Optimal duration of adjuvant ADT depends on the type of radiation used for high-risk patients

No one wants to have androgen deprivation therapy (ADT), even if it is for a limited time. It has been known for a long time that it improves oncological outcomes when given with ("adjuvant to") radiation therapy in patients with high-risk prostate cancer. Several randomized clinical trials (RCTs) have tried to find the best duration to use it, but it is difficult to arrive at reliable optimization points- it would involve varying the duration for a large number of high risk patients. Kishan et al. have taken an innovative approach to solving this problem by combining several RCTs and a multi-institutional observational study. Unlike typical "meta-analyses," they compared similar patients across three studies.

The three studies they used in their analysis were:

1. The high-risk patients in the DART 01.05 GICOR RCT (see this link), which randomized patients to 28 months or 4 months of adjuvant ADT in patients getting high dose external beam radiation (EBRT-only). They found that 28 months is better than 4 months, but is there a duration that is less than 28 months for EBRT-only?
2. The patients in the TROG 03.04 RADAR RCT (see this link), which randomized patients to 18 months or 6 months of adjuvant ADT in patients getting varying doses of EBRT or high dose rate brachy boost therapy (BBT). They found that 18 months is better than 6 months for BBT, but is there a duration that is less than 18 months for BBT?
3. The patients in a multi-institutional (retrospective, non-randomized) study who received varying durations of adjuvant ADT and EBRT-only or brachy boost therapy for their high risk PCa (see this link).

They used distant metastasis-free survival (DMFS) as the endpoint of interest because it has been found to correlate well with eventual overall survival. They went back to the original patient-level data to extract comparable patients when comparing them across studies. This retained many of the advantages of each of the three studies. While this innovative approach does not constitute the highest level of evidence (Level 1), it offers a degree of reliability that goes beyond simple observational studies.

They used two statistical methods to look at the data. In one analysis, they divided the durations into three parts: 

• ≤6mo.
• >6 - 18 mos
• >18 mos

In another analysis (called "cubic splines") they found the best fit for the continuous data. Both analyses led to similar conclusions.

The best estimates for the best minimum adjuvant ADT duration are:

• at least 26.3 months for EBRT-only
• at least 12 months for BBT

But, one might object, didn't Nabid's PCS IV trial show that 18 months is as good as 36 months (see this link)? Kishan points out that only about half of the cohort in that trial who were supposed to get 36 months of ADT actually got that much. And nearly a quarter of the 36-month cohort actually received less than 21 months. The only data we've seen so far has been analyzed by the dose they were intended to get, not by what they actually got. Also, why were the drop-out rates so high? The DART RCT had 95% compliance with the full 28 months, even though the radiation doses given were much higher.

There is a trade-off: BBT can come with severe late-term urinary side effects (among 19% in the ASCENDE-RT RCT), while the late-term urinary side effects are milder for EBRT-only (only 2.5% in DART). Only the patient can decide if he is willing to take on 12 months of ADT with BBT vs over twice as long for EBRT-only, given the higher expected radiation toxicity with BBT.

Which is better: EBRT+2 years of ADT or BBT+1 year of ADT?

Patel et al. looked at the use of the two therapies at many of the top cancer centers. They found there was no significant difference in the occurrence of distant metastases or in prostate cancer-specific survival.

There are several unanswered questions:

• As we have seen (see this link), brief intense use of abiraterone or other advanced hormone therapy may obviate the need for longer ADT.
• Decipher genomic analysis may indicate which patients may be able to get away with less hormone therapy, and which need more. The PREDICT-RT RCT will eventually answer this question.
• Does SBRT monotherapy or HDR brachy monotherapy still require adjuvant ADT? Those therapies can have almost as high a biologically effective dose as BBT but with fewer side effects. This study suggests that 12 months of ADT is beneficial with even the highest dose radiation, but future clinical trials will give a more reliable answer.
• Standard-of-care dictates 2-3 years of adjuvant ADT when enlarged pelvic lymph nodes are found by CT or MRI. What is the optimum duration when cancerous pelvic lymph nodes are only detected with a PSMA PET scan and not by CT? What about when they are too small to be detected by any kind of imaging, and their presence is only suggested by risk characteristics?
• What duration of adjuvant ADT minimizes biochemical recurrence-free survival and the need for any salvage treatment?
• Will these estimates hold up if tested in an RCT?