By Ryan Grover, MD, Radiation Oncologist, Scripps Proton Therapy Center
Head and neck cancers can be challenging to treat as they are often adjacent to multiple critical structures, such as the optic nerves or the brain stem. At Scripps, we are successfully treating these complex cancers using multi-field optimization intensity-modulated proton therapy (MFO-IMPT) and volumetric cone beam computed tomography (CBCT) for patient positioning and treatment monitoring.
IMPT has the capability of providing a highly conformal dose, which is particularly important for head and neck cancers that require a high dose gradient between the tumor volume and organs at risk. MFO techniques allow us to take full advantage of IMPT capabilities by optimizing multiple beams simultaneously to treat hard-to-reach tumors, such as those wrapped around healthy tissue.
However, these benefits also make MFO-IMPT highly sensitive to variations in the range of the proton beam; daily setup errors; anatomical changes due to weight loss; different organ filling levels; intrafractional patient motion; and changes in tumor volume and/or shape. Monitoring the patient’s treatment with the use of CBCT and adapting treatment plans/approaches when necessary to fit each patient is critical.
At Scripps, we have developed effective strategies to monitor patient changes and consequent effects on delivered dose. CBCT, in combination with regularly scheduled verification CTs, allows us to track changes in a patient’s geometry throughout the course of treatment, better determine actual dose delivered, and decide if and when plans should be adapted.
Patient case study
Recently, we treated a 28 year-old woman who was diagnosed with chondrosarcoma of the ethmoid sinus in 2015 and had the tumor removed surgically. She ultimately chose Scripps for post-surgical proton therapy based on our growing reputation in the San Diego community and the advanced technology we offered.
We successfully treated her cancer in 35 treatments at 70 Gy over the course of 7.5 weeks, using Varian’s ProBeam® proton therapy system and developing MFO plans using Varian’s Eclipse™ treatment planning system for proton. The total time for each treatment was about 20 minutes, including about 5 minutes for patient setup, CBCT acquisition, and verification orthogonal KV images.
MFO-IMPT treatment plan generation
Due to the risk of cancer cells that may have remained after surgery, we planned to treat the area from which the tumor was removed as well as immediately surrounding, at-risk areas. This case was challenging because the tumor had been located right between both optic nerves, just underneath the brain and front of the brain stem. Getting a high dose into an area surrounded by so many sensitive, critical structures (including all of the optics), can be challenging with traditional radiation therapy. Also, differing levels of air or fluid in the sinus cavity have the potential to change the intended dose delivered throughout the course of treatment, so these must be monitored closely.
Our treatment plan came together fairly easily because we have established approaches for choosing beam angles in such situations. We utilized a four-beam configuration consisting of two posterior oblique beams, an anterior-posterior beam, and a superior-anterior oblique beam. We used MFO in this case to simultaneously deliver a higher dose to the highest-risk area and a lower dose to the adjacent at-risk tissues. When using the MFO technique, each beam preferentially delivers dose to portions of the tumor that are better and more safely accessible from that particular beam angle. This results in optimal organ sparing and excellent dose coverage.
In addition, the ProBeam system’s small spot size allowed us to effectively “squeeze in” dose through each beam, and best avoid critical structures within each beam path. Ultimately, we were able to achieve a dose distribution that was homogeneous, stable, and met our tolerances for critical structures..
Treatment monitoring in real-time with high-quality volumetric cone beam CT
Throughout the course of treatment, this patient received regular treatment-room CBCT scans as well as confirmatory, treatment-plan-verification CTs over the course of her 7-week treatment. We’ve been using in-room CBCT scanners to image IMPT patients, daily in many cases. As would be expected, this allows us to verify position of bony and soft tissue structures, and monitor changes in a patient’s anatomy or tumor that might indicate the need for adaptive planning.
We closely monitored the fluid levels in her sinuses and her positional stability, observing no significant changes from baseline. Augmented by treatment-plan-verification CT data, we were confident the dose we were delivering was accurately placed and consistent. In her case, no mid-course re-planning was needed. This information was extremely reassuring to the patient as well, as we were able to show her images that confirmed the delivery of her treatment. She finished her treatment in August 2016, having tolerated the treatment extremely well without major side effects*.
Future adaptive planning and dose tracking
We are refining the process regarding CBCT treatment monitoring and CT acquisition for plan verification. In general, we do a verification CT at the beginning of treatment to verify setup is accurate and another one mid-course; other verification CTs might be triggered by the CBCT or clinical factors.
In the future, we look forward to implementing developments in Varian’s Velocity™ oncology software platform that will allow us to create a synthetic CT from a CBCT, and enable us to perform verification planning without having to acquire a separate verification CT. We anticipate the capability to calculate the daily delivered dose based on the geometry of the day as captured by CBCT. With this information we could compare the dose actually delivered to the dose initially intended and derive information needed to inform adaptive plan generation. This provides exciting possibilities to improve radiotherapy for our patients at the point of care.
* Varian as a medical device manufacturer cannot and does not recommend specific treatment approaches. Individual treatment results may vary.