Validation of Monte Carlo dose calculation algorithm for CyberKnife multileaf collimator.

PURPOSE: To commission and evaluate the Monte Carlo (MC) dose calculation algorithm for the CyberKnife equipped with a multileaf collimator (MLC). METHODS: We created a MC model for the MLC using an integrated module of the CyberKnife treatment planning software (TPS). Two parameters could be optimized: the maximum energy and the source full width at half-maximum (FWHM). The optimization was performed by minimizing the differences between the measured and the MC calculated tissue phantom ratios and profiles. MLC plans were calculated in the TPS with the MC algorithm and irradiated on different phantoms. The dose was measured using an A1SL ionization chamber and EBT3 Gafchromic films, and then compared to the TPS dose to obtain dose differences (ΔD). Finally, patient-specific quality assurances (QA) were performed with global gamma index criteria of 3%/1 mm. RESULTS: The maximum energy and source FWHM showing the best agreement with measurements were 6.4 MeV and 1.8 mm. The output factors calculated with these parameters gave an agreement within ±1% with measurements. The ΔD showed that MC model systematically underestimated the dose with an average of -1.5% over all configurations tested. For depths deeper than 12 cm, the ΔD increased, up to -3.0% (maximum at 15.5 cm depth). CONCLUSIONS: The MC model for MLC of CyberKnife is clinically acceptable but underestimates the delivered dose by an average of -1.5%. Therefore, we recommend using the MC algorithm with the MLC only in heterogeneous regions and for shallow-seated tumors.

Dose indicator for CyberKnife image-guided radiation therapy.

PURPOSE: The purpose of this study was to calculate dose distributions from CyberKnife image-guided radiation therapy (IGRT) for brain, H&N, lung, and pelvis treatment regions and use them to extract the corresponding effective dose and estimate-related risk. METHODS: We developed a CyberKnife IGRT kV beam model in a standard treatment planning system and validated it against measurements in heterogeneous phantoms. Five brain, five head and neck, five thorax, and 10 (five male and five female) pelvis patient computed tomographies (CTs) were contoured. The dose distribution resulting from different CyberKnife IGRT protocols was calculated. From them, the effective dose was calculated according to ICRP publication Nr 103, using the average dose to contoured organs. The corresponding risk factors were calculated. Entrance surface dose (ESD) was also calculated and compared with existing data. RESULTS: The maximum effective dose produced by CyberKnife IGRT protocols was 0.8 mSv (brain), 1.9 mSv (H&N), 20.2 (pelvis), and 42.4 mSv (thorax) per fraction for a risk estimate of 0.004% (brain), 0.01% (H&N), 0.1% (pelvis), and 0.2% (thorax). Calculated ESD were compatible with existing data. CONCLUSIONS: Dose calculation models for CyberKnife IGRT kV beams were implemented in a clinical treatment planning system and validated in water and heterogeneous phantoms. We determined the effective dose and the related risk estimate resulting from CyberKnife IGRT protocols for brain, head and neck, thorax, and pelvis cases. The effective doses calculated for CyberKnife IGRT protocols were similar to those obtained for cone beam CT protocols on conventional C-arm linear accelerators, except for extreme irradiation conditions for thorax cases (140 kV X-ray tube tension).

Development of a synthetic single crystal diamond dosimeter for dose measurement of clinical proton beams.

The scope of this work was to develop a synthetic single crystal diamond dosimeter (SCDD-Pro) for accurate relative dose measurements of clinical proton beams in water. Monte Carlo simulations were carried out based on the MCNPX code in order to investigate and reduce the dose curve perturbation caused by the SCDD-Pro. In particular, various diamond thicknesses were simulated to evaluate the influence of the active volume thickness (e AV) as well as the influence of the addition of a front silver resin (250 µm in thickness in front of the diamond crystal) on depth-dose curves. The simulations indicated that the diamond crystal alone, with a small e AV of just 5 µm, already affects the dose at Bragg peak position (Bragg peak dose) by more than 2% with respect to the Bragg peak dose deposited in water. The optimal design that resulted from the Monte Carlo simulations consists of a diamond crystal of 1 mm in width and 150 µm in thickness with the front silver resin, enclosed by a water-equivalent packaging. This design leads to a deviation between the Bragg peak dose from the full detector modeling and the Bragg peak dose deposited in water of less than 1.2%. Based on those optimizations, an SCDD-Pro prototype was built and evaluated in broad passive scattering proton beams. The experimental evaluation led to probed SCDD-Pro repeatability, dose rate dependence and linearity, that were better than 0.2%, 0.4% (in the 1.0-5.5 Gy min-1 range) and 0.4% (for dose higher than 0.05 Gy), respectively. The depth-dose curves in the 90-160 MeV energy range, measured with the SCDD-Pro without applying any correction, were in good agreement with those measured using a commercial IBA PPC05 plane-parallel ionization chamber, differing by less than 1.6%. The experimental results confirmed that this SCDD-Pro is suitable for measurements with standard electrometers and that the depth-dose curve perturbation is negligible, with no energy dependence and no significant dose rate dependence.

Contribution of image-guided adaptive brachytherapy to pelvic nodes treatment in locally advanced cervical cancer.

PURPOSE: With the increasing use of simultaneous integrated boost in the treatment of cervical cancer, there is a need to anticipate the brachytherapy (BT) contribution at the level of the pathologic pelvic lymph nodes. This study aimed to report the dose delivered at their level during BT. METHODS AND MATERIALS: Patients with pelvic nodal involvement and treated with a combination of chemoradiation followed by image-guided adaptive pulsed-dose-rate BT were selected. On per BT three-dimensional images, pelvic lymphadenopathies were delineated, without planning aim. For the purposes of the study, D100, D98, D90, and D50 were reviewed and converted in 2-Gy equivalent doses, using the linear quadratic model with an α/ß of 10 Gy. RESULTS: Ninety-one patients were identified, allowing evaluation at the level of 226 lymphadenopathies. The majority of them were external iliac (48%), followed by common iliac (25%), and internal iliac (16%) regions. The 2-Gy equivalent doses D98 were 4.4 ± 1.9 Gy, 5.4 ± 3.1 Gy, and 4.3 ± 2.1 Gy for the obturator, internal iliac, and external iliac, respectively, and 2.8 ± 2.5 Gy for the common iliac. The contribution to the common iliac nodes was significantly lower than the one of external and internal iliac (p < 0.001). CONCLUSIONS: BT significantly contributes to the treatment of pelvic nodes at the level of approximately 5 Gy in the internal, external, and obturator areas and 2.5 Gy in the common iliac, allowing the anticipation of nodal boost with the simultaneous integrated boost technique. However, important individual variations have been observed, and evaluation of the genuine BT contribution should be recommended.

Brachytherapy Combined With Surgery for Conservative Treatment of Children With Bladder Neck and/or Prostate Rhabdomyosarcoma.

PURPOSE: To report the results of a conservative strategy based on partial surgery combined with brachytherapy in a prospective cohort of children with bladder-prostate rhabdomyosarcoma (BP RMS). METHODS AND MATERIALS: We prospectively documented the outcome of children treated in our department between 1991 and 2015 for BP RMS and undergoing a multimodal approach combining conservative surgery (partial cystectomy and/or partial prostatectomy) and perioperative interstitial low-dose-rate or pulse-dose-rate brachytherapy. Before brachytherapy, children had received chemotherapy with modalities depending on their risk group of treatment. RESULTS: A total of 100 patients were identified, with a median age of 28 months (range, 5.6 months-14 years). According to the Intergroup Rhabdomyosarcoma Study (IRS) group, 84 were IRS-III, and 12 were IRS-IV tumors. Four patients were treated at relapse. The median number of chemotherapy cycles before local therapy was 6 (range, 4-13). After surgery, 63 patients had a macroscopic tumor residuum. Five patients underwent a brachytherapy boost before pelvic external beam radiation therapy because of nodal involvement, and 95 had exclusive brachytherapy. Median follow-up was 64 months (range, 6 months-24.5 years). Five-year disease-free and overall survival rates were 84% (95% confidence interval 80%-88%) and 91% (95% confidence interval 87%-95%), respectively. At last follow-up most survivors presented with only mild to moderate genitourinary sequelae and a normal diurnal urinary continence. Five patients required a secondary total cystectomy: 3 for a nonfunctional bladder and 2 for relapse. CONCLUSION: Brachytherapy is effective as part of a conservative strategy for BP RMS, with a relatively low delayed toxicity as compared with previously published studies using external beam radiation therapy. Longer follow-up is required to ensure that the functional results are maintained over time.

Brachytherapy for Conservative Treatment of Invasive Penile Carcinoma: Prognostic Factors and Long-Term Analysis of Outcome.

PURPOSE: To report the largest experience with brachytherapy as a conservative approach for the treatment of penile carcinoma. METHODS AND MATERIALS: We examined the outcomes of 201 patients treated at our institution over 45 years for invasive squamous cell carcinoma of the glans penis by brachytherapy. RESULTS: With a median follow-up of 10.7 years, local relapse as first failure was reported in 37 patients (18.9%), and 24 of 31 patients (77.4%) with local failure only were in complete remission after new treatment. At last follow-up 25 patients (12.4%) underwent partial surgery and 7 (3.5%) total penectomies for relapse. Fifty patients (24.8%) presented urethral stenosis requiring at least 1 dilatation, and 14 (7%) required limited surgeries for toxicities. At 5 years the estimated overall survival rate was 79% (95% confidence interval 73%-85%). The estimated original local control rate was 82% (95% confidence interval 76%-88%). Presence of inguinal lymph node metastasis and tumor size correlated with a poorer overall and disease-free survival in univariate and multivariate analyses. In univariate analysis, neutrophilia at diagnosis correlated with a higher probability of distant relapse (P=.025), and a dose ≥62 Gy correlated with better local control in N0 patients (P=.038). The risk of complication correlated with the dose, treated volume, and dose rate. CONCLUSION: This large institutional experience confirms the high local control achieved with brachytherapy for penile carcinoma, with the advantage of organ preservation. Most local relapses are efficiently salvaged by second-intent surgery.