Understanding Radiotherapy: Defining the Roles of Dosimetry, Medical Physicists

The roles of medical physicists and dosimetrists in radiation oncology treatment are not limited to initial treatment planning and verification; with highly contoured, image-guided adaptive radiation therapy techniques and hypofractionated treatment strategies, their work is more than ever woven throughout every facet of radiotherapy. The precision of dosimetric and beam placement planning has improved dramatically over recent years but is not error-free, making patient set-up, immobilization and adaptation of beam placement plans, and patient positioning between doses key to successfully delivering therapeutic radiation doses to tumor tissue while minimizing irradiation of healthy, nontarget tissues.

Were it not for the problem of nontarget tissue irradiation, maximal and curative doses of ionizing radiation could be delivered to most tumors.

The limitation “is not our ability to blast the tumor with radiation, but it is the normal tissues surrounding a tumor,” noted Sotirios Stathakis, PhD, DABR, an associate professor in the Department of Radiation Oncology and Radiology at the University of Texas Health Science Center in San Antonio, during a presentation at the Oncology Nursing Society (ONS) Bridge virtual conference.¹

“Radiation comes at a cost,” Dr Stathakis said. “The side effects of radiation therapy occur because healthy tissue near the tumor is affected as well as the cancerous tissue. Most side effects are localized to the area of treatment, although some effects, such as fatigue can occur body wide. Our problem is not imparting dose to the tumor but sparing normal tissue in the process.”

Modality and Dose Calculations

The optimal radiotherapy modality varies depending on tumor type, position, and depth from the body surface. Electron beam radiotherapy can target superficial tumors without penetrating deeply into the patient’s healthy tissues beneath the malignancies, whereas proton beams can deliver energy deeper within the body with relatively little deposition or scatter of energy in the healthy tissues between the tumor and beam source or behind the tumor. X-ray beams penetrate much more deeply, Dr Stathakis explained.

“Photon beams are the most common form of radiation therapy modality,” he said. “Nowadays we have more options with proton beams being more available in the US, worldwide as well. We consider them fairly new. The advantage of a proton beam is that most of the dose at the depth, won’t spread in the tissue before [reaching the tumor].”

The peak dose delivered is called the Bragg peak. Newer radiotherapy modalities are becoming available as well, including carbon and other heavy ion treatments. But all radiotherapy modalities operate by delivering energy to DNA strands, causing double-strand DNA breaks. 

“When the DNA breaks, 3 outcomes are possible,” Dr Stathakis said. “One, the damage is small and then the cell is repaired. [Two,] if the damage is moderate, then the cell might mis-repair and die later or mutate. And [three] if the damage is maximum, then the cell will not be able to repair the DNA and die.”

But calculating beam paths through differently radiosensitive healthy patient tissues onto complex tumor contours and calculating margins of error around the gross tumor volumes (GTVs) is not simple.

“Calculating the dose in patients is a complex process.” Dose calculations are usually calculated from first principles and also with a lot of assumptions built into equations, Dr Stathakis cautioned. “There are several complex calculations and the probability of error can be high.” Therefore, ensuring that patient positioning and immobilization not add to the inherent dose distribution imprecision is crucial.

Patient Position and Immobilization

The patient should be positioned for every fraction radiotherapy session with millimeter accuracy because if the patient is not in the correct position, normal tissue will be irradiated.

“If the patient moves during treatment, then the treatment will not be delivered accurately,” Dr Stathakis explained. “We use immobilization devices for most of our patients, to reduce their motion. Those devices quickly, accurately, and reproducibly place the patient in the treatment position. They should be light-weight, durable, have minimal effect on treatment, and not cause image artifacts.”

Patient comfort is central to effective, sustained immobilization during irradiation, and patients should be consulted regarding their comfort repeatedly during setup and immobilization. Pillow wedges, roles, rings, arm struts, and head rests help position and immobilize patients in part because they distribute pressure in a manner that minimizes discomfort and the level of patient effort required to sustain planned body positions. Immobilization devices such as these, SBRT compression paddles, and wing and breast boards restrict motion during irradiation and allow reproducible patient positioning during set-up.

“Immobilization for brain and head and neck patients can be invasive as in [when] a stereotactic frame is mounted on the patient’s skull and the patient couch; some plastic masks are also locked in the couch,” Dr Stathakis added. “A thermoplastic mask covers down to the shoulders. We need the [head and neck cancer] patient’s shoulders to always be in the same location, because otherwise the beam penetration could be altered if one day, they are higher and the next day lower than they’re supposed to be.

“Immobilization is an art,” he noted.

SBRT back lock systems involve covering the patient in a clear membrane, and then removing the air between that membrane and the patient’s skin to restrict patient motion.  Diaphragm plates can be used to help with constricted breathing.