DOS 541 - Week 3 Discussion
Discuss how this information relates to your practice as a medical dosimetrist. Discuss implementation in clinics and/or in protocols. Provide at least one credible reference to support your views.
Initial Post: Normal Tissue Tolerances
A medical dosimetrist’s goal is to design a plan to deliver a therapeutic radiation dose to a target lesion while simultaneously accounting for the dose tolerances of intervening and surrounding tissues, their relative importance, and the likelihood and severity of side effects if damaged. Beyond this, the plan must account for possible inaccuracies in the detection and delineation of the lesion, errors in patient positioning, interfraction and intrafraction motion of targets and organs at risk, and a host of other uncertainties. There are multitudes of strategies for dealing with uncertainties, but the central two questions that must be answered are always “how much dose dose the lesion need to receive?” and “how much dose can uninvolved tissues tolerate?”
The answer to the first question is not always “as much as possible” because not all tumors are structurally distinct from healthy tissues, and may actually share space with some normal cells and some malignant cells in the same area. A strategy of delivering radiation at a rate and level that is intolerable to malignant cells but tolerable to healthy cells is the basis of many forms of radiation therapy. This strategy depends on accurate knowledge of the radiation tolerance of normal tissues, so that the treatment plan can push the dose as high as safely possible.
Foundational efforts to formally quantify normal tissue tolerances were by Rubin & Cassarett in 1972 and Emami et al in 1991.1,2 These practice review and literature review projects were based on resources available at the time of each of their publications, and they highlighted as many gaps in knowledge as they filled. More importantly, they laid the foundation for later and larger efforts such as Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC), which aimed to produce predictive models for the relationship between dose and normal tissue complications.
In my experience in dosimetry, concerns about normal tissue dose are the main limiting factors in many plans. This has particularly been true in intracranial and head & neck planning, since there are a variety of sensitive structures such as the brain, eyes, ears, and the apparatus for the upper sections of the respiratory and digestive systems. The physicians I have worked with have often cited QUANTEC as the basis for their therapeutic dose and tissue tolerance prescriptions.
During my planning and planning obervations, lesions in the anterior part of the head have been especially troublesome because of the close proximity of organs at risk to each other, which significantly limits available beam configuration options. I have had to replan cases several times with different techniques to achieve acceptable dose levels to organs at risk while still maintaining adequate target coverage. I have also witnessed many cases in which normal tissue tolerances had to be intentionally exceeded in order to achieve a therapeutic plan. These cases have been particularly informative during my training, because they opened up uncomfortable conversations about the difficult choices that often come along with cancer care. Sometimes there is no good option and a patient and physician must work together to choose the least bad option. In these cases, QUANTEC is useful for predicting the types of damage and their associated symptoms that can occur, as well as their likelihood based on the dose delivered.
- Rubin P, Cassarett G. A direction for clinical radiation pathology. In: Vaeth JM, et al., eds. Frontiers of Radiation Therapy and Oncology VI. Baltimore, MD: University Park Press, 1972:1-16.
- Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys. 1991;21(1):109–122.
- Emami B. Tolerance of normal tissue to therapeutic radiation. Reports of Radiotherapy and Oncology. 2013;1(1):35-48.