Dose escalation study of rhenium-186 hydroxyethylidene diphosphonate in patients with metastatic prostate cancer

Rhenium-186 hydroxyethylidene diphosphonate (¹⁸⁶Re-HEDP) has been used for the palliative treatment of metastatic bone pain. A phase 1 dose escalation study was performed using ¹⁸⁶Re-HEDP Twenty-four patients with hormone-resistant prostate cancer entered the study. Each patient had at least four bone metastases and adequate haematological function. Groups of at least three consecutive patients were treated with doses starting at 1295 MBq and increasing to 3515 MBq (escalated in increments of 555 MBq). Thrombocytopenia proved to be the dose-limiting toxicity, while leucopenia played a minor role. Early death occurred in one patient (10 days after administration) without clear relationship to the ¹⁸⁶Re-HEDP therapy. Transient neurological dysfunction was seen in two cases. Two patients who received 3515 MBq ¹⁸⁶Re-HEDP showed grade 3 toxicity (thrombocytes 25–50 × 10⁹/1), defined as unacceptable toxicity. After treatment alkaline phosphatase levels showed a transient decrease in all patients (mean: 26% ± 10% IUA; range: 11%–44%). Prostate-specific antigen values showed a decline in eight patients, preceded by a temporary increase in three patients. From this study we conclude that the maximally tolerated dose of ¹⁸⁶Re-HEDP is 2960 MBq. A placebo-controlled comparative study on the efficacy of ¹⁸⁶Re-HEDP has been initiated.     

Dose escalation study with rhenium-188 hydroxyethylidene diphosphonate in prostate cancer patients with osseous metastases

The aim of this study was to determine the maximum tolerated dose of rhenium-188 hydroxyethylidene diphosphonate (HEDP) in prostate cancer patients with osseous metastases who are suffering from bone pain. Twenty-two patients received a single injection of escalating doses of carrier-added 188Re-HEDP [1.3 GBq (35 mCi), 2.6 GBq (70 mCi), 3.3 GBq (90 mCi) and 4.4 GBq (120 mCi)]. Blood counts and biochemical parameters were measured weekly over a period of 8 weeks. Haematological toxicity (WHO grading) of grade 3 or 4 was considered unacceptable. Clinical follow-up studies including methods of pain documentation (medication, pain diary) were performed for 6 months after treatment. In the 1.3-GBq group, no haematological toxicity was observed. First haematotoxic results were noted in those patients with a dose of 2.6 GBq 188Re-HEDP. In the 3.3-GBq group, one patient showed a reversible thrombopenia of grade 1, one a reversible thrombopenia of grade 2 and three a reversible leukopenia of grade 1. In the 4.4-GBq group, thrombopenia of grades 3 and 4 was observed in one and two patients (baseline thrombocyte count <200x10(9)/l), respectively, and leukopenia of grade 3 was documented in one patient. The overall nadir of thrombopenia was at week 4. The individual, maximum percentage decrease in thrombocytes in the 1.3-, 2.6-, 3.3- and 4.4-GBq groups was 17%, 40%, 60% and 86%, respectively. In two patients, a transient increase in serum creatinine was observed (max. 1.6 mg/dl). Pain palliation was reported by 64% of patients, with a mean duration of 7.5 weeks. The response rate seemed to increase with higher doses, reaching 75% in the 4.4-GBq group. It is concluded that in prostate cancer patients, the maximum tolerated dose of 188Re-HEDP is 3.3 GBq if the baseline thrombocyte count is below 200x10(9)/l. In patients with thrombocyte counts significantly above 200x10(9)/l, a dose of 4.4 GBq might be tolerable. Thrombo- and leukopenia are the most important side-effects. Pain palliation can be achieved in 60%-75% of patients receiving a dose of 2.6 GBq or more of 188Re-HEDP. Studies in a larger patient population are warranted to evaluate further the palliative effect of 188Re-HEDP.     

Dose escalation study with rhenium-188 hydroxyethylidene diphosphonate in prostate cancer patients with osseous metastases

The aim of this study was to determine the maximum tolerated dose of rhenium-188 hydroxyethylidene diphosphonate (HEDP) in prostate cancer patients with osseous metastases who are suffering from bone pain. Twenty-two patients received a single injection of escalating doses of carrier-added ¹⁸⁸Re-HEDP [1.3 GBq (35 mCi), 2.6 GBq (70 mCi), 3.3 GBq (90 mCi) and 4.4 GBq (120 mCi)]. Blood counts and biochemical parameters were measured weekly over a period of 8 weeks. Haematological toxicity (WHO grading) of grade 3 or 4 was considered unacceptable. Clinical follow-up studies including methods of pain documentation (medication, pain diary) were performed for 6 months after treatment. In the 1.3-GBq group, no haematological toxicity was observed. First haematotoxic results were noted in those patients with a dose of 2.6 GBq ¹⁸⁸Re-HEDP. In the 3.3-GBq group, one patient showed a reversible thrombopenia of grade 1, one a reversible thrombopenia of grade 2 and three a reversible leukopenia of grade 1. In the 4.4-GBq group, thrombopenia of grades 3 and 4 was observed in one and two patients (baseline thrombocyte count <200×10⁹/l), respectively, and leukopenia of grade 3 was documented in one patient. The overall nadir of thrombopenia was at week 4. The individual, maximum percentage decrease in thrombocytes in the 1.3-, 2.6-, 3.3- and 4.4-GBq groups was 17%, 40%, 60% and 86%, respectively. In two patients, a transient increase in serum creatinine was observed (max. 1.6 mg/dl). Pain palliation was reported by 64% of patients, with a mean duration of 7.5 weeks. The response rate seemed to increase with higher doses, reaching 75% in the 4.4-GBq group. It is concluded that in prostate cancer patients, the maximum tolerated dose of ¹⁸⁸Re-HEDP is 3.3 GBq if the baseline thrombocyte count is below 200×10⁹/l. In patients with thrombocyte counts significantly above 200×10⁹/l, a dose of 4.4 GBq might be tolerable. Thrombo- and leukopenia are the most important side-effects. Pain palliation can be achieved in 60%–75% of patients receiving a dose of 2.6 GBq or more of ¹⁸⁸Re-HEDP. Studies in a larger patient population are warranted to evaluate further the palliative effect of ¹⁸⁸Re-HEDP. Received 7 July and in revised form 6 October 1999     

Dual radiobiological interpretations of retrospective clinical data: the time factor

Dual interpretations are different radiobiological mechanisms that explain theoretically the same observed results. Radiobiological interpretations of the time factor are most frequently based on changes in total dose that produce a given effect. If this dose is increased by different mechanisms (e.g. increasing overall time and decreasing dose per fraction) at the same time, proposals for altered fractionation schemes based on the choice of one or the other mechanism, in principle, can lead to erroneous predictions of outcome. This is especially the case when the analyses are based on retrospective clinical data, where the influence of patient selection is unknown. Examples of dual interpretations taken from the literature on head and neck, melanoma and prostate cancer are discussed.     

Dual radiobiological interpretations of retrospective clinical data: the time factor

Dual interpretations are different radiobiological mechanisms that explain theoretically the same observed results. Radiobiological interpretations of the time factor are most frequently based on changes in total dose that produce a given effect. If this dose is increased by different mechanisms (e.g. increasing overall time and decreasing dose per fraction) at the same time, proposals for altered fractionation schemes based on the choice of one or the other mechanism, in principle, can lead to erroneous predictions of outcome. This is especially the case when the analyses are based on retrospective clinical data, where the influence of patient selection is unknown. Examples of dual interpretations taken from the literature on head and neck, melanoma and prostate cancer are discussed.     

Bad neighbours: hypoxia and genomic instability in prostate cancer

Prostate cancer (PCa) is a clinically heterogeneous disease and has poor patient outcome when tumours progress to castration-resistant and metastatic states. Understanding the mechanistic basis for transition to late stage aggressive disease is vital for both assigning patient risk status in the localised setting and also identifying novel treatment strategies to prevent progression. Subregions of intratumoral hypoxia are found in all solid tumours and are associated with many biologic drivers of tumour progression. Crucially, more recent findings show the co-presence of hypoxia and genomic instability can confer a uniquely adverse prognosis in localised PCa patients. In-depth informatic and functional studies suggests a role for hypoxia in co-operating with oncogenic drivers (e.g. loss of PTEN) and suppressing DNA repair capacity to alter clonal evolution due to an aggressive mutator phenotype. More specifically, hypoxic suppression of homologous recombination represents a “contextual lethal“ vulnerability in hypoxic prostate tumours which could extend the application of existing DNA repair targeting agents such as poly-ADP ribose polymerase inhibitors. Further investigation is now required to assess this relationship on the background of existing genomic alterations relevant to PCa, and also characterise the role of hypoxia in driving early metastatic spread. On this basis, PCa patients with hypoxic tumours can be better stratified into risk categories and treated with appropriate therapies to prevent progression.     

射波刀治疗前列腺癌初步临床观察

目的 探讨超大分割立体定向放射治疗（SBRT）前列腺癌的有效性和安全性。方法 回顾性分析2010年5月至2018年5月治疗的26例前列腺癌患者。中位年龄69（57~87）岁。临床分期为局限期14例,转移期12例。放疗方案：18例为35.0~37.5 Gy/5次,其中1例5年前行调强放射治疗（IMRT）72 Gy;8例为前列腺（25 Gy/5次）+盆腔（48.0~50.4 Gy/28次）。SBRT为隔日治疗,中位处方剂量线69.5%（65%~80%）。内分泌治疗：2例行去势手术,其余采用化学去势疗法。主要危及器官限量：直肠V_{35 Gy} <1 cm³、膀胱V_{35 Gy} <5 cm³。主要观察指标：放射损伤、前列腺特异性抗原（PSA）评价、局部控制、症状缓解。次要观察指标为无进展生存（PFS）和总生存（OS）。结果 中位随访时间22.44个月,26例患者均顺利完成治疗,未出现≥3级早期和晚期放射损伤,1、2级早期放射损伤发生率分别为38.4%和19.2%,1、2级晚期放射损伤发生率分别为30.8%和3.8%。放疗后效果评价症状缓解及局部控制情况良好。放疗后1、3、6、12个月,局限期患者PSA值与放疗前相比均下降明显（Z=2.900,2.794,2.510,2.090,P<0.05）,但转移期患者组差异均无统计学意义（P>0.05）。结论 超大分割立体定向放射治疗前列腺癌局部控制效果好,症状缓解率高,无严重不良反应。     

LDR vs. HDR brachytherapy for localized prostate cancer: the view from radiobiological models

PURPOSE: Permanent LDR brachytherapy and temporary HDR brachytherapy are competitive techniques for clinically localized prostate radiotherapy. Although a randomized trial will likely never be conducted comparing these two forms of brachytherapy, a comparative radiobiological modeling analysis proves useful in understanding some of their intrinsic differences, several of which could be exploited to improve outcomes. METHODS AND MATERIALS: Radiobiological models based upon the linear quadratic equations are presented for fractionated external beam, fractionated (192)Ir HDR brachytherapy, and (125)I and (103)Pd LDR brachytherapy. These models incorporate the dose heterogeneities present in brachytherapy based upon patient-derived dose volume histograms (DVH) as well as tumor doubling times and repair kinetics. Radiobiological parameters are normalized to correspond to three accepted clinical risk factors based upon T-stage, PSA, and Gleason score to compare models with clinical series. Tumor control probabilities (TCP) for LDR and HDR brachytherapy (as monotherapy or combined with external beam) are compared with clinical bNED survival rates. Predictions are made for dose escalation with HDR brachytherapy regimens. RESULTS: Model predictions for dose escalation with external beam agree with clinical data and validate the models and their underlying assumptions. Both LDR and HDR brachytherapy achieve superior tumor control when compared with external beam at conventional doses (<70 Gy), but similar to results from dose escalation series. LDR brachytherapy as boost achieves superior tumor control than when used as monotherapy. Stage for stage, both LDR and current HDR regimens achieve similar tumor control rates, in agreement with current clinical data. HDR monotherapy with large-dose fraction sizes might achieve superior tumor control compared with LDR, especially if prostate cancer possesses a high sensitivity to dose fractionation (i.e., if the alpha/beta ratio is low). CONCLUSIONS: Radiobiological models support the current clinical evidence for equivalent outcomes in localized prostate cancer with either LDR or HDR brachytherapy using current dose regimens. However, HDR brachytherapy dose escalation regimens might be able to achieve higher biologically effective doses of irradiation in comparison to LDR, and hence improved outcomes. This advantage over LDR would be amplified should prostate cancer possess a high sensitivity to dose fractionation (i.e., a low alpha/beta ratio) as the current evidence suggests.     

Efficacy of dose escalation on TCP,recurrence and second cancer risks: a mathematical study

Objective:

We investigated the effects of conventional and hypofractionation protocols by modelling tumour control probability (TCP) and tumour recurrence time, and examined their impact on second cancer risks. The main objectives of this study include the following: (a) incorporate tumour recurrence time and second cancer risks into the TCP framework and analyse the effects of variable doses and (b) investigate an efficient protocol to reduce the risk of a secondary malignancy while maximizing disease-free survival and tumour control.

Methods:

A generalized mathematical formalism was developed that incorporated recurrence and second cancer risk models into the TCP dynamics.

Results:

Our results suggest that TCP and relapse time are almost identical for conventional and hypofractionated regimens; however, second cancer risks resulting from hypofractionation were reduced by 22% when compared with the second cancer risk associated with a conventional protocol. The hypofractionated regimen appears to be sensitive to dose escalation and the corresponding impact on tumour recurrence time and reduction in second cancer risks. The reduction in second cancer risks is approximately 20% when the dose is increased from 60 to 72 Gy in a hypofractionated protocol.

Conclusion:

Our results suggest that hypofractionation may be a more efficient regimen in the context of TCP, relapse time and second cancer risks. Overall, our study demonstrates the importance of including a second cancer risk model in designing an efficient radiation regimen.

Advances in knowledge:

The impact of various fractionation protocols on TCP and relapse in conjunction with second cancer risks is an important clinical question that is as yet unexploredClinically, it is observed that over half of all cancer patients undergo radiotherapy over the course of their treatment, either as a primary treatment modality or in an adjuvant or a neoadjuvant context. In current radiotherapy treatments, tumours are often irradiated with a heterogeneous dose distribution throughout the treatment volume. The probability that all cancerous cells are removed from the system immediately post treatment is known as tumour control probability (TCP).¹ The design and complexity of any treatment regimen in terms of improving therapeutic efficacy can be deduced (to some extent) from TCP values. Although a given heterogeneous dose distribution to the target volume locally controls the disease to a large extent (for a given radiation regimen), there are still shortcomings associated with the currently used radiation protocol. Of these, side effects are of great importance and can be classified based on the time to clinical presentation, with shorter-acting side effects arising from irritation of the skin or mucosa, or irradiation of tissues with sensitive adjacent structures. Late toxicities of radiation are known to manifest after a period of 10–15 years, and one of the major late toxicities is the appearance of a secondary malignancy. Moreover, owing to the gains made in cancer care and patient management, there has been a marked increase in the number of survivors of childhood cancers, or cancers at young ages, and these patients are therefore at increased risk for the delayed consequences of radiotherapy. Several clinical studies have reported tumour recurrence or relapse within a span of 5 years² and late toxicities in the form of secondary malignancies within 5–20 years^3–7 post irradiation. These clinical investigations have been carried out on several types of tumours across a variety of treatment regimens and have also indicated that tumour relapse is a leading cause of death along with radiation-induced second cancers.Several pre-treatment factors such as age at diagnosis, gender and stage of tumour may impact tumour relapse. Relapse probability and time may vary depending on the treatment modality. In our work, for simplicity, we consider the effect of single treatment modality, namely, radiotherapy on TCP along with analysis of time to relapse. Also, escalating the dose to the target volume will eliminate tumours^8–10 and may have an impact on the relapse time (either very long ideally or an increase in the recurrence time) depending on the radiation protocol. Moreover, dose escalation may elevate radiation-induced second cancer risks, but the degree to which it increases these risks remains to be determined.Clinically, it is widely believed that radiotherapy-induced cancer risks are owing to scattered doses and radiation leakage from the linear accelerator, as well as irradiation to the healthy tissues adjacent to the target volume. Several systematic clinical investigations have indicated that radiation therapy is a significant causative factor of second cancers. Numerous case–control and cohort–control studies have also suggested that there is an increased risk of secondary malignancies with young cancer survivors, for example, survivors of Hodgkin''s lymphoma (HL).^11–13 Therefore, it is of paramount importance to reduce radiation doses to healthy tissue, while at the same time improve dose conformity to the target volume. The critical organs that get irradiated can be located in-beam (known as serial organs) or out-beam (also known as parallel organs) to the radiation beams and occasionally also receive the same integral dose. It is therefore critical to minimize radiation dosage and schedule radiation doses such that there is minimal impairment of delivery to the critical organs around the primary treatment volume, a probable increase in the relapse time (or with a long relapse time ideally), but also with the goal of reducing the risk of a secondary malignancy. It should be noted that in this work, we concentrated on modelling late complications due to radiotherapy, that is, radiation-induced second cancers only and not on normal tissue complication probability (NTCP) modelling.The main objective of this work is to develop a generalized mathematical framework that can incorporate relapse dynamics into a TCP model in conjunction with a second cancer risk model. This was carried out to understand the effects of dose escalation on recurrence and second cancer risks. Our minimal model also proposes an efficient paradigm in terms of regimen that may provide insights to be confirmed by future clinical investigations.     

Dose-volume and radiobiological dependence on the calculation grid size in prostate VMAT planning

This study evaluated the effects of dose-volume and radiobiological dependence on the calculation grid size in prostate volumetric-modulated arc therapy (VMAT) planning. Ten patients with prostate cancer were selected for this retrospective treatment planning study. Prostate VMAT plans were created for the patients using the 6 MV photon beam produced by a Varian TrueBEAM linac with the calculation grid size equal to 1, 2, 2.5, 3, 4, and 5?mm. Dose-volume histograms (DVHs) of targets and organs at risk were generated for different grid sizes. We calculated the radiobiological parameters of the tumor control probability (TCP) of clinical target volume (CTV) and planning target volume (PTV), and the normal tissue complication probability (NTCP) of organs at risk (rectal wall, rectum, bladder wall, bladder, left femur, and right femur). The homogeneity, conformity, and gradient indexes of CTV and PTV were calculated for different grid sizes. The TCP of PTV was found decreasing with a rate of 0.06%/mm when the calculation grid size increased from 1 to 5?mm. On the other hand, both NTCPs of rectal wall and rectum were found decreasing with rates of 0.03%/mm and 0.05%/mm, respectively, with an increase of grid size. The homogeneity index of PTV increased with a rate of 0.57/mm of the calculation grid size, whereas the conformity index of PTV decreased with a rate of 0.0075/mm. The gradient index of PTV was found increasing with a rate equal to 0.05/mm. In prostate VMAT planning, variations of dose-volume and radiobiological parameters with calculation grid size on PTV, rectal wall, and rectum were more significant than those of CTV and other organs at risk such as bladder wall, bladder, and femurs. Results in this study are important in the treatment planning quality assurance when the calculation grid size is varied to compromise a shorter dose computing time.     

Helical CT of prostate cancer: early clinical experience

OBJECTIV:. This study was undertaken to determine whether helical CT can reveal carcinoma of the prostate detected at transrectal sonographically guided biopsy. MATERIALS AND METHODS: Helical CT of the prostate was performed in 35 patients: 25 with proven prostate cancer (group I) and 10 without cancer detected at biopsy (group II). All patients in group I had cancer in the peripheral zone, and three of these showed foci of cancer in the transitional zone. All patients of group II had undergone at least two sets of biopsy before CT. In group I, areas of contrast enhancement in the peripheral zone of the prostate were defined as suggestive of cancer and correlated with the histopathologic findings. RESULTS: Helical CT revealed cancer in 22 (88%) of 25 patients with proven prostate cancer. Transrectal sonographically guided biopsy detected 102 cancer sites in the peripheral zone and three in the transitional zone in these 25 patients. Helical CT accurately revealed 59 peripheral zone cancer sites (58%) but did not reveal 43 cancer sites (42%). Abnormal contrast enhancement in the peripheral zone that was not caused by cancer was seen in 10% of suspicious lesions. The three cancer sites in the transitional zone were indistinguishable from benign nodular changes. CONCLUSION: Prostate cancer detected at transrectal sonographically guided biopsy appears on helical CT of the prostate as focal or diffuse areas of contrast enhancement in the peripheral zone. A prospective study has been initiated to determine the accuracy, sensitivity, and specificity.     

Image-guided intensity-modulated radiotherapy for prostate cancer: Dose constraints for the anterior rectal wall to minimize rectal toxicity

Rectal adverse events (AEs) are a major concern with definitive radiotherapy (RT) treatment for prostate cancer. The anterior rectal wall is at the greatest risk of injury as it lies closest to the target volume and receives the highest dose of RT. This study evaluated the absolute volume of anterior rectal wall receiving a high dose to identify potential ideal dose constraints that can minimize rectal AEs. A total of 111 consecutive patients with Stage T1c to T3a N0 M0 prostate cancer who underwent image-guided intensity-modulated RT at our institution were included. AEs were graded according to the Common Terminology Criteria for Adverse Events, version 4.0. The volume of anterior rectal wall receiving 5 to 80 Gy in 2.5-Gy increments was determined. Multivariable Cox regression models were used to identify cut points in these volumes that led to an increased risk of early and late rectal AEs. Early AEs occurred in most patients (88%); however, relatively few of them (13%) were grade ≥2. At 5 years, the cumulative incidence of late rectal AEs was 37%, with only 5% being grade ≥2. For almost all RT doses, we identified a threshold of irradiated absolute volume of anterior rectal wall above which there was at least a trend toward a significantly higher rate of AEs. Most strikingly, patients with more than 1.29, 0.73, or 0.45 cm³ of anterior rectal wall exposed to radiation doses of 67.5, 70, or 72.5 Gy, respectively, had a significantly increased risk of late AEs (relative risks [RR]: 2.18 to 2.72; p ≤ 0.041) and of grade ≥ 2 early AEs (RR: 6.36 to 6.48; p = 0.004). Our study provides evidence that definitive image-guided intensity-modulated radiotherapy (IG-IMRT) for prostate cancer is well tolerated and also identifies dose thresholds for the absolute volume of anterior rectal wall above which patients are at greater risk of early and late complications.     

Antibody mass escalation study in patients with castration-resistant prostate cancer using 111In-J591: lesion detectability and dosimetric projections for 90Y radioimmunotherapy.

J591, a monoclonal antibody that targets the external domain of the prostate-specific membrane antigen, has potential as an agent for radioimmunotherapy. A pilot trial was performed in patients with prostate cancer using repetitive administrations of escalating masses of J591. An analysis was performed to assess lesion detectability by (111)In-J591 gamma-camera imaging compared with standard imaging methods and the effect of increasing antibody mass on lesion detectability, biodistribution, and dosimetry. METHODS: Fourteen patients with metastatic prostate cancer received escalating amounts (10, 25, 50, and 100 mg) of J591 in a series of administrations each separated by 3 wk. All antibody administrations included a fixed amount of the radiolabeled antibody (111)In-1,4,7,10-tetraazacyclododecane-N,N',N',N'-tetraacetic acid-J591 ((111)In-DOTA-J591) (2 mg of J591 labeled with 185 MBq [5 mCi] of (111)In via the chelating agent DOTA). Three whole-body gamma-camera scans with at least 1 SPECT scan together with multiple whole-body counting-rate measurements and serum activity-concentration measurements were obtained in all patients. Images were analyzed for distribution and lesion targeting. Estimates of clearance rates and liver and lesion uptake were made for each treatment cycle. These estimates were used to generate dosimetric projections for radioimmunotherapy with (90)Y-labeled J591. RESULTS: A total of 80 lesions in 14 patients were detected. Both skeletal and soft-tissue diseases were targeted by the antibody as seen on (111)In-J591 scans. The antibody localized to 93.7% of skeletal lesions detected by conventional imaging. Clearance of radioactivity from the whole body, serum, and liver was dependent on antibody mass. Normalized average values of the ratio of residence times between lesion and liver for 10, 25, 50, and 100 mg of antibody were 1.0, 1.9, 3.2, and 4.0. Dosimetric projections for radioimmunotherapy with (90)Y-labeled J591 suggested similar absorbed doses to lesions for treatment at the maximally tolerated activity (MTA), irrespective of antibody mass. However, absorbed doses to liver at the MTA would be antibody mass-dependent with estimates of 20, 10, 7, and 5 Gy for 10, 25, 50, and 100 mg of J591. CONCLUSION: The proportion of the amount of antibody increased in lesions and decreased in the liver with increasing mass of administered antibody up to a dose of 50 mg. Proportional hepatic uptake continued to decrease with increasing antibody mass up to 100 mg. The optimal antibody mass for radioimmunotherapy would therefore appear to be greater than or equal to 50 mg.