LCD L35075: Proton Beam Therapy

ABSTRACT

DESCRIPTION

Proton Beam Therapy (PBT) is a technology for delivering conformal external beam radiation with positively charged atomic particles to a well-defined treatment volume. PBT is approved by the U.S. Food and Drug Administration.

Due to its unique dose deposition characteristics, PBT can, in certain situations, deliver the prescribed target dose while giving a lower dose to normal tissues as compared to photon-based forms of external beam radiotherapy.

Photon beams deposit their greatest amount of energy beneath the patient's surface with a gradual reduction in energy deposition along the beam path as photons pass through the target and then through an exit point out of the body. In contrast, the physical profile of a beam of proton particles allows for the majority of its energy to be deposited over a very narrow range of tissue at a depth largely determined by the energy of the proton beam. A proton beam deposits relatively less radiation energy upon entering the body compared to a photon beam. The energy deposition of the proton beam then rapidly increases over a narrow range of tissue at a desired depth to produce an intense dose distribution pattern called the Bragg peak. Beyond the Bragg peak, energy and dose deposition rapidly decrease, resulting in the absence of any significant exit dose deposited in normal tissue beyond the target.

TREATMENT

PBT Treatment Planning

PBT can allow for radiation treatment plans that are highly conformal to the target volume. PBT planning defines the necessary field sizes, gantry angles and beam energies needed to achieve the desired radiation dose distribution.

An assessment of patient suitability for PBT is an important step in the process of care. Changes in the density and composition of tissues in the path of the beam have much greater impact on the delivered dose for protons than photons. Tissue interfaces, especially those with large differences in electron density, can lead to larger or unacceptable dosimetric uncertainties in PBT for certain patients.

PBT treatment planning is a multi-step process and shares functions common to other forms of external beam radiotherapy planning:

• Simulation and Imaging: Three-dimensional image acquisition of the target region by simulation employing CT, CT/ PET and/or MR scanning equipment is an essential prerequisite to PBT treatment planning. If respiratory or other normal organ motion is expected to produce significant movement of the target region during radiotherapy delivery, the radiation oncologist may additionally elect to order multi-phasic treatment planning image sets to account for motion when rendering target volumes.

• Contouring: Defining the target and avoidance structures is a multi-step process:

• The radiation oncologist reviews the three-dimensional images and outlines the treatment target on each slice of the image set. The summation of these contours defines the Gross Tumor Volume (GTV).

• The radiation oncologist draws a margin around the GTV to generate a Clinical Target Volume (CTV) which encompasses the areas at risk for microscopic disease (i.e., not visible on imaging studies).

• A final margin is then added to create a Planning Target Volume (PTV).

• Nearby normal structures that could potentially be harmed by radiation (i.e., "organs at risk'; or OARs) are also contoured.

• Radiation Dose Prescribing: The radiation oncologist assigns specific dose coverage requirements for the CTV which will be met even in the presence of expected positional and range uncertainties. A typical prescription may define a dose that will be delivered to at least 99% of the CTV. Additionally, PBT prescription requirements routinely include dose constraints for the OARs (e.g., upper limit of mean dose, maximum allowable point dose, and/or a critical volume of the OAR that must not receive a dose above a specified limit).

• Dosimetric Planning and Calculations: The qualified medical physicist or a supervised dosimetrist calculates a treatment plan to deliver the prescribed radiation dose to the CTV and simultaneously satisfy the normal tissue dose constraints by delivering significantly lower doses to nearby organs. Delivery mechanisms vary, but regardless of the delivery technique, all delivery parameters and/or field specific hardware are developed by a medical physicist or supervised dosimetrist and an expected dose distribution is calculated for the treatment plan.

• Patient Specific Dose Verification: An independent dose calculation and/or measurement should confirm that the intended dose distribution for the patient is physically verifiable and feasible.

Documentation of all aspects of the treatment planning process is essential.

PBT Treatment Delivery

Proton delivery methods can be described in one of two forms: scattering or scanning.

• In scattered deliveries, the beam is broadened by scattering devices, beam energies are combined by mechanical absorbers and the beam is shaped by placing material such as collimators and compensators into the proton path.

• In scanning deliveries, the beam is swept laterally over the target with magnets instead of with scattering devices. Collimators and range compensators are still sometimes used for lateral and distal beam shaping, but field specific hardware is not always required because the scanning magnets allow the lateral extent of the beam to be varied with each energy level, a technique sometimes called intensity-modulated proton therapy (IMPT).

The basic requirement for all forms of PBT treatment delivery is that the technology must accurately produce the calculated dose distribution described by the PBT plan. PBT dose distributions are sensitive to changes in target depth and shape and thus, changes in patient anatomy during treatment may require repeat planning. Such a change must be documented.

Precise delivery is vital for proper treatment. Therefore, imaging techniques such as stereoscopic X-ray or CT scan (collectively referred to as Image Guided Radiation Therapy or IGRT) should be utilized to verify accurate and consistent patient and target setup for every treatment fraction.

Indications For Coverage

PBT is considered reasonable in instances where sparing the surrounding normal tissue cannot be adequately achieved with photon-based radiotherapy and is of added clinical benefit to the patient. Examples of such an advantage might be:

• The target volume is in close proximity to one or more critical structures and a steep dose gradient outside the target must be achieved to avoid exceeding the tolerance dose to the critical structure(s).

• A decrease in the amount of dose inhomogeneity in a large treatment volume is required to avoid an excessive dose "hotspot" within the treated volume to lessen the risk of excessive early or late normal tissue toxicity.

• A photon-based technique would increase the probability of clinically meaningful normal tissue toxicity by exceeding an integral dose-based metric associated with toxicity.

• The same or an immediately adjacent area has been previously irradiated, and the dose distribution within the patient must be sculpted to avoid exceeding the cumulative tolerance dose of nearby normal tissue.

PBT may offer dosimetric advantages as well as added complexity over conventional radiotherapy, 3D Conformal Radiation Therapy (3-D CRT) or Intensity Modulated Radiation Therapy (IMRT). Before applying PBT techniques, a comprehensive understanding of the benefits and consequences is required. In addition to satisfying at least one of the four selection criteria noted above, the radiation oncologist's decision to employ PBT requires an informed assessment of the benefits and risks including:

• Determination of patient suitability for PBT allowing for reproducible treatment delivery

• Adequate definition of the target volumes and OARs

• Equipment capability, including ability to account for organ motion when relevant

• Physician, physicist and staff training

• Adequate quality assurance procedures.

It is important to note that normal tissue dose volume histograms (DVHs) must be demonstrably improved with a PBT plan to validate coverage. Therefore, coverage decisions must extend beyond ICD-10 codes to incorporate additional considerations of clinical scenario and medical necessity with appropriate documentation. The final determination of the appropriateness and medical necessity for PBT resides with the treating radiation oncologist who should document the justification for PBT for each patient.

Group 1

On the basis of the above medical necessity requirements and published clinical data, disease sites that frequently support the use of PBT include the following:

• Ocular tumors, including intraocular melanomas

• Tumors that approach or are located at the base of skull, including but not limited to:

• Chordoma

• Chondrosarcomas

• Primary or metastatic tumors of the spine where the spinal cord tolerance may be exceeded with conventional treatment or where the spinal cord has previously been irradiated

• Unresectable benign or malignant central nervous system tumors to include but not be limited to primary and variant forms of astrocytoma, glioblastoma, medulloblastoma, acoustic neuroma, craniopharyngioma, benign and atypical meningiomas, pineal gland tumors, and arteriovenous malformations

• Primary hepatocellular cancer treated in a hypofractionated regimen

• Primary or benign solid tumors in children treated with curative intent and occasional palliative treatment of childhood tumors when at least one of the four criteria noted above apply

• Patients with genetic syndromes making total volume of radiation minimization crucial such as but not limited to NF-1 patients and retinoblastoma patients

• Pituitary neoplasm

• Advanced staged (e.g., T4) and/or unresectable malignant lesions of the head and neck

• Malignant lesions of the paranasal sinus, and other accessory sinuses

• Unresectable retroperitoneal sarcoma.

PBT is one of the acceptable forms of external beam radiation therapy that may be used to administer Stereotactic Body Radiation Therapy (SBRT) or Stereotactic Radiosurgery (SRS). When PBT is used to administer SBRT or SRS, the delivery and management codes relevant for SBRT or SRS apply, and the same clinical indications apply as for those treatment strategies.

Group 2

Coverage of proton beam therapy in Group 2 is limited to providers who have demonstrated experience in data collection and analysis with a history of publication in the peer-reviewed medical literature.

• Unresectable lung cancers and upper abdominal/peri-diaphragmatic cancers

• Advanced stage, unresectable pelvic tumors including those with peri-aortic nodes or malignant lesions of the cervix

• Breast cancers

• Unresectable pancreatic and adrenal tumors

• Skin cancer with macroscopic perineural/cranial nerve invasion of skull base

• Unresectable malignant lesions of the liver, biliary tract, anal canal and rectum

• Prostate cancer, without distant metastases

• Hodgkin or Non-Hodgkin Lymphoma involving the mediastinum or in non-mediastinal sites where PBT has the potential to reduce the risk of pneumonitis or late effects of radiation therapy (secondary malignancy, cardiovascular disease, or other chronic health conditions)

• Re-irradiation where prior radiation therapy to the site is the governing factor necessitating PBT in lieu of other radiotherapy.

Prostate Cancer

Coverage and payments of proton beam therapy for prostate cancer will require:

• Physician documentation of patient selection criteria (stage and other factors as represented in the NCCN guidelines);

• Documentation and verification that the patient was informed of the range of therapy choices, including risks and benefits.

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