Prostate Seed Implant, IMRT or SBRT for Prostate Cancer Treatment?
Recently, a dear friend of mine, a scientist, called and told me he had been diagnosed with early stage prostate cancer and needs to undergo radiation therapy for his treatment. His physician had gave him three options: prostate seed implant (PSI), Intensity Modulated Radiation Therapy (IMRT), and Stereotactic Body Radiation Therapy (SBRT) via Cyberknife. He asked me for advice. I gave him my thoughts, but didn’t recommend one particular option over another; I told him he should make his final decision with his physician. Regarding PSI versus IMRT, I referred him to my post on mdphysics.com without getting into the specifics over the phone. I also brought up the same point left by a recent commenter (medphysphd) that uncertainty in treatment planning exists with all the treatment modalities. However, the human error will be higher in PSI than other modalities. Regarding choosing IMRT over SBRT/Cyberknife (or vice-versa), below is a summary of what I told my friend. I am also posting this here as a response to the recent comment left by a reader inquiring about this very issue:
1- Therapeutic ratio which is defined as Tumor Control/Complication increases by fractionating radiotherapy. Fractionation increases the effectiveness of dose to the tumor and reduces the late effect in normal tissue.
2- There is evidence that dose escalation in prostate radiation therapy improved prostate tumor control. Therefore, these days the prostate is treated with IMRT at a much higher dose than before.
3- In addition to dose escalation, there is evidence that hypofractionation may also increase the therapeutic ratio by increasing better tumor control, as in SBRT using Cyberknife (based on radiobiological outcomes of prostate treatment from HDR and LDR).
4- However, high doses of radiation used either in the case of IMRT or SBRT is limited by the dose to normal tissue and sensitive proximity organs to the prostate (such as bladder and rectum), therefore, the dose delivery should be controlled by IGRT (in case of IMRT) or real-time correction (in case of SBRT).
- In IMRT delivery, the target is localized everyday using either fidutial markers and cone beam CT or trans-abdominal ultrasound imaging.
- In SBRT, however, the system corrects, in real-time, for organ position and targets motion during dose delivery. So, it does control dose to proximity organs and delivers accurate target dose in real time.
5- King at el. examines the rationale and technical feasibility of Cyberknife radiotherapy for localized prostate cancer. Conformal isodose curves and dose volume histograms (DVH) are used to compare with an optimized IMRT plan. They correlated and adjusted the dose in SBRT to compare it with IMRT at 74 Gy. They found:
- Both plans are very conformal, and the 74 Gy isodose provides complete coverage of the target and minimal overlap with the rectum.
- The bladder and rectum DVHs show significantly improved sparing with the Cyberknife as compared to the IMRT plan.
- The GTV coverage is similar for both IMRT and Cyberknife, although one can see that the Cyberknife would deliver a slightly higher mean dose to the prostate.
- Considering the improved normal tissue sparing with the Cyberknife compared with IMRT, the Cyberknife could allow further dose-escalation while keeping normal tissue under current tolerances.
6- Fractioned radiotherapy is based on the shape of the dose-response relationship for early and late-responding tissue and the fact that the α/β ratio for most tumors and normal tissue are quite different (normally 10 Gy for tumors and 3 Gy for late-effects). Therefore, the tumor BED (biological effective dose) is maximized while keeping the late-effects at tolerance doses when smaller dose per fraction is used. If normal tissue late-effects and tumors have the same α/β ratio then this rationale for small fractions disappears. For further information on BED and fractionation, go to Chapter 13 of “Radiobiologists for Radiologists” by Eric Hall.
7- Cyberknife is more convenient for patients than IMRT since the number of fractions is much lower than IMRT.
8- Cyberknife controls the target motion in real time rather than for each treatment as in IMRT.
9- If an error occurs for a whole fraction, it contributes to a much higher error in Cyberknife than in IMRT, since the fraction size in Cyberknife is much higher than in IMRT.
10- There are already more outcomes and evidence for dose-escalation in IMRT than for hypofractionation in Cyberknife (since SBRT is a relatively new technique compared to IMRT).
11- The cost of SBRT versus IMRT for prostate cancer treatment needs to be evaluated as well.
References that I used in advising my friend in his decision are as follows (I also suggest these references to anyone looking to learn more about IMRT, SBRT and Cyberknife):
IMRT – IGRT – SBRT, John Meyer. “Prostate Cancer Therapy with Stereotactic Body Radiation Therapy,” Todd Pawlicki, Cristian Cotrutz, and Christopher King. Frontiers Of Radiation Therapy and Oncology, Vol 40, pages 395-406.
Christopher R. King, et al. CyberKnife Radiotherapy For Localized Prostate Cancer: Rationale And Technical Feasibility. Technology in Cancer Research and Technology, Volume 2, Number 1, February (2003). Pages 25-29. http://www.tcrt.org/
Radiobiology for the Radiologists, Eric Hall. Chapter 13, pages 211-229.
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