Analysis of Proton Therapy Technique Robustness for Prostate Plans Based on CT Surveillance

Reporter: J. Taylor Whaley, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: May 21, 2012

Presenter: Curtiland Deville, Maura Kirk
Presenter's Institution: Hospital of the University of Penn, Philadelphia, PA


  • Prostate cancer is the most common non-skin cancer found in men, with approximately 200,000 cases occurring per year in the United States.
  • Proton therapy is an accepted form of treatment for prostate cancer. Due to the improved dosimetric profile and potential for diminished toxicity compared to x-ray therapy, interest and use continues to rise. Historically, prostate cancers are treated with passive scattered protons using opposed lateral beams, often with one field per day. The motivations for single-field daily treatment include treatment efficiency as well as decreased organ motion.
  • Previous studies have demonstrated intrafraction motion is largely related to the time on the treatment table. Because proton therapy takes longer for each treatment, intrafraction motion is a concern.
  • Because of the sensitivity of protons to organ and set-up motion, attempts to decreased intrafraction motion and diminish heterogeneity are an ongoing area of research. At the Hospital of the University of Penn, patients with prostate cancers are treated with a full bladder and an endorectal balloon to maximize dosimetric advantages.
  • Additionally, fiducial implants within the prostate are utilized to manage interfraction motion and aid in daily alignment.
  • With the implementation of protons rising, radiation treatment planning with proton therapy is an increasing area of interest as technology and knowledge regarding this field evolves. Because of the limitations of passively scattered protons, particularly conformity of dose to the target volume, proton therapy with Pencil Beam Scanning has emerged as a possible solution; however, not all treatment plans are equally robust.
  • The purpose of this study was to evaluate the conformity and robustness of proton plans delivered with uniform scanning (US), pencil beam scanning (PBS) with single field uniform dose optimized to target coverage (SFUD), pencil beam scanning with single field uniform dose optimized to avoidance of organs at risk(OAR), and intensity modulated proton therapy (IMPT).
  • Additionally, the study evaluates the robustness of individual fields to determine the feasibility of delivering one field daily.


  • 10 patients with prostate cancer treated with PBS proton beam therapy in the fixed beam room received weekly verification CTs during the course of treatment.
  • 5 patients were treated on the standard fractionation protocol with prescription doses of 79.2 Gy in 44 fractions, while 5 patients were treated on the hypofractionated protocol with prescription doses of 70.0 Gy in 28 fractions.
  • Plans using US, SFUD, OAR, and IMPT were created using Varian Eclipse for the original CT scan as well as with subsequent weekly CT scans.
  • Verification scans were obtained following treatment and were rigidly registered to the planning CT based on bony alignment and location of fiducials, mimicking treatment alignment.
  • CTV included prostate and seminal vesicles for all patients. PTV included a 5 mm uniform expansion.
  • All plans met target coverage overall and for individual fields with 99% of the CTV covered by 98% of the prescribed dose and 98% of the PTV covered by 95% of the prescribed dose.
  • The original plans were all normalized so the mean dose to the PTV was 100% of the prescription.
  • Conformity was evaluated with plans for original CT as well as subsequent verification scans. Conformity index was defined as CI=V95/VPTV.


  • At initial planning, PBS showed improved conformity compared to US for 2- and single-field proton plans.
  • For evaluation of robustness, all techniques met the CTV planning targets with 2-fields. However, there was improved PTV coverage with US vs. the PBS planning techniques when the prostate and SV were the target volumes. When the target volume included the prostate only, all techniques met the PTV robustness target.
  • For US, robustness maintained from 2- to 1-field delivery regardless of SV inclusion.
  • For PBS with SFUD, the robustness of the plan was maintained if SV not included in the target.
  • The target coverage was more variable when the target volume included the seminal vesicles. This is because the seminal vesicles have a tendency to have more motion than the prostate. Additionally, air in the rectum is also known to distort the seminal vesicle locations.
  • When using a single-field, robustness was worse with the PBS plan optimized for OAR technique. Robustness was met for <90%, even for two fields a day delivery.
  • In order to preserve robustness with PBS plans for >90%, an additional 2mm is neeed for the PBS target optimization volume.

Authors' conclusions

  • In treatment planning for prostate cancer, Uniform Scanning plans are the least conformal but most robust plans for target coverage with single-field delivery.
  • For Pencil Beam Scanning plans, SFUD optimized for target coverage produces the most robust single- and 2-field target coverage.
  • For PBS, an additional 2 mm margin is needed to optimize the volume and improve robustness of single-field plans.
  • Further investigation is required to OAR-optimized PBS plans. Additionally, IMPT plans were not as robust as SFUD plans for proton therapy in prostate cancer.

Scientific/Clinical Implications

  • The authors present their experience with treatment planning for proton radiotherapy for prostate cancer, and the presentation is certainly a valuable contribution to the literature.
  • Planning for proton therapy remains in its early stages and continues to evolve as experience increases.
  • Because of the complexity associated with proton therapy, it is critical that significant attention is given to detailed treatment planning.
  • Certainly, additional studies and clinical experience are needed to continue to improve the delivery of proton therapy.


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