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Protons with Free Breathing Versus Photons with Deep Inspiration Breath Hold in Post-Mastectomy Patients: A Dosimetric Comparative Analysis
Reporter: Jacob E. Shabason, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: September 26, 2013
Presenting Author: Sagar Patel, MD
Presenting Author Affiliation: Massachusetts General Hospital, Boston, MA
- In select patients with locally advanced breast cancer, post-mastectomy radiation to the chest wall improves survival.
- This treatment can result in excess radiation dose to the heart, coronary arteries and lungs, which may lead to an increase in long-term morbidity. With conventional techniques, sparing these organs at risk without compromising target coverage can be impossible.
- Prior dosimetric studies of post-mastectomy patients with left sided cancers showed an improvement in heart and lung sparing with proton therapy as compared to photons with partially wide tangent fields.
- However, modern photon delivery techniques, specifically deep inspiration breath hold (DIBH), can further spare normal tissue, particularly the heart. These techniques may be more challenging when protons are used due to the required time needed to delivery proton therapy compared to photon.
- As such, the authors sought to compare photons with DIBH to free breathing protons delivered to the chest wall in post-mastectomy patients with locally advanced left sided breast cancer.
- The study included 15 women with locally advanced left-sided breast cancers status post-mastectomy treated with proton radiotherapy on a prospective trial.
- At the time of simulation each patient underwent a free breathing and breath hold scan. Proton plans were generated on the free breathing scans and photon plans were generated using the breath hold scans.
- 50.4 Gy (RBE) was prescribed to the chest wall and 45-50.4 Gy (RBE) to regional lymphatics, including the internal mammary nodes. Plans were generated with the goal of achieving 95% coverage of target volumes while maximally sparing cardiopulmonary structures.
- Doses to target structures and organs at risks were compared between the two techniques.
- Paired t-test and linear regression were used for statistical comparison of the two modalities.
- Proton and photon plans had comparable coverage with a relatively equivalent percentage of target volume receiving the prescription dose:
- Chest Wall: 96.8% (protons) vs. 98.3% (photons), p=0.11.
- Internal mammary nodes: 95.9 % (protons) vs. 98.5% (photons), p=0.09.
- Supraclavicular nodes: 97.0 % (protons) vs. 99.1% (photons), p=0.15.
- However, free breathing proton plans, achieved greater homogeneity with less severe hot and cold spots:
- Chest wall maximum dose: 53.19 (protons) vs. 58.76 (photons), p<0.00001.
- Chest wall minimum dose: 29.31 (protons) vs. 16.23 (photons), p=0.001.
- In addition, proton plans were better able to spare the heart (particularly the left anterior descending artery) and lung when analyzing multiple dosimetric parameters:
- Dose to heart: 0.33 Gy vs. 2.24 Gy, p< 0.01.
- V20 Heart: 0.45% vs. 2.42%, p< 0.01.
- Dose to left ventricle: 0.09 Gy vs. 3.61 Gy, p< 0.01.
- V20 left ventricle: 0.02% vs. 3.31%, p< 0.01.
- Dose to lung: 5.82 Gy vs. 13.12 Gy, p< 0.01
- V20 lung: 12.46% vs. 26.02%, p< 0.01.
- Left anterior descending artery max dose: 7.13 Gy vs. 46.47 Gy, p<0.0001
- For left sided post-mastectomy radiation, free breathing proton therapy achieved greater cardiopulmonary sparing, particularly to the left anterior descending artery, as compared to conventional photon therapy treated at breath hold.
- Both treatment modalities had comparable target coverage, however proton plans were more homogenous with less extreme cold and hot spots.
- The authors plan to assess proton plans treated with DIBH in an attempt to further spare critical cardiopulmonary structures
- The authors present a well done and important dosimetric study showing the potential benefit of proton therapy in post mastectomy radiation for locally advance left sided breast cancer.
- Improved sparing of critical cardiopulmonary structures should decrease the risk of radiation induced cardiopulmonary toxicity. In particular, the decreased dose to the left anterior descending artery should prevent late coronary stenosis.
- Although the doses to critical normal structures in proton plans presented in the study were very favorable, treating with protons using DIBH has the potential to even further spare critical structures.
- It is important to note that the internal mammary lymph nodes were treated in all patients in this study. Because treatment of internal mammary nodes remains somewhat controversial, it would be interesting and clinically relevant to evaluate the differences in proton and photon plans that excluded internal mammary node radiation to see if the conclusions still remain true for these patients.
- Future studies evaluating clinical outcomes with proton therapy in this specific clinical situation will be of great interest, while the data presented here remain hypothesis-generating.