Dosimetry Modeling for Focal Low-Dose-Rate Prostate Brachytherapy.
OBJECTIVE
Focal brachytherapy targeted to an individual lesion(s) within the prostate may reduce side effects experienced with whole-gland brachytherapy. The outcomes of a consensus meeting on focal prostate brachytherapy were used to investigate optimal dosimetry of focal low-dose-rate (LDR) prostate brachytherapy targeted using multiparametric magnetic resonance imaging (mp-MRI) and transperineal template prostate mapping (TPM) biopsy, including the effects of random and systematic seed displacements and interseed attenuation (ISA).


METHODS
Nine patients were selected according to clinical characteristics and concordance of TPM and mp-MRI. Retrospectively, 3 treatment plans were analyzed for each case: whole-gland (WG), hemi-gland (hemi), and ultra-focal (UF) plans, with 145-Gy prescription dose and identical dose constraints for each plan. Plan robustness to seed displacement and ISA were assessed using Monte Carlo simulations.


RESULTS
WG plans used a mean 28 needles and 81 seeds, hemi plans used 17 needles and 56 seeds, and UF plans used 12 needles and 25 seeds. Mean D90 (minimum dose received by 90% of the target) and V100 (percentage of the target that receives 100% dose) values were 181.3 Gy and 99.8% for the prostate in WG plans, 195.7 Gy and 97.8% for the hemi-prostate in hemi plans, and 218.3 Gy and 99.8% for the focal target in UF plans. Mean urethra D10 was 205.9 Gy, 191.4 Gy, and 92.4 Gy in WG, hemi, and UF plans, respectively. Mean rectum D2 cm(3) was 107.5 Gy, 77.0 Gy, and 42.7 Gy in WG, hemi, and UF plans, respectively. Focal plans were more sensitive to seed displacement errors: random shifts with a standard deviation of 4 mm reduced mean target D90 by 14.0%, 20.5%, and 32.0% for WG, hemi, and UF plans, respectively. ISA has a similar impact on dose-volume histogram parameters for all plan types.


CONCLUSIONS
Treatment planning for focal LDR brachytherapy is feasible. Dose constraints are easily met with a notable reduction to organs at risk. Treating smaller targets makes seed positioning more critical.
Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom.
Bashar
B
Al-Qaisieh
Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom. Electronic address: joshua.mason@nhs.net.
Josh
J
Mason
Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom.
Peter
P
Bownes
Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom.
Ann
A
Henry
Division of Surgery and Interventional Science, University College London, London, United Kingdom; Department of Radiology, Northwick Park Hospital, London North West NHS Trust, London, United Kingdom.
Louise
L
Dickinson
Division of Surgery and Interventional Science, University College London, London, United Kingdom; University College London Hospital, London, United Kingdom.
Hashim U
HU
Ahmed
University College London Hospital, London, United Kingdom.
Mark
M
Emberton
St Luke's Cancer Centre, Guildford, United Kingdom.
Stephen
S
Langley
D000368:Aged; D001706:Biopsy; D001918:Brachytherapy / Q000379:methods*; D005240:Feasibility Studies; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D008875:Middle Aged; D009010:Monte Carlo Method; D058958:Organs at Risk / Q000528:radiation effects*; D011467:Prostate / Q000473:pathology / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012007:Rectum / Q000528:radiation effects; D014521:Urethra / Q000528:radiation effects