Kevin Martell

Parameters predicting for prostate specific antigen response rates at one year post low-dose-rate intraoperative prostate brachytherapy

Purpose To develop a model for prostate specific antigen (PSA) values at one year among patients treated with intraoperatively planned 125I prostate brachytherapy (IOPB). Material and methods Four hundred and deven patients treated with IOPB for prostate adenocarcinoma were divided into four groups: those with PSA values ≥ 3 ng/ml; < 3 and ≥ 2; < 2 and ≥ 1 or PSA < 1 between 10.5 and 14.5 months post implantation (1yPSA). Ordinal regression analysis was then performed between patient, tumor, and treatment characteristics. 1yPSA values were also compared with toxicity outcomes. Results Median 1yPSA was 0.77 (0.04-17.36). Thirty-two patients (8%) had a PSA ≥ 3; 35 (9%) had PSA < 3, ≥ 2; 87 (21%) had PSA < 2, ≥ 1, and most patients 254 (62%) had PSA < 1. PSA response was independent of gland volume, Gleason score, clinical stage, seed activity, V90, V200, D90, or number of needles and seeds used. Older patients had significantly lower 1yPSA; median ages 65.1 (46.5-81.0), 62.1 (50.4-79.5), 60.5 (47.1-80.3), and 58.1 (45.1-74.2) years for each of the 1yPSA groups respectively (p < 0.001). Also, both implant V150 (p < 0.001) and initial PSA values (p = 0.04) were predictive of 1yPSA values. There was no correlation between 1yPSA values and toxicity encountered. Conclusions PSA response at 1 year post IOPB appears to be dependent on patient age, initial PSA, and implant V150. Our results provide reassurance that parameters other than biochemical failure influence 1yPSA values.

165: A Practical Energy Modulation Technique to Avoid Enucleation for Advanced Periocular Cancers

Purpose: Consider a 2x2x1.5 cm basal cell cancer invading right medial canthus periocular embryonic fusion plane. Usual techniques fail: an irregular PTV 4x4x2.5 cm deep, a concave surface, a deep tumour, and adjacent ocular structures. Oculoplastics/Mohs risk enucleation. Electrons with an internal eye shield require bolus and limit energy to 9 MeV. High energy conformal RT risks medial retinal damage. Systemic agents may palliate but do not cure. We describe low energy electron RT (e) with an orthovoltage (ortho) bump. “Bump” modulates energy by replacing some e-dose with ortho to increase surface dose and optimize dose distribution. Bump applies to any anatomic location to a depth of 2-3 cm. Bump can use ewith a tungsten eye shield and ortho for maximal eye-sparing. With orbit invaded, morbidity follows. Radiotherapy may be the best eyepreserving option. Methods and Materials: Central-axis dose calculation using measured % depth dose were compared with central and offcentral axis dose calcs using kVDoseCalc, a dose engine validated in kV cone-beam and ortho therapy; and Monte Carlo for eoffaxis dose calc. We compare conformal RT, arcs, and bump, for periocular cancer cases. We compared central axis data for a 4x4 cm field with: 1) 9 MeV alone; 2) 9 MeV with 0.7 cm custom wax; 3) 9 MeV, 80% of dose, 100 kV DXR bump, SSD 10 cm, 20% of dose; 4) 9 MeV, 80% of dose, 200 kV DXR bump, SSD 50 cm, 20% of dose. Patients treated at our institution in 10 or 20 treatments received 8 or 16 electron treatments (prescribed to account for REB of electrons) and 2 or 4 photons treatments, for a total dose of 45 Gy in 10 fractions, or 50 Gy in 20 fractions. Results: For the case above tables based on measured dose give: Surface dose (1) 86%; (2) 90%; (3) 100%; (4) 94% Dmax (100%) (1) 2.0 cm; (2) 1.3 cm; (3) 2.0 cm; (4) 2.0 cm Dose @ 2.7 cm (1) 89%; (2) 58%; (3) 87%; (4) 91% Surface and depth refer to skin surface. Dose is normalized: Dmax = 100%. REB and geometry are not included. Comparing dynamic conformal ARCs, VMAT, electrons +/bolus or tantalum mesh, and bump show the benefits of 9 MeV with 100-200 kV bump. Dose drop off is swift at ~40%/cm beyond D90%. Dose spares eye. Low SSD, low kV bump results in best homogeneity and surface dose; high kV bump gives best dose at depth. Patients can be scanned with a 3D printer wax replica eye shield to reduce artifact and enable accurate dose calculation. Actual patient results are illustrated with isodose distributions; for three clinical cases, the dose above 80% to retina was 2.5 cc for conformal treatment, 1.0 cc for dynamic conformal arc and < 0.5 cc for bumps, demonstrating excellent shielding for the bump technique. Conclusions: Energy modulation with ortho and electrons can result in improved dose distribution. Benefits include: increased treatment depth, improved dose homogeneity, no bolus, increased shield effectiveness, and reduced penumbra; important when treating near the eye.