Physics and the fight against cancer
DOI: 10.1063/PT.5.010014
At first glance, cancer might seem defenseless against the weapons devised by physicists or built from their discoveries.
The resolution of magnetic resonance imaging scanners, for instance, is a few cubic millimeters. Nascent tumors can in principle be detected when they’re small, isolated, and easiest to treat. And when a tumor is found, high-energy x rays, gamma rays, protons, and other forms of radiation can be directed at tumors in beam patterns of increasing sophistication and effectiveness.
But having written about cancer over the years, I’ve learned that there’s more to beating the disease than locating and zapping tumors.
One of the biggest challenges arises from the nature of cancer. Ironically, given that the disease is characterized by rampant cell division, tumors grow slowly and, for the most part, stealthily. The ability to resolve a lentil-sized tumor is little help if you don’t know where to look.
And that ability is even less help if you don’t know whether to look in the first place. In rich countries, cancer ranks below heart disease and noncancerous respiratory diseases as a leading cause of death. Most people don’t die of cancer. Routine, image-based screening for the general population would reveal too few tumors to offset its huge cost.
Compounding the detection problem, especially for breast cancer, is the number of false positives. Five years ago, in the course of looking for research to write about, I came across a paper in the Proceedings of the National Academy of Sciences by MIT’s Michael Feld and his collaborators. Its introduction began with this striking and somewhat depressing paragraph (with my emphasis added):
In the United States, ≈216 000 new cases of breast cancer are diagnosed each year, and 40 000 women die from the disease. Mammography, the most common technique for detecting nonpalpable, highly curable breast cancer, employs x-rays to quantitatively probe density changes in breast tissue. Because these density changes are not uniquely correlated with breast cancer, mammography serves as a screening technique rather than a diagnostic tool. Thus, a lesion found through either clinical breast examination or mammography is always biopsied. Because of current limitations, 7090% of mammographically detected lesions are found to be benign upon biopsy. Breast biopsy is most often performed by surgical excision that removes the entire lesion or by core needle biopsy that removes 512 cores of tissue, typically 1 mm in diameter and several centimeters long, to ensure proper sampling. The complete diagnostic process, from start to finish, may take months and may include multiple biopsies.
But if you can detect cancer early, the prognosis following prompt treatment is good. Table 8-2 in The Biological Basis of Cancer (Cambridge U. Press) compiles three-, five-, and ten-year survival rates for seven malignancies. All except the ten-year survival rate for breast cancer are above 57%.
How could physics help to achieve a survival rate of 100%? As a former astronomer, I’m not qualified to answer, but as a writer, I can imagine what an ideal treatment might look like.
Early detection would be carried out through a noninvasive, nonimaging method. A blood test would be ideal. To kill the tumor, the patient would ingest an agent that would make its way to the tumor, stick to it, then inject a cancer-killing drug into the tumor cells. A blood test after the treatment could confirm its success.
How fanciful is that scenario? And where does physics fit in? I’m not sure about the first question, but the second is easier to answer because physicists are already working with scientists in other disciplines to solve it.
A blood test might arise from the statistical analysis of the cancer genome. Nanoparticles that can adhere to tumor cells have already been developed. Research that seeks to discover how viruses inject their RNA or DNA into cells could point to how artificial viruses could accomplish the same thing with anticancer drugs. Physicists are also working on how to repair p53, a key, cancer-fighting protein that fails to protect cells when it suffers certain mutations.
But before that research bears fruit, physicists are improving current therapies and diagnostics. The latest innovation that I’ve come across, stereotactic body radiation therapy , entails treating tumors with short, intense bursts of radiation.
From my inexpert perspective, the prospects for defeating cancer look good.
