  • Model-estimated risk of kidney injury for the same-side kidney: average 82% with IMRT plans and 1.1% with IMPT plans (p = 0.001).
  • Model-estimated risk of radiation-induced liver disease was lower with IMPT. With the Michigan model it was 0% with IMPT vs 5.8% with IMRT, not statistically significant (p = 0.16). With the Child-Pugh Grade A model it was 1.8% vs 14.3% (p = 0.03). The Grade B model gave 36.9% vs 65.1% (p = 0.003).
  • Model-estimated risk of acute esophagitis: average 14% with IMPT and 30.5% with IMRT (p less than 0.001). The second figure is misprinted as IMPT in the text.
  • In the 3 patients re-planned with larger proton spots (6 and 9 mm), target coverage decreased slightly and organ sparing was generally slightly worse. All larger-spot plans still covered at least 95% of the target with 95% of the dose.
• The authors' conclusion: The authors concluded that, in these plans, IMPT allowed better sparing of organs at risk than IMRT, mainly the liver, the same-side kidney and the opposite lung. They also concluded that proton spot sizes larger than 3 x 3 mm (at least up to 9 x 9 mm) did not compromise the dose results and shortened delivery time.
• Limitations the authors state:
  • The study did not directly address air cavities in the treated area, which can change in density and affect the dose protons deliver.
  • There were no phase III randomized trials, even for IMRT, and no clinical study using protons in this disease. The authors called for prospective controlled studies.
  • Lung complication risks were not estimated, because the published model parameters came from diseases other than mesothelioma.

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, Pleural Mesothelioma, Intensity-Modulated Radiation Therapy (IMRT).

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