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Proton Therapy Delivery and Its Clinical Application in Select Solid Tumor Malignancies
Summary February 6th, 2019
The fundamentals of radiation planning and delivery for proton therapy using prostate cancer as a model are presented. The application of these principles to other selected disease sites highlights how proton radiotherapy may enhance clinical outcomes for cancer patients.
Transcript
It is important to understand the potential, and evolving role, of proton radiation therapy for treating certain cancers over the more traditional photon radiation therapy. An important challenge is to identify and understand the cancer disease states that may be particularly suitable for and benefit from proton radiation therapy. As with other forms of external beam radiation therapy, proton therapy is delivered in a non-invasive fashion, using an external robotic device that moves around the patient.
The location that the proton therapy beam terminates within the body can be controlled with a high degree of precision. Making it ideal for tumors adjacent to critical healthy organs. Before beginning the simulation, insert the extended table top into the treatment table and insure that it is locked.
Place an inflated immobilization cushion over the table top and the indexing bar for the leg mold at that level of the patient's knee. Place a solid head rest at the top of the table and confirm that the patient has completed the full bladder process by drinking 16 to 24 ounces of fluid 45 minutes prior to the scheduled scan time. After registering the patient into the computed tomography, or CT Patient Registration System, select the prostate scanning protocol with a slice thickness of three millimeters.
Ask the patient to sit on the table and assist the patient into a supine, headfirst position. Place the patients legs into the inflated immobilization cushion so that the mold will envelope the side of the legs and create a barrier between the legs and position the patient's arms on the chest. Confirm the patient alignment with the laser system and connect the dual vacuum pump to the nozzle of the immobilization cushion.
Once the patient is in a maintainable position, use the Qfix dual vacuum pump remove the air from the immobilization cushion to form a solid mold around their legs and feet. Manually adjust the treatment table so that the laser crosshairs are at the level of the hips and at the midline of the hips and abdomen at the level of the hip joint. Use a marking pen to indicate the locations of the crosshairs and place localization markers onto the marks to designate a starting point for the verification simulation during the radiation delivery.
Set the appropriate CT scanning parameters to include the pelvic region from the L3 spine to the mid femur. Then scan the patient using the prostate scanning protocol, and export the digital scan file to the treatment planning software before notifying the symmetry. For radiation treatment planning, use the treatment planning system contouring tools to define all of the relevant geometric volumes based on the acquired CT images.
Contour the first clinical target volume, or CTV1 to include the prostate, seminal vesicles, and involved lymph nodes. The CTV1 will be prescribed 45 grays and will have the appearance of a U-shaped structure on the axial images. The small bowel, rectum, and bladder normal tissues will reside within the U-shaped target volume.
Contour the CTV2 to include the prostate and seminal vesicles. The CTV2 will be prescribed a total dose of 34.2 grays. Select three beams at gantry angles of 90, 180, and 270 degrees for the treatment of the CTV1 volume.
And select only the 90 and 270 degree beam angles for the CTV2 treatment. Measure to confirm the accuracy of the planned dose using ionization chamber arrays and to verify the accuracy of the dose calculation using a secondary independent dose calculation software. If the plans and dose distributions meet the accepted dose constraint guidelines and robustness, obtain physician approval before exporting all of the treatment planning data to the treatment delivery system for the patient treatment.
On the day of the treatment arrange the treatment room to reproduce the patient setup used during the CT simulation. Insure that the immobilization cushion label matches the patient ID and place the cushion on the treatment table with the correct indexing and with the appropriate headrest at the head of the table. Confirm that the patient has completed the full bladder process and escort the patient into the treatment room.
Help the patient into the supine position onto the treatment table with hands clasped across the chest and the legs in the immobilization cushion. Electronically move the treatment table from the load position toward the isocenter to align the patient to the marks that were placed during simulation. Adjusting the table to correct any gross errors in the patient positioning as necessary.
Once the patient is properly aligned to the simulation marks, complete the shifts from the start position, to those determined during the dosimetric treatment planning process, to align the patient to the desired treatment isocenter. Perform orthogonal kilovoltage imaging to insure proper internal patient alignment to the pelvic bones and fiducial markers previously placed by urology within the prostate. Overlay the acquired kilovoltage images on digitally reconstructed radiographs from the planning CT simulation scan to determine if positional adjustments are required and apply any necessary shifts to insure alignment.
If the kilovolt images demonstrate excessive bowel gas, ask the patient to expel the air while lying on the treatment table if possible and realign and reimage. Once acceptable kilovoltage images are acquired and confirmed, perform a cone beam ct scan to assess bladder and rectal filling applying additional patient positioning adjustments based on the scan as necessary. Then initiate the treatment delivery with audible verification between two therapists of gantry angle, monitor units, number of scanning spots and layers, and snow position for each treatment angle, marking the treatment isocenter for daily alignment and removing the marks after the treatment.
Due to the large areas targeted during breast cancer radiation, photon based radiation treatment techniques result in substantial irradiation exposure to thoracic structures, including the lungs, heart, and contralateral breast. These regions may be spared from excess radiation with proton therapy. Multiple irradiation fields are required for photon therapy-based craniospinal irradiation.
Since the target CTV for craniospinal irradiation includes the entire cerebrospinal fluid space, extending from the brain vertex, to the spinal canal, through the cauna equida at the level of the S2, S3 vertebral junction. The benefit of using proton therapy for craniospinal treatment, is that the therapy substantially limits the radiation dose to the thoracic abdominal structures anterior to the spinal cord, compared to the area of the dosage experienced with photon based therapy, for the same time of tumor treatment. It is important that every patient is placed in a comfortable position during the CT simulation.
So that the same position can be reproduced for every radiation session. Proton therapy can be thoroughly improved by taking advantage of the unique radiobiological properties of protons such as changes in linear energy transfer, or combining protons with other treatment modalities. Challenges include selecting the patients who would most benefit from proton therapy and how to utilize protons to deliver four column more nerve sparing prostate cancer treatment.
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