High prevalence of late adverse events in malignant bone tumour survivors diagnosed at adult age.

BACKGROUND: Late treatment-related adverse events are particularly prevalent in survivors of childhood bone cancer because of the combination of cytotoxic drugs, major surgery and radiotherapy. Existing studies for late toxicity in survivors of Ewing's sarcoma (ES) and osteosarcoma (OS) diagnosed at adult age have focused on specific sequelae. We investigated a broad spectrum of potential late effects in these patients. METHODS: Relapse-free OS and ES patients aged ≥ 16 at diagnosis and treated at the Radboud University Medical Centre (1982-2007) were invited for systematic late toxicity screening. This included history taking, physical examination, echocardiogram, bone densitometry, audiogram, and serum and urine screening for renal toxicity and infertility. Adverse events were graded according to the Common Terminology Criteria for Adverse Events version 3.0. RESULTS: In 24 survivors (63% male, mean age at screening 45.7 years, mean follow-up 10.9 years, 70% OS) we found a median of eight adverse events. Frequent findings included abnormal gait, osteoporosis, pain, left ventricular systolic dysfunction, obesity and nephropathy. The maximum grade of any adverse event was mild in four (17%), moderate in 11 (46%), severe in six (25%), and disabling in three cases (13%). There was a trend towards more events in patients diagnosed at an older age. CONCLUSION: The incidence of late adverse events in this study of survivors of bone tumours diagnosed at adult age is higher than in any previously published childhood cancer survivorship study. Older patients seem to be particularly at risk. Our findings underscore the need for systematic screening of late effects in bone cancer survivors of adult age at diagnosis.

Microdontia after chemotherapy in a child treated for neuroblastoma.

OBJECTIVE: Chemotherapy used on paediatric oncology patients often causes disturbances in dental development. Aim of this case report is to present the late effects of chemotherapy on dental development in a patient treated for neuroblastoma at early age. DESIGN: Case report. RESULTS: This paper presents a female patient treated at early age with surgery and chemotherapy for a neuroblastoma (stage IVS) in the right thorax and massive liver metastases. The examination of the patient at age 11.7 years showed microdontia of six teeth. In three of them size and form of the crown were affected, while in the other three the size was reduced but the form was not affected. CONCLUSIONS: Chemotherapy on children treated for neuroblastoma can adversely influence tooth development. This has to be taken into consideration by the dentist when monitoring the development of the dentition and occlusion.

Treatment strategy and results in children treated on three Dutch Childhood Oncology Group acute myeloid leukemia trials.

This report describes the long-term follow-up data of three consecutive Dutch Childhood Oncology Group acute myeloid leukemia (AML) protocols. A total of 303 children were diagnosed with AML, of whom 209 were eligible for this report. The first study was the AML-82 protocol. Results were inferior (5-year probability of overall survival (pOS) 31%) to other available regimes. Study AML-87 was based on the BFM-87 protocol, with prophylactic cranial irradiation in high-risk patients only, and without maintenance therapy. This led to a higher cumulative incidence of relapse than that reported by the Berlin-Frankfurt-Münster (BFM), but survival was similar (5-year pOS 47%), suggesting successful retrieval at relapse. The subsequent study AML-92/94 consisted of a modified BFM-93 protocol, that is, without maintenance therapy and prophylactic cranial irradiation. However, all patients were to be transplanted (auto- or allogeneic), although compliance was poor. Antileukemic efficacy was offset by an increase in the cumulative incidence of nonrelapse mortality, especially in remission patients, and survival did not improve (5-year pOS 44%). Our results demonstrate that outcome in childhood AML is still unsatisfactory, and that further intensification of therapy carries the risk of enhanced toxicity. Our patients are currently included in the MRC AML studies, based on the results of their AML 10 trial.

Prognostic value of tumor size, metastases, extension into bone, and increased tumor marker in children with malignant sacrococcygeal germ cell tumors: a prospective evaluation of 71 patients treated in the German cooperative protocols Maligne Keimzelltumoren (MAKEI) 83/86 and MAKEI 89.

PURPOSE: To evaluate the prognostic value of metastases, extension into bone, and alpha-fetoprotein (AFP) elevation in children with malignant sacrococcygeal germ cell tumors (GCTs) prospectively collected in two cooperative Maligne Keimzelltumoren (MAKEI) protocols (83/86 and 89). PATIENTS AND METHODS: Between October 1983 and October 1995, 76 of 210 registered patients with sacrococcygeal primaries presented either with pure yolk sac tumor, embryonal carcinoma (EC), or yolk sac tumor and EC mixed with immature and mature teratoma elements. Stages T1 and T2 disease were diagnosed in 15 and 61 children, respectively, 41 patients had metastases, and 35 children presented with extension into bone. At diagnosis, 22 children had an AFP elevation of less than 10,000 ng/mL. Thirty-six children showed an AFP level between 10,000 and 100,000 ng/mL, and 12 patients had values of greater than 100,000 ng/mL. Five patients died of complication during treatment and were excluded from further evaluation. Seventy-one patients could be analyzed. RESULTS: The 5-year relapse-free survival rate (RFS, Kaplan-Meier) was 0.76 +/- 0.03 (54 of 71 patients; median observation time, 54 months after diagnosis). The RFS of patients with and without metastases was different, but not significantly so (0.71 v 0.82). The outcome of patients with extension into bone (n = 31) and without this extension (n = 40) was 0.71 versus 0.80 (RFS, 5 years). Above-normal AFP level had no prognostic significance (P =.52). CONCLUSION: In children with malignant sacrococcygeal GCTs treated with an intensive, short-interval, platinum-based regimen, the stage, extent of metastases, extension into bone, and AFP level had no prognostic significance.

BFM-oriented treatment for children with acute lymphoblastic leukemia without cranial irradiation and treatment reduction for standard risk patients: results of DCLSG protocol ALL-8 (1991-1996).

Modern treatment strategies, consisting of intensive chemotherapy and cranial irradiation, have remarkably improved the prognosis for children with acute lymphoblastic leukemia. However, patients with a potential for cure are at risk of severe acute and late adverse effects of treatment. Furthermore, in 25-30% of patients treatment still fails. The objectives of the DCLSG study ALL 8 were to decrease the toxicity and to increase the effectivity of BFM-oriented treatment. Decrease of toxicity was aimed at by confirmation of the results of the previous DCLSG study ALL-7, showing that the majority (94%) of children with ALL can successfully be treated with BFM-oriented therapy without cranial irradiation, and by reduction of treatment for standard risk (SRG) patients. To increase the cure rate in medium risk (MRG) patients the efficacy of high doses of intravenous 6-mercaptopurine (HD-6MP) during protocol M and in SRG patients the efficacy of high doses of L-asparaginase (HD-L-ASP) during maintenance treatment was studied in randomized studies. Patient stratification and treatment were identical to protocol ALL-BFM90, with the following differences: no prophylactic cranial irradiation, SRG patients received only phase 1 of protocol I. Four hundred and sixty-seven patients entered the protocol: 170 SRG, 241 MRG and 56 HRG patients. The 5 years event-free survival rate for all patients was 73% (s.e. 2%); for SRG, MRG and HRG patients 85% (s.e. 3%), 73% (s.e. 3%) and 39% (s.e. 7%), respectively. In patients >1 year of age at diagnosis unfavorable prognostic factors were male sex, >25% blasts in the bone marrow at day 15 and initial white blood cell count (WBC) >50 x 10(9)/l. The cumulative risk of CNS relapse rate was 5% (s.e. 1%) at 5 years. These results confirm that the omission of cranial irradiation in BFM-oriented treatment does not jeopardize the overall good treatment results, nor does early reduction of chemotherapy in SRG patients. No benefit was observed from treatment intensification with HD-L-ASP in SRG patients, nor from HD-6MP in MRG patients.

Analysis of handwriting of children during treatment for acute lymphoblastic leukemia.

BACKGROUND: Children treated for acute lymphoblastic leukemia (ALL) often complain about handwriting problems. PROCEDURE: Using a computerized writing task, we have prospectively studied the processes necessary for the production of handwriting movements in 11 children (5-12 years old) during treatment for ALL. Children were tested at time points closely related to the vincristine administration. RESULTS AND CONCLUSIONS: Children treated for ALL drew slower, with longer pause durations and increased drawing pressure. Children were able to overcome the problems, except for a consistently increased drawing pressure. This increased drawing pressure may be an attempt of the children to obtain sufficient kinesthetic information and thus can be seen as an adequate adaptation mechanism in case of peripheral neuropathy due to the neurotoxic effects of vincristine. However, neurotoxic effects of other cytostatic drugs cannot be excluded.

SIOP treatment guidelines for renal tumours in small infants: fact or fantasy?

AIMS: Since as far back as 1980, SIOP (Société Internationale d>>Oncologie Pédiatrique) have advocated primary nephrectomy (PN) only for unilateral renal tumours in patients > tumour (WT). Fourteen of the 25 patients (56%) were treated with PN, including four patients with CMN. In group B there was one patient (2%) with CMN and 40 patients with WT. Thirteen of the patients (31%) were treated with PN. A total of 15 patients were treated before 1980 and 26 after 1980. Eight of 15 (53%) patients were treated with PN before 1980 and 21/26 (81%) were pre-treated after 1980, according to the protocol. CONCLUSION: Despite the SIOP recommendations, only 56% of patients

Mild axonal neuropathy of children during treatment for acute lymphoblastic leukaemia.

Neurophysiological functioning was studied prospectively in children treated for acute lymphoblastic leukaemia with a low dose vincristine regime (8 x 1.5 mg/m2/dose), to obtain more insight into vincristine neuropathy. A WHO neurotoxicity score was estimated and vibration sense and electrophysiological measurements were taken at standardized times during vincristine treatment. The WHO neurotoxicity score showed decreased or disappearance of Achilles tendon reflexes, and mild sensory disturbances, but a grade 3-4 neurotoxicity was not demonstrated by any of the children. Vibration perception thresholds increased progressively during treatment and amplitudes of action potentials of peroneal and sensory ulnar and median nerves decreased, whereas nerve conduction velocities stayed unchanged. Both vibration perception thresholds and the electrophysiological findings hardly exceeded the limits of normality. We conclude that children treated for acute lymphoblastic leukaemia with a low dose vincristine regimen have mild axonal neuropathy which may be responsible for the motor problems in these children.

The saddle prosthesis in pelvic primary and secondary musculoskeletal tumors: functional results at several postoperative intervals.

The first purpose of this study was to evaluate the saddle prosthesis in patients with periacetabular tumors in terms of the functional results obtained after several postoperative intervals. The second purpose was to evaluate the complications and how they might be prevented in the future. Functional results according to the MSTS functional rating system were evaluated at several postoperative intervals in 15 patients treated with internal hemipelvectomy and reconstruction with the saddle prosthesis because of periacetabular primary (n = 9) or secondary (n = 6) malignancies. All complications were evaluated. Three months postoperatively, 7/9 patients with a primary tumor and 2/4 patients with a secondary tumor were able to walk outside without pain. Median functional results 3 and 6 months postoperatively were 40% and 50%, respectively. Deep infection occurred in 4 patients and fracture of the iliac remnant in 2. Heterotopic ossifications along the interpositional component were seen in 5 patients, but they did not negatively influence the functional outcome. Three (relative) contraindications to reconstruction with the saddle prosthesis could be ascertained: osteoporosis, extended involvement of the iliac wing by tumor, and insufficient soft-tissue quality after previous procedures. (Short-term) functional results after reconstruction with the saddle prosthesis are satisfactory if the above-mentioned contraindications are taken into consideration.

Osteosarcoma: oncologic and functional results. A single institutional report covering 22 years.

BACKGROUND AND OBJECTIVES: The oncologic and functional results in patients treated because of osteosarcoma (OS) were evaluated. METHODS: Fifty-one patients with high-grade OS were treated between 1974 and 1996 at our hospital. All patient records were studied, and the surviving patients were evaluated according to the American Musculoskeletal Tumor Society functional rating system. The majority of patients received adjuvant chemotherapy (prior to 1983) or neoadjuvant chemotherapy (from 1983). Until 1987, all patients with extremity OS had ablative surgery; from 1987, the majority had limb-saving surgery. Lung metastases were resected in most cases. RESULTS: Overall 2-year and 5-year disease-free survival (DFS) rates were 27 of 51 and 16 of 42, respectively. Patients with vertebral or pelvic OS or contaminated margins after resection had a very bad outcome. In all other subgroups, including patients with various types of chemotherapy, response to chemotherapy, diameter of tumor, presence or absence of metastatic spread, and location of tumor, a 5-year DFS of about 50% was found. Recurrent disease in patients who had achieved a 2-year disease-free interval was relatively low (4/23 patients). CONCLUSIONS: Survival in our series was worse than in most other studies. A very bad outcome was found in patients with vertebral or pelvic OS or with contaminated margins after resection.

Intensive treatment of children with acute lymphoblastic leukemia according to ALL-BFM-86 without cranial radiotherapy: results of Dutch Childhood Leukemia Study Group Protocol ALL-7 (1988-1991).

In The Netherlands from July 1988 to October 1991, children (0 to 16 years of age) with de novo acute lymphoblastic leukemia (ALL) were treated according to protocol ALL-7 of the Dutch Childhood Leukemia Study Group (DCLSG). In this protocol, chemotherapy and treatment stratification were identical to the ALL-BFM-86 protocol (Reiter et al, Blood 84:3122, 1994), but cranial irradiation was restricted to patients with initial central nervous system (CNS) involvement. Patients were stratified into 3 risk groups, based on leukemia cell mass and response to initial treatment: standard-risk group (SRG), risk group (RG), and experimental group (EG). As in ALL-BFM-86, a randomized study on late intensification (protocol S) was performed in RG patients, and during the study (since October 1990), early reinduction treatment (protocol II) was introduced for SRG patients. Treatment duration for all patients was 18 months. Two hundred eighteen children entered the study: 74 SRG, 127 RG, and 17 EG patients. The overall complete remission (CR) rate was 98%. The 5-year event-free survival (EFS) for all DCLSG ALL-7 patients was 65. 3% (standard error [SE] 3.2%), which was significantly different from the 73% (SE 1%) 5-year EFS achieved in the ALL-BFM-86 study (P =.02, Z-test). However, restricting the analysis to SRG patients receiving protocol II with a total duration of treatment of 18 months, the 5-year EFS rates were 64.6% (SE 4.0%) and 67% (SE 4%), respectively, and no significant difference could be established (P =.67, Z-test). The 5-year EFS rates for SRG, RG, and EG patients were 63.5% (SE 5.6%), 66.6% (SE 4.2%), and 63.3% (SE 12.0%), respectively. SRG patients receiving protocol II fared better than patients not receiving protocol II (5-year EFS 76.7% [SE 7.7] and 54. 5% [SE 7.5], respectively). No difference in 5-year EFS was observed in RG patients randomized to receive or not to receive late intensification with protocol S. The overall CNS relapse rate at 5 years was 5.5%. The incidence rate at 5 years was 11.4% in SRG patients not receiving protocol II, whereas no CNS relapses occurred in SRG patients receiving protocol II. Six children died in first complete remission and 2 children developed a second malignancy (thyroid carcinoma and acute nonlymphoblastic leukemia). Systemic high-dose methotrexate (MTX) and intrathecal chemotherapy is a safe and effective method of CNS prophylaxis in the context of BFM-oriented treatment for all children with ALL, regardless of the risk group (with the possible exception of T-ALL patients with high white blood cell counts). The results of the DCLSG ALL-7 study confirm those of the ALL-BFM-86 study showing that early reinduction with protocol II is essential in the treatment of SRG patients and that late intensification with protocol S does not improve the prognosis for RG patients.

Teratomas in infancy and childhood.

BACKGROUND AND PROCEDURE: Outcomes in children with teratomas collected between October 1982 and December 1995 in cooperative protocols of the German Society of Pediatric Oncology and Hematology (GPOH) were analyzed. Teratomas were diagnosed in 329 (42%) of 780 registered patients with germ cell tumors. The annual incidence was 0.24/100,000. Main primary sites were coccygeal (n = 132, 2.2:1 female predominance), ovary (n = 81), testis (n = 40) and brain (n = 15, 2.8:1 male predominance). RESULTS: Two hundred seventy cases of extracranial non-testicular teratoma were evaluated: In mature teratomas (n = 154) the observed relapse rate was 10%. Incomplete resection was the main risk factor for relapse. After complete resection, the relapse-free survival (RFS, Kaplan-Meier-estimation) was 0.96 +/- 0.01 (n = 126, observation time 18-155 months) in comparison to an RFS of 0.56 +/- 0.09 in incompletely resected teratomas grade 0 (n = 28, observation time 28-94 months) (P < 0.01). Im-mature teratomas were treated by surgery alone in 76 cases and by surgery and adjuvant chemotherapy in 40 cases. The observed relapse risk was 18%. Main risk factors for relapse were incomplete tumor resection (n = 38) as well as immaturity in incompletely resected teratomas. Fifteen of 29 relapsing patients presented with malignant tissue in the recurrent tumor (mainly yolk sac tumor); in contrast, seven of 40 patients with immature teratoma relapsed despite adjuvant chemotherapy without showing malignant components (P = 0.014). Nine of 36 (25%) relapsing patients died of disease. Eleven of the 27 (41%) surviving children suffered from mutilation after repeated surgery. COMMENTS: It is suggested that an international randomized trial for patients with incompletely resected high risk teratoma be initiated to evaluate the effect of adjuvant chemotherapy on specific end-points: 1) influence on relapse rate in general; 2) reduction of the proportion of malignant relapses; 3) avoidance of mutilating surgery.

Inhibition of DNA methylation in malignant MOLT F4 lymphoblasts by 6-mercaptopurine.

Treatment of MOLT F4 lymphoblasts with 6-mercaptopurine (6-MP) resulted in a decrease of ATP and a depletion of S-adenosylmethionine (AdoMet). To investigate whether this might affect the methylation of DNA, we treated MOLT F4 lymphoblasts with increasing concentrations of 6-MP, followed by labeling with [methyl-14C]methionine and [methyl-3H]thymidine. After DNA isolation, we measured the incorporated radioactivity and determined the 14C/3H ratio as a measure for the methylation of newly formed DNA. The 14C/3H ratio was decreased by 17% with 1 mumol/L 6-MP; treatment with increasing concentrations of 6-MP up to 10 mumol/L showed a further decrease to 70%, in comparison with untreated cells. To demonstrate that the methylation of deoxycytidine residues in DNA was reduced, we quantified hydrolyzed DNA by HPLC. The 14C/3H ratio showed a decrease with increasing 6-MP concentrations, indicating that treatment with 6-MP resulted in hypomethylation of DNA.

DNA methylation patterns in the calcitonin gene region at first diagnosis and at relapse of acute lymphoblastic leukemia (ALL).

Aberrant DNA methylation can occur early in neoplastic transformation and may lead to the development of cancer. We describe the alterations of methylation patterns at the DNA sequence level which occurred in the 5' region of the calcitonin gene in lymphoblasts from 14 pediatric patients with acute lymphoblastic leukemia (ALL). The DNA methylation status of 25 CpG sites was determined by sequence analysis after bisulfite treatment of the DNA. This method showed that 13 out of 14 patients had increased numbers of methylated CpG sites in the calcitonin gene region at initial diagnosis when compared to control DNA from healthy individuals. The 5' region of the calcitonin gene appears to be methylated to a significantly higher degree in T lineage ALL compared to B lineage ALL (P < 0.01). Each of six ALL patients who were investigated at initial diagnosis and at relapse showed alterations in DNA methylation between the two stages. These six cases were also investigated by Southern blot analysis with methylcytosine-sensitive restriction enzymes and this method showed an increase in DNA methylation in only four of the six cases. The DNA sequencing method thus appears to be better suited to assess alterations of DNA methylation than Southern blot analysis. There are marked regional differences in the frequency of methylation of individual CpG sites and in the frequency of alterations between the two stages. Our results show that alterations in DNA methylation continue to occur from the initial stage to the relapse stage of ALL, suggesting that aberrant DNA methylation may play a role in tumor progression.

Metabolism of intravenously administered high-dose 6-mercaptopurine with and without allopurinol treatment in patients with non-Hodgkin lymphoma.

PURPOSE: We investigated the metabolism of high dose 6 mercaptopurine (HD-6MP) infusions and its influence on the metabolism by allopurinol, an inhibitor of xanthine oxidase, the enzyme that catabolizes 6MP into thioxanthine and thiouric acid. PATIENTS AND METHODS: Nine patients (aged 2-11 years) with non-Hodgkin lymphoma (NHL) were treated with HD-6MP (1300 mg/m(2).24h) within a therapeutic window after diagnosis. Four patients received oral allopurinol (200 mg/m(2).day) to prevent urate nephropathy, and five did not. Plasma and RBC were isolated before and 4, 20, 24, 28, and 48h after the start of the infusion. All measurements were performed with HPLC. RESULTS: Considerable variations were found in the plasma levels of 6MP, thioxanthine, and thiouric acid and of RBC-MeTIN levels. 6MP-riboside was not detectable, and MeMP and MeMPR levels were <1.3 muM in the plasma. In general, 6MP, thioxanthine, and MeMP levels in plasma were higher, and thiouric acid plasma levels and RBC-MeTIN levels were lower in the patients treated with allopurinol compared to those who did not receive allopurinol. CONCLUSIONS: 6MP is extensively metabolized in patients with NHL treated with HD-6MP. Thiopurine methylation, at the levels of nucleotide, nucleoside, and base, is an important metabolic pathway after HD-6MP. Co-administration of allopurinol can result in both a decreased catabolism and anabolism of 6MP compared to treatment with HD-6MP alone. This observation may have consequences for the therapeutic efficacy and toxic effects of 6MP in combination with allopurinol.

Effects on transmethylation by high-dose 6-mercaptopurine and methotrexate infusions during consolidation treatment of acute lymphoblastic leukemia.

6-mercaptopurine (6MP) cytotoxicity is caused by thioguanine and methylthioinosine nucleotides. Thiopurine methylation occurs to a large extent in vivo and in vitro. In this reaction, S-adenosyl-L-methionine (AdoMet), produced from methionine and ATP, is converted into S-adenosyl-L-homocysteine (AdoHcy) which, in turn, is hydrolyzed into homocysteine. Remethylation of homocysteine into methionine is inhibited by methotrexate (MTX). In cultured lymphoblasts, AdoMet: AdoHcy ratio and DNA methylation decrease after incubation with 6MP. The aim of the present study was to investigate the influence of high-dose 6MP on the methylation capacity in children with acute lymphoblastic leukemia. Five patients received 4 courses with high-dose intravenous MTX (5' g.m-2 in 24 hr) immediately followed by high-dose 6MP (1300 mg.m-2 in 24 hr). Five control patients received high-dose MTX and oral 6MP (25 mg.m -2 daily for 8 weeks). Leucovorin rescue was started at 36 hr in both groups. In the intravenous 6MP group, 6-methylmercaptopurine, its riboside, and 6-methylmercapto-8-hydroxypurine were detectable in plasma in concentrations of 0.3-2.6 muM (6MP steady state levels: 11.6 muM). In red blood cells, mean methylthioinosine nucleotide levels were one third of those of ATP (13.1 nmol/10(8)). AdoHcy levels (10 pmol/10(8)) remained constant in both groups and AdoMet was not detectable ( < 20 pmol/10(8)). In both groups, plasma homocysteine increased and methionine decreased following administration of MTX. The delay in the recovery of methionine in the intravenous 6MP group after MTX infusion is probably the result of an increased demand on methyl groups during 6MP infusion.

Thiopurine methyltransferase: a review and a clinical pilot study.

Thiopurine methyltransferase (TPMT) is an important enzyme in the metabolism of 6-mercaptopurine (6MP), which is used in the treatment of acute lymphoblastic leukemia (ALL). TPMT catalyzes the formation of methylthioinosine monophosphate (MetIMP), which is cytotoxic for cultured cell lines, and it plays a role in detoxification of 6MP. Population studies show a genetic polymorphism for TPMT with both high and low activity alleles. About 1 of 300 subjects is homozygous for the low activity. The function TPMT plays in detoxification or therapeutic efficacy of 6MP in vivo is not clear. In this article the genetic polymorphism of TPMT is reviewed and the contribution of TPMT to the cytotoxic action, or detoxification, of 6MP in children with ALL is discussed. Induction of TPMT activity has been described during the treatment for ALL. We performed a pilot study on the influence of high-dose 6MP infusions (1300 mg/m2 in 24 h) on TPMT activity of peripheral blood mononuclear cells (pMNC) of eleven patients with ALL. The TPMT activities were in, or, above the normal range. There was no statistically significant difference between the TPMT activities before and after the 6MP infusions. MetIMP levels in pMNC increased during successive courses. This might be explained by TPMT induction, but other explanations are plausible as well. Twenty five percent of the TPMT assays failed, because less than the necessary 5.10(6) pMNC could be isolated from the blood of leukopenic patients. Red blood cells can not be used for TPMT measurements, since transfusions are frequently required during the treatment with 6MP infusions. Therefore, the influence of high-dose 6MP infusions on TPMT activity can only be investigated further when a TPMT assay which requires less pMNC has been developed.