A Comparison of 3-D Conformal Proton Therapy, Intensity Modulated Proton Therapy, and Intensity Modulated Photon Therapy for Retroperitoneal Sarcoma

Reviewer: John P. Plastaras, MD, PhD
Abramson Cancer Center of the University of Pennsylvania
Last Modified: November 9, 2006

Presenter: Christine S. Chung
Presenter's Affiliation: Harvard Radiation Oncology Program, Boston, MA
Type of Session: Scientific

Background

The treatment of retroperitoneal sarcoma usually involves a combination of surgery and radiotherapy.
Post-operative radiotherapy can entail treating very large, complex volumes located in close proximity to critical normal tissues. This has limited the maximum dose to 45-50 Gy.
Technological advances in radiotherapy delivery may improve the ability to deliver high doses while sparing normal structures.
- Proton radiotherapy can be very conformal, because there is no exit dose and there is minimal side scatter
- To date, most proton therapy has used passive scattering, which can control the shape of the distal edge of a planning volume, but not the proximal edge.
- Using intensity modulated proton therapy (IMPT) with scanning beam protons, both the proximal and the distal edge of a planning volume can be shaped.

Materials and Methods

This dosimetric planning study compared 3 different treatment methods:
- 3D conformal proton therapy (3D CPT)
- intensity modulated proton therapy (IMPT)
- intensity modulated photon therapy (IMXT)
Patients:
- 10 patients undergoing surgery for retroperitoneal sarcoma
- Radiotherapy was planned pre-operatively in 6 patients and post-operatively in 4 patients
Clinical tumor volume (CTV) included:
- Post-operatively: structures considered at risk for microscopic residual disease
- Pre-operatively: gross tumor with 1.5 cm volume expansion where possible
Target volume doses:
- 50.4 GyE in 1.8 Gy/ fraction
- Pre-op Boost of 900 cGy (total 59.4 GyE)
- Post-op Boost of 1620 cGy (total 66.6 GyE)
Dosimetric constraints:
- Liver: 50% < 30 GyE
- Small Bowel: 90% < 45 GyE
- Stomach, Colon, Duodenum: max 50 GyE
- Kidney: 50% < 20 GyE

Results

All three planning techniques were able to get dose to the planning target volume
3D CPT had the best dose homogeneity to the planning target volume:
- Mean inhomogeneity coefficient: 3D CPT 4.0%, vs. IMXT: 6.8% and IMPT 7.5%
IMPT had the lowest mean doses to organs at risk:
- Liver: IMXT: 11.8 Gy, 3D CPT 6.6 Gy, IMPT 5.7 Gy
- Stomach: IMXT: 15.4 Gy, 3D CPT 11.8 Gy, IMPT 7.8 Gy
- Small bowel: IMXT: 13.4 Gy, 3D CPT 7.1 Gy, IMPT 5.8 Gy

Author's Conclusions

This is the largest treatment planning study for retroperitoneal sarcomas to date.
IMPT, 3D CPT, and IMXT provide excellent target coverage within normal tissue constraints, to doses of 50.4 Gy.
IMPT and IMXT plans result in higher inhomogeneity within the target, but all techniques yield clinically acceptable outcomes.
IMPT achieves the closest conformity.
IMPT and 3D CPT provide the lowest radiation dose to organs at risk.
Selective dose escalation to retroperitoneal margins of tumors was achievable with IMPT.

Clinical/Scientific Implications

This dosimetric study compared IMXT with either standard 3D proton planning or IMPT for management of retroperitoneal sarcomas. This is a traditionally very difficult tumor type and body site to treat with radiotherapy, given the large, complex target volumes near organs at risk.
IMPT using scanning proton therapy is the next generation of highly conformal radiotherapy. Not only is IMPT more conformal, it is also associated with lower neutron contamination than either IMXT or passive scattering proton therapy.
Several centers in the U.S. are preparing to use IMPT with scanning proton therapy, but its availability will be limited in the next several years.
Long-term clinical data on patients treated with IMPT will be required to determine if these dosimetric gains can be translated into clinical gains