Prostate IMRT Dose Constraints

[radoncmonkey]

Can anyone recommend a good source for IMRT dose constraints for the following two scenarios:

1) Post-prostatectomy with + margin, treating to 66 Gy
2) Post-HDR to 19Gy, boosting with 45 Gy IMRT to total dose 64 Gy

Much love appreciated.

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[medgator]

Can anyone recommend a good source for IMRT dose constraints for the following two scenarios:

1) Post-prostatectomy with + margin, treating to 66 Gy

Much love appreciated.

You can the check the RTOG's website for the current protocol looking at treating pelvic LN + prostate vs. prostate alone plus hormones after a rising PSA post-prostatectomty

http://rtog.org/members/protocols/0534/0534.pdf

 

[deleted4401]

Unsure whether there is data on this yet ... people are just probably tweaking on their on.

For post-prostatectomy to 66 Gy, I'd say just use the same volume #s, and subtract 10 from the dose. The reason being is that ~76 Gy is a typical dose for localized intact disease. I.e., instead of 70 Gy/25% for rectal, use 60 Gy/25%, and instead of 65 Gy/35%, use 55Gy/35%. That way you account for the fact you are using a lower dose, and you keep the same dose per fraction to the normal structure.

For the HDR case ... man, that get's complicated. I just tried to figure out what I'd do, but keep screwing it up. Simply, I would calculate the contribution to rectal dose from HDR, and then try to maintain the same constraints (70/25, 65/35, or your constraint du jour) when adding the EBRT.

Or, is that totally ridiculous?
S

 

[medgator]

For post-prostatectomy to 66 Gy, I'd say just use the same volume #s, and subtract 10 from the dose. The reason being is that ~76 Gy is a typical dose for localized intact disease. I.e., instead of 70 Gy/25% for rectal, use 60 Gy/25%, and instead of 65 Gy/35%, use 55Gy/35%. That way you account for the fact you are using a lower dose, and you keep the same dose per fraction to the normal structure.

Im not sure if the RTOG's constraints take it into account, but I wonder if you may need to be even tighter since there was prior surgical manipulation in the area that's being treated. It's a completely theoretical concern --- anyone have any insights?

 

[Palex80]

Unsure whether there is data on this yet ... people are just probably tweaking on their on.

For post-prostatectomy to 66 Gy, I'd say just use the same volume #s, and subtract 10 from the dose. The reason being is that ~76 Gy is a typical dose for localized intact disease. I.e., instead of 70 Gy/25% for rectal, use 60 Gy/25%, and instead of 65 Gy/35%, use 55Gy/35%. That way you account for the fact you are using a lower dose, and you keep the same dose per fraction to the normal structure.

For the HDR case ... man, that get's complicated. I just tried to figure out what I'd do, but keep screwing it up. Simply, I would calculate the contribution to rectal dose from HDR, and then try to maintain the same constraints (70/25, 65/35, or your constraint du jour) when adding the EBRT.

Or, is that totally ridiculous?
S

I am a newbie when it comes to prostate HDR, but would it be wise to use dose constraints like the one used for treating cancer of the uterine cervix?
For example according to the EORTC guidelines, one can use 70-75 Gy EQD2 for 2cc of Rectum and 85-90 Gy Gy EQD2 for 2cc Bladder with an a/b=3.

 

[TarHeelRadOnc]

Unsure whether there is data on this yet ... people are just probably tweaking on their on.

For post-prostatectomy to 66 Gy, I'd say just use the same volume #s, and subtract 10 from the dose. The reason being is that ~76 Gy is a typical dose for localized intact disease. I.e., instead of 70 Gy/25% for rectal, use 60 Gy/25%, and instead of 65 Gy/35%, use 55Gy/35%. That way you account for the fact you are using a lower dose, and you keep the same dose per fraction to the normal structure.

For the HDR case ... man, that get's complicated. I just tried to figure out what I'd do, but keep screwing it up. Simply, I would calculate the contribution to rectal dose from HDR, and then try to maintain the same constraints (70/25, 65/35, or your constraint du jour) when adding the EBRT.

Or, is that totally ridiculous?
S

This is a great question. Rectal DVH parameters are much better defined in the definitive setting than in the post-op setting. However, with the recently reported survival benefit with post-op RT in patients with positive margins or pT3 disease in the SWOG 8794 study (ASTRO and AUA 2008), we seem to be seeing an increasing proportion of patients after RP than ever before. Despite the increased eagerness to take patients at intermediate or high risk of extraprostatic disease to surgery in light of these data, RT-related rectal toxicity appears to be increased in the post-prostatectomy setting. Cheng et al (Red J 2008) reported an increased rate of acute grade >=2 GI toxicity with post-op IMRT (using tomotherapy) compared to patients treated in the definitive setting (41% vs 25%), despite lower mean prescription doses (78.9Gy vs 68.8Gy) and similar mean rectal doses (40Gy v 39.5Gy). The authors calculated a dose modification factor (DMF) to account for the difference in biologic effect on the rectal wall (1.17-1.24), meaning that a mean rectal wall dose of 40Gy in the post-operative setting carried a similar risk of grade 2 rectal toxicity as a mean dose of 48Gy in the definitive setting. I think that this is a useful rule of thumb for converting definitive dose constraints to the post-op setting. For definitive IMRT to a dose of 75.6Gy, my rectal dose constraints are: V77 <10%, V70 <25%, V47 <50%, V32 <80%. The V70 number comes from the MDACC trial. The V77 and V47 numbers come from an MSKCC dosimetric study (Skwarchuk et al. Red J 2000) and the V32 number comes from an MDACC retrospective study (Tucker et al Red J 2004). For a prescription dose of 66Gy in the postop setting, a 20% downward dose adjustment yields the following constraints: V66 <10%, V58 <25%; V39 <50%; V26 <80%. I think that the intermediate dose parameters are important predictors of late rectal toxicity, and may be even more important at the lower doses used in the post-op setting. Moreover, including intermediate dose constraints in the prescription (V47 and V32) helps ensure a good IMRT plan with a steep dose gradient moving posteriorly through the rectum.

Similar to the post-op setting, few good dosimetric studies are available describing predictors of rectal toxicity after HDR brachy. Most current protocols use two fraction HDR (RTOG 0321 used 9.5Gy x 2, which is from the William Beaumont experience). In patients treated to 45-50.4Gy with EBRT followed by two fraction HDR boost, I would limit the maximum rectal wall dose to <6Gy per fraction. This comes from a German dosimetric study (Pinkawa et al Radiother Oncol 2006). Moreover, partial volume dose constraints may also be important. One study suggested that the percent of the rectum receiving 10%, 30%, and 50% of the prescription dose is predictive of late rectal bleeding (Akimoto et al. Red J 2006). To keep things simple, you can use a rule of 7s: Limit the rectal wall volume receiving 50% of the prescription dose to <7% and the volume receiving 10% of the prescription dose to <70%.