Inclusion Criteria:

          1. Male or female aged ≥18 years.

          2. Histologically confirmed DLBCL (WHO classification).

          3. Received at least one prior line of therapy including immuno-chemotherapy.

          4. In first or subsequent relapse, or refractory to the last treatment (defined as less
             than a complete metabolic response to the last treatment, or within 6 months from the
             last treatment).

          5. Not suitable for, or declined high dose chemotherapy and autologous stem cell
             transplantation (ASCT).

          6. Presence of radiographically measurable lymphadenopathy or extranodal lymphoid
             malignancy (at least one objectively bi-dimensionally measurable (nodal) lesion (>2 cm
             in its largest dimension by CT scan).

          7. Negative human anti-murine antibody (HAMA) test.

          8. Life expectancy of at least 3 months.

          9. Bone marrow tumour infiltration <25% tumour cells.

         10. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2

         11. Normal organ and bone marrow function defined as:

               1. Absolute neutrophil count ≥1.5 x 109/L;

               2. Platelet count ≥150 x 109/L;

               3. Haemoglobin ≥9 g/dL;

               4. Total bilirubin ≤1.5 x upper limit of normal (ULN) (except patients with
                  documented Gilbert's syndrome);

               5. Liver enzymes: Aspartate transaminase (AST); Alanine transaminase (ALT) or
                  Alkaline phosphatase (ALP) ≤2.5 x ULN (or ≤5.0 x ULN if liver involvement by
                  primary disease);

               6. Adequate renal function as demonstrated by a serum creatinine ≤1.5 mg/dL or a
                  creatinine clearance >60 mL/min;

               7. Normal coagulation parameters (elevated international normalized ratio (INR),
                  prothrombin time or activated partial thromboplastin time (APTT) ≤1.3 ULN range
                  acceptable).

         12. Women of childbearing potential must

               1. understand that the study medication may have teratogenic risk

               2. have a negative serum pregnancy test at screening and before Betalutin injection

               3. commit to continued abstinence from heterosexual intercourse (excluding periodic
                  abstinence or the withdrawal method) or begin two acceptable methods of birth
                  control with a Pearl-Index ≤ 1%. without interruption from 4 weeks before
                  starting study drug, throughout study drug therapy and for 12 months after end of
                  study drug therapy, even if she has amenorrhoea. Apart from abstinence,
                  acceptable methods of birth control are:

                    -  Combined (estrogen and progestogen containing) hormonal contraception
                       associated with inhibition of ovulation

                    -  oral

                    -  intravaginal

                    -  transdermal

                    -  Progestogen-only hormonal contraception associated with inhibition of
                       ovulation

                    -  oral

                    -  injectable

                    -  implantable

                    -  Intrauterine device (IUD)

                    -  Intrauterine hormone-releasing system ( IUS)

                    -  Bilateral tubal occlusion

                    -  Vasectomised partner

         13. Male patients must agree to use condoms during intercourse throughout study drug
             therapy and the following 12 months.

         14. Ability to give written, informed consent prior to any study-specific screening
             procedures, with the understanding that the consent may be withdrawn by the patient at
             any time without prejudice.

         15. Capable of understanding the protocol requirements, is willing and able to comply with
             the study protocol procedures, and has signed the informed consent document.

         16. A negative Hepatitis B test (HBsAg and anti-HBc) and negative HIV test during
             screening

        Exclusion Criteria:

          1. Prior hematopoietic allogenic stem cell transplantation.

          2. Prior autologous stem cell transplantation.

          3. Previous total body irradiation, or irradiation of >25% of the patient's bone marrow.

          4. Prior anti-lymphoma therapy (chemotherapy, immunotherapy or other investigational
             agent) within 4 weeks prior to start of study treatment (corticosteroid treatment at
             doses of ≤20 mg/day, G-CSF or GM CSF are permitted up to 2 weeks prior to start of
             study treatment.). Note: excluding pre-treatment with rituximab as part of this study

          5. Patients who are receiving any other investigational agents.

          6. Patients with known or suspected central nervous system involvement of lymphoma.

          7. History of a previous treated cancer except for the following:

               1. adequately treated local basal cell or squamous cell carcinoma of the skin;

               2. cervical carcinoma in situ;

               3. superficial bladder cancer;

               4. localized prostate cancer undergoing surveillance or surgery;

               5. localised breast cancer treated with surgery and radiotherapy but not including
                  systemic chemotherapy;

               6. other adequately treated Stage 1 or 2 cancer currently in complete remission;

          8. Pregnant or breastfeeding women.

          9. Exposure to another CD37 targeting drug.

         10. Allergy to X ray contrast agents.

         11. A known hypersensitivity to rituximab, HH1, Betalutin or murine proteins or any
             excipient used in rituximab, HH1 or Betalutin.

         12. Has received a live attenuated vaccine within 30 days prior to enrolling in the study.

         13. Evidence of severe or uncontrolled systemic diseases:

               1. uncontrolled infection including evidence of ongoing systemic bacterial, fungal,
                  or viral infection (excluding viral upper respiratory tract infections) at the
                  time of initiation of study treatment;

               2. pulmonary conditions e.g. unstable or uncompensated respiratory disease

               3. hepatic, renal neurological or metabolic conditions - which in the opinion of the
                  investigator would compromise the protocol objectives.

               4. psychiatric conditions e.g. patients unlikely to comply with the protocol, e.g.
                  mental condition rendering the patient unable to understand the nature, scope,
                  and possible consequences of participating in the study

               5. history of erythema multiforme, toxic epidermal necrolysis or Stevens-Johnson
                  syndrome;

               6. cardiac conditions, including

                    -  history of acute coronary syndromes (including unstable angina)

                    -  class II, III, or IV heart failure as defined by the New York Heart
                       Association (NYHA) functional classification system;

                    -  known uncontrolled arrhythmias (except sinus arrhythmia) in the past 24
                       weeks.