My reasoning is that about ~50% of radiotherapy is curative, and the majority of this (e.g. breast) is in the adjuvant post-op setting. Who's to say that in the next 10 years we don't discover biomarkers that will stratify which early stage patients actually need post-op radiotherapy for local control? We know that after WLE only ~30% of patients fail locally. So at the extreme end of the spectrum, one day we could see treatment such that only 30% of breast patients get adjuvant radiotherapy, greatly reducing utilization.
I doubt that we will ever be in place to predict exactly which patients will and which ones will not need radiotherapy. I just don't see these "magic" biomarkers appearing.
A couple of thoughts on the subject:
1. In radiotherapy we have yet to find a single biomarker, that can safely allow us to ommit radiotherapy. In systemic therapy there are several markers already in routine use to predict the response of the disease to a systemic treatment (for example K-ras mutation for EGFR-antagonists).
2. As you pointed out only 30% of our patients fail locally, thus we treat 70% of our patients without needing to. Is however treating 70% of all the patients in vain a figure too high or unacceptable and do we actually need to make this figure better?
Look at systemic chemotherapy in breast cancer. It provides an overall survival benefit of about 5% in most cases. Yet our fellow medical oncologists hardly think twice when they offer this treatment to their patients, knowing than 95% of them receive it in vain. Surely we have genetic biomarkers emerging for breast cancer than are supposed to predict who's to get systemic chemotherapy and who's not to get it, but they are not considered standard yet. At least in Europe we don't use them. The medical oncologists still look only at T-/N-stage, grading, ER/PR/Her2neu-status and (newly) Ki-67-index.
3. I expect more indications for radiotherapy to emerge in the coming years and utilization to increase.
a) Although I have been working only for 5 years in radiation oncology, I already see a trend of retreating patients with radiotherapy. The amount of patients sent to us for retreatment is increasing and I have the impression, that systemic therapy is currently managing to keep the disease extremely long under control, thus more and more patients are sent to us for treatment due to local problems.
b) While some indications will disappear in the next 10 years, other will arise.
I expect that by 2020 we will only treat rectal cancer in the neoadjuvant setting if it's a cN+ or the tumor is situated in the lower rectum. The standard cT3 cN0 patients will probably get neoadjuvant FOLFOX with some "catchy" antibody without radiation.
On the other hand we may start treating Stage II+III NSCLC (perhaps only Adeno?) with prophylactic WBRT. I also expect radiation therapy to make a come back in the field of gastric cancer, probably in the neoadjuvant setting together with chemo. SBRT will also make a serious leap ahead and it may even substitute lobectomy for early NSCLC.
There may be a "market" for oligometastatic disease too, with Tomotherapy-like solutions leading the way there.
c) In 10 years we don't even know what kind of technology we will have in our hands. If the Tomotherapy attempt with CPAC to provide a rather "cheap" proton solution works out, then we may all have protons by 2020. This will allow us to escalate dose and hopefully minimize side-effects, all without investing 200 million dollars for a proton gantry. That could result into a major boost for radiation therapy.
