Cervical HDR: EQD2 Assessment and Source Decay
A 52-year-old with FIGO IIB cervical cancer receives 45 Gy in 25 fractions to the pelvis with concurrent cisplatin, followed by MRI-guided HDR brachytherapy using a tandem-and-ring applicator. The prescription is 28 Gy in 4 fractions to the HR-CTV, one fraction per week. At the first fraction, MRI-based planning gives HR-CTV D90 = 6.3 Gy, bladder D2cc = 5.0 Gy, rectum D2cc = 4.0 Gy, and sigmoid D2cc = 4.5 Gy per fraction. The same geometry is assumed for all four fractions. The iridium-192 source was calibrated at 10.0 Ci on January 1; the first implant is February 15. Half-life is 73.8 days. The original plan was calculated for 10.0 Ci and had a 12-minute treatment time. The physicist is asked whether to accept the plan, and the resident asks why the treatment time is longer.
1. Calculate the total EQD2 for HR-CTV D90 and the OAR D2cc, using alpha/beta = 10 Gy for tumor and 3 Gy for OAR. Is the plan acceptable?
Read the worked answer
Use EQD2 = D x (d + alpha/beta)/(2 + alpha/beta), summing EBRT and HDR contributions.
HR-CTV (alpha/beta = 10): EBRT: 45 x (1.8 + 10)/12 = 44.25 Gy. HDR: 4 x 6.3 = 25.2 Gy; 25.2 x (6.3 + 10)/12 = 34.23 Gy. Total = 78.5 Gy. The EMBRACE II target is about 85 Gy EQD2, so the HR-CTV is underdosed by roughly 6.5 Gy.
Bladder (alpha/beta = 3): EBRT: 45 x (1.8 + 3)/5 = 43.2 Gy. HDR: 4 x 5.0 = 20 Gy; 20 x (5 + 3)/5 = 32 Gy. Total = 75.2 Gy, below the 90 Gy limit.
Rectum: EBRT 43.2 Gy; HDR 4 x 4.0 = 16 Gy; 16 x (4 + 3)/5 = 22.4 Gy; total 65.6 Gy, below 75 Gy.
Sigmoid: EBRT 43.2 Gy; HDR 4 x 4.5 = 18 Gy; 18 x (4.5 + 3)/5 = 27 Gy; total 70.2 Gy, below 75 Gy.
The OAR doses are acceptable, but the HR-CTV coverage is not. I would not accept the plan without improving target dose.
2. What options do you have to improve HR-CTV coverage without exceeding OAR limits?
Read the worked answer
Because the OAR doses have margin, I would first reoptimize dwell times to increase HR-CTV D90 while keeping bladder, rectum, and sigmoid D2cc below their limits. I would review the MRI at each fraction because applicator position and tumor regression change geometry. If dwell-time optimization is insufficient, I would consider interstitial needles for lateral or parametrial extension, add a fifth HDR fraction, or increase the EBRT boost component. I would not simply increase all dwell times uniformly, because that would raise OAR dose. The choice depends on tumor geometry, OAR tolerance, and the overall treatment schedule. The plan should be re-evaluated with MRI-based dosimetry before acceptance.
3. Calculate the source activity on February 15 and the new treatment time, and explain why it matters.
Read the worked answer
Decay from January 1 to February 15 is 45 days. A = A0 x exp(-ln2 x t/T1/2) = 10.0 x exp(-0.693 x 45/73.8) = 10.0 x exp(-0.423) = 6.55 Ci.
The treatment time scales inversely with activity if the same dose is prescribed: 12 min x 10.0/6.55 = 18.3 min. The plan must be recalculated with the current source strength and the treatment time verified independently. Treating with the old 12-minute time would underdose the patient by about 35%. This is why source calibration, decay correction, and a second-person check of treatment time are essential in HDR brachytherapy.