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Brachytherapy grays

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User
Posted 28 May 2022 at 12:39

I was wondering if there was information that " converts" the brachytherapy grays into its external beam equivalent .

Im having 15 grays brachytherapy and 15 x 3 gys external beam. This is less than the gys people on ext beam get. I guess  the 15 gys hdr boost obviously outweighs that difference, but  i wonder whats its equivalent?

J.

User
Posted 28 May 2022 at 13:34
I think the 'standard' dose for prostate cancer radiotherapy is 60 grays, whether it be spread over 20 sessions or 37. So that all adds up.

Hermit

User
Posted 28 May 2022 at 14:05
They can't be compared in a linear way. The traditional treatment is 37 fractions at 2Gy (74Gy) but increasingly, men are given 19 fractions at 3.2Gy or 20 fractions at 3Gy (60.8Gy / 60Gy) and there is a current trial of 5 or 6 fractions at 5Gy (25 - 30Gy) but all versions deliver approximately the same radiation load to the cancer cells. There isn't an 'equivalent' to 15Gy brachy - it will give approximately the same load where it is needed as any of the other combinations.
"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard

User
Posted 28 May 2022 at 15:56

Originally Posted by: Online Community Member
They can't be compared in a linear way. The traditional treatment is 37 fractions at 2Gy (74Gy) but increasingly, men are given 19 fractions at 3.2Gy or 20 fractions at 3Gy (60.8Gy / 60Gy) and there is a current trial of 5 or 6 fractions at 5Gy (25 - 30Gy) but all versions deliver approximately the same radiation load to the cancer cells. There isn't an 'equivalent' to 15Gy brachy - it will give approximately the same load where it is needed as any of the other combinations.

This is correct, although it seems strange that a fewer number of Gy given at higher dose equal more fractions of a lower dose with greater total of Gy.  There is a formula for this but I could not work it out. The amount of radiation is generally set at a point where it is effective.  If a greater dose than the usually adopted one is given it can be more effective but result in increased side effects.  Clearly, if it can be established that 5 or 6 fractions of EBRT at yet higher dose work as well as 19 or 20, which figures where only implemented about a decade go following the CHHiP Hypofractionated trial, it will mean fewer visits for RT advantaging both patient and those setting up linacs for patients.

It does get even more involved where different forms of radiation are used  in combination as with Brachy + EBRT or as in my case where I had 30 fractions of EBRT each of 2Gy + 6 fractions of Carbon ions each of 3 Gy, albeit in a trial. 

Barry
User
Posted 28 May 2022 at 19:12

At 15 Gy, you are referring to HDR Brachy. (LDR Brachy is usually 170Gy.)

A full dose of HDR as a monotherapy would normally be 2 x 15Gy, so your 1 x 15Gy is a half dose.

A full dose of 3Gy External beam fractions would be 20 x 3Gy = 60Gy, so you're having ¾ of the dose of External Beam.

Your treatment regime is called HDR Boost or Brachytherapy Boost, and this is to get a high dose into the prostate, but a lower dose into the other areas to be treated, such as areas around the prostate. Seminal Vesicles can be included in the HDR if necessary (T3b), but will probably be included in the external beam anyway (as is the prostate itself). Sometimes they include the pelvic lymph nodes in the external beam field to mop up any micro-mets (mets too small to show up on scans) which might have already escaped into them. (I had this.) Brachytherapy Boost is regarded as a good compromise for hitting cancer in the prostate hard without generating excessive side effects which would result from doing it all with external beam alone.

As to the total Gy used, the higher the power of the individual doses (and hence fewer number of sessions/fractions, known as hypofractionation), the lower the total dose needed to achieve the same result. So at one extreme, LDR brachy alone which treats very slowly over around 200 days (think of it as 200 sessions/fractions) is around 170Gy, 37 sessions of external beam total 74Gy, 20 sessions external beam total 60Gy, 5 sessions SABR total 37.5Gy, 2 sessions HDR brachy 30Gy. These are all roughly the same effective treatment power.

Edited by member 28 May 2022 at 19:18  | Reason: Not specified

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User
Posted 28 May 2022 at 13:34
I think the 'standard' dose for prostate cancer radiotherapy is 60 grays, whether it be spread over 20 sessions or 37. So that all adds up.

Hermit

User
Posted 28 May 2022 at 14:05
They can't be compared in a linear way. The traditional treatment is 37 fractions at 2Gy (74Gy) but increasingly, men are given 19 fractions at 3.2Gy or 20 fractions at 3Gy (60.8Gy / 60Gy) and there is a current trial of 5 or 6 fractions at 5Gy (25 - 30Gy) but all versions deliver approximately the same radiation load to the cancer cells. There isn't an 'equivalent' to 15Gy brachy - it will give approximately the same load where it is needed as any of the other combinations.
"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard

User
Posted 28 May 2022 at 14:46

Thank you

User
Posted 28 May 2022 at 15:56

Originally Posted by: Online Community Member
They can't be compared in a linear way. The traditional treatment is 37 fractions at 2Gy (74Gy) but increasingly, men are given 19 fractions at 3.2Gy or 20 fractions at 3Gy (60.8Gy / 60Gy) and there is a current trial of 5 or 6 fractions at 5Gy (25 - 30Gy) but all versions deliver approximately the same radiation load to the cancer cells. There isn't an 'equivalent' to 15Gy brachy - it will give approximately the same load where it is needed as any of the other combinations.

This is correct, although it seems strange that a fewer number of Gy given at higher dose equal more fractions of a lower dose with greater total of Gy.  There is a formula for this but I could not work it out. The amount of radiation is generally set at a point where it is effective.  If a greater dose than the usually adopted one is given it can be more effective but result in increased side effects.  Clearly, if it can be established that 5 or 6 fractions of EBRT at yet higher dose work as well as 19 or 20, which figures where only implemented about a decade go following the CHHiP Hypofractionated trial, it will mean fewer visits for RT advantaging both patient and those setting up linacs for patients.

It does get even more involved where different forms of radiation are used  in combination as with Brachy + EBRT or as in my case where I had 30 fractions of EBRT each of 2Gy + 6 fractions of Carbon ions each of 3 Gy, albeit in a trial. 

Barry
User
Posted 28 May 2022 at 19:12

At 15 Gy, you are referring to HDR Brachy. (LDR Brachy is usually 170Gy.)

A full dose of HDR as a monotherapy would normally be 2 x 15Gy, so your 1 x 15Gy is a half dose.

A full dose of 3Gy External beam fractions would be 20 x 3Gy = 60Gy, so you're having ¾ of the dose of External Beam.

Your treatment regime is called HDR Boost or Brachytherapy Boost, and this is to get a high dose into the prostate, but a lower dose into the other areas to be treated, such as areas around the prostate. Seminal Vesicles can be included in the HDR if necessary (T3b), but will probably be included in the external beam anyway (as is the prostate itself). Sometimes they include the pelvic lymph nodes in the external beam field to mop up any micro-mets (mets too small to show up on scans) which might have already escaped into them. (I had this.) Brachytherapy Boost is regarded as a good compromise for hitting cancer in the prostate hard without generating excessive side effects which would result from doing it all with external beam alone.

As to the total Gy used, the higher the power of the individual doses (and hence fewer number of sessions/fractions, known as hypofractionation), the lower the total dose needed to achieve the same result. So at one extreme, LDR brachy alone which treats very slowly over around 200 days (think of it as 200 sessions/fractions) is around 170Gy, 37 sessions of external beam total 74Gy, 20 sessions external beam total 60Gy, 5 sessions SABR total 37.5Gy, 2 sessions HDR brachy 30Gy. These are all roughly the same effective treatment power.

Edited by member 28 May 2022 at 19:18  | Reason: Not specified

User
Posted 28 May 2022 at 21:12

Thanks Andy 

 
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