Radiotherapy in Burkitt's lymphoma: conventional or superfractionated regime--early results

Response to hyperfractionated accelerated radiotherapy in chemotherapy-refractory non-Hodgkin lymphoma

Patients with chemotherapy-refractory non-Hodgkin lymphoma (NHL) have a poor prognosis with a median overall survival (OS) of only 10 months. To investigate the role of radiotherapy (RT) in such patients, we conducted a retrospective review of 17 patients with biopsy-proven refractory NHL who received hyperfractionated accelerated RT between 2000 and 2017. Forty-seven percent had stages I and II and 53% had stages III and IV disease. Majority (59%) had diffuse large B-cell lymphoma. One-year local control rate was 82%. Fifty-nine percent proceeded to hematopoietic stem cell transplantation (HSCT). At a median follow-up time of 8.8 months (range: 13 days to 17.4 years), 10 were alive with five in remission. Six patients were long-term survivors with a median OS of 8.1 years. Hyperfractionated accelerated RT in chemotherapy-refractory NHL provides durable local disease control in the majority of cases. Combined with HSCT, the RT regimen may also provide long-term disease remission in a subset of patients.

Hyperfractionated radiation therapy in Burkitt's lymphoma: a reconsideration aspect

Burkitt's lymphoma (BL) is an aggressive non-Hodgkin's B-cell lymphoma with an extremely short doubling time that often presents in extra nodal sites or as an acute leukaemia. Nowadays, with the rapid response to chemotherapy and the diffuse nature of BL, there is no established role for radiation therapy (RT) even in localized disease. Regarding the relapsed/refractory BL, the treatment recommendations remain undefined. We present a 56-year-old woman, diagnosed with BL refractory to 6 cycles of R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone), who had disease progression on R-DHAP (rituximab, dexamethasone, high dose cytarabine and cisplatin) with intrathecal methotrexate, then a partial response on RICE (rituximab, ifosfamide, carboplatin and etoposide). Patient received high dose chemotherapy and autologous haematopoietic stem cell transplantation. Then, she was treated with hyperfractionated involved-field RT regimen. Currently, the patient remains disease free for around 2 years after remission. We acknowledge that RT is not a standard treatment of BL, especially in patients who attain complete response (CR) after first-line multi-agent chemotherapy or even in those who have a CR after second-line chemotherapy pre-transplant. Yet, the use of a superfractionated regimen of consolidative radiation could be justified in the treatment of recurrent/refractory localized BL who do not achieve a CR even with second-line salvage chemotherapy. Radiation therapy in this context, given that it is a well-tolerated treatment, is a modality worthy of being re-considered in relapsed/refractory BL. Copyright © 2016 John Wiley & Sons, Ltd.

Outcome of hyperfractionated radiotherapy in chemotherapy-resistant non-Hodgkin’s lymphoma

PURPOSE

Patients with chemotherapy-resistant lymphoma have rapidly progressive disease and a poor prognosis. Local symptoms are treated with radiotherapy (RT) for local control. We have reviewed local control and toxicity in patients treated with hyperfractionated accelerated RT.

METHODS AND MATERIALS

A total of 34 patients received hyperfractionated RT between 1997 and 2003. The radiation dose was 39.9-40.5 Gy in 30 fractions. The median treatment time was 22 days with twice-daily involved-field RT. The median follow-up was 4.4 years. Response was assessed <3 months after RT and was classified as a complete response, a complete response-unconfirmed, a partial response, or no response. Local control was defined as maintenance of local complete response, complete response-unconfirmed, or lack of local progression with a partial response. Recurrence or progression outside the RT volume was regarded as distant disease.

RESULTS

The median age was 53 years; 20 patients were men and 14 were women. The initial diagnosis was Stage I-II in 56% and Stage III-IV in 44%. The disease bulk was > or =10 cm in 35% (n = 12). The histologic features at diagnosis were follicular in 11 (Grade 1 in 4, Grade 2 in 3, and Grade 3 in 4), diffuse large B-cell in 14, peripheral T-cell lymphoma in 2, Burkitt-like in 1, mantle cell in 2, natural killer cell in 2, plasmacytoma/lymphoma in 1, and T-cell lymphoblastic in 1. The initial treatment was chemotherapy in 32 patients (94%); 71% were refractory to initial chemotherapy and 29% developed a relapse after an initial response. The RT response was complete in 24% (n = 8), complete, unconfirmed in 26% (n = 9), partial in 47% (n = 16), and none in 3% (n = 1). The local control rate was 73% at 1, 2, and 3 years. Grade 1 dermatitis was the most common side effect.

CONCLUSION

Hyperfractionated RT provided good local control and was well tolerated. This encouraging result requires additional study with comparison to conventional fractionation regimens.

The hyper-CVAD regimen in adult acute lymphocytic leukemia

The regimen of hyperfractionated cyclophosphamide, vincristine, doxorubicin, and dexamethasone (hyper-CVAD) has demonstrated significant activity in adult lymphocytic leukemia (ALL) and in other hematologic malignancies, including Burkitt's disease, lymphoblastic lymphoma, mantle cell lymphoma, and multiple myeloma. This article presents the rationale for the development of this regimen, describes the program, summarizes the results of the large clinical trials developed at the University of Texas M. D. Anderson Cancer Center, and discusses strategies to improve the results.

Repopulation of tumour cells between cycles of chemotherapy: a neglected factor

Repopulation of clonogenic tumour cells during fractionated radiation treatment is recognised as an important factor affecting local control. Given the longer intervals between cycles and longer total duration of treatment, the impact of repopulation is likely to be greater following chemotherapy. Limited data from experimental models suggest that, after chemotherapy, there is a 'lag period', followed by variable but rapid rates of repopulation of tumour cells, possibly accelerating between cycles. Modelling of these properties indicates that after the initial response, accelerated repopulation between cycles can lead to tumour regrowth without any change in the drug sensitivity of the tumour cells. The importance of repopulation may be comparable with that of intrinsic or acquired cellular resistance in determining the effective resistance of tumours to chemotherapy. Biological agents with rapid onset and short duration of action, which can selectively inhibit tumour-cell repopulation, administered between cycles of chemotherapy, might improve the therapeutic index.

Accelerated radiation therapy for primary lymphoma of the brain

BACKGROUND AND PURPOSE

We report the results of a pilot study of the use of accelerated radiation therapy for 10 patients with primary lymphoma of the brain.

MATERIALS AND METHODS

There were five females and five males with a median age of 60 years (range 31-77 years) and a median Karnofsky performance status of 60 (range 30-90). Nine patients underwent biopsies only and one patient underwent a partial resection. The radiation therapy consisted of 50 Gy in 25 fractions over 13 consecutive days to the whole brain including all meningeal surfaces. Two fractions were delivered daily with a minimum interval of 6 h between fractions. Treatment was delivered daily right through the weekend with no treatment interruptions.

RESULTS

To date nine patients have died. The median survival was 17 months. Seven patients have demonstrated definite evidence of recurrence (six in the brain and one with positive CSF cytology only). There have been two autopsies, one of which demonstrated diffusely recurrent tumor and the other showing residual disease and radiation demyelination and small areas of necrosis in the pons which undoubtedly was the cause of death at 5 months. One patient alive and free of recurrence at 69 months has bilateral radiation retinopathy and an undiagnosed degenerative neurologic condition.

CONCLUSION

We conclude that this accelerated schedule of radiation therapy is ineffective in improving survival in primary lymphoma of the brain and is associated with unacceptable increased toxicity.

Regrowth resistance as a likely significant contributor to treatment failure in drug-sensitive neoplastic diseases

Attempts to improve the effectiveness of therapy for neoplastic diseases have largely focused on increasing the cytotoxic efficacy of therapy. While this approach is logical, there is another approach, based on the concept of regrowth resistance, which offers an alternate means of improving treatment outcome. The term "regrowth resistance" refers to the reduction in treatment efficacy resulting from the regrowth of neoplastic cells between courses of therapy or even between doses of radiation therapy. Regrowth resistance is likely to play a significant role in determining the outcome of treatment in rapidly proliferating neoplasms. A reduction in the rate of tumor regrowth would increase the net effectiveness of cytotoxic therapy and would also inhibit the development of resistance to cytotoxic therapies.

Local control of carcinoma of the tonsil by radiation therapy: an analysis of patterns of fractionation in nine institutions

PURPOSE

To investigate the importance to outcome of treatment for squamous cell carcinomas of the tonsillar fossa, of dose per fraction, overall treatment duration, and total dose.

METHODS AND MATERIALS

A collaborative retrospective study was undertaken in nine centers that used widely different dose-fractionation patterns for external beam radiation therapy.

RESULTS

There were 676 eligible cases treated only with photon beams during the years 1976-1985. The probability of local control (of the tonsillar fossa primary) was influenced by both T-stage and N-stage. Significant treatment parameters were total dose and overall treatment duration, but not dose per fraction. Over the range of about 40 to 90% and for a constant overall treatment duration, local tumor control probability increased by nearly 2% for each 1 Gy increase in total dose. For a constant total dose there was a decrease in the probability of local control associated with prolongation of overall treatment duration, presumed to result from accelerated regrowth of surviving tumor clonogens during the course of treatment. If it is assumed that accelerated regrowth occurred at a constant rate and began within 9 days of the start of treatment, an average of 0.53 Gy extra dose per day's extension of treatment would be required to maintain a constant probability of local control. Correspondingly, the probability of local control from a constant dose would be lowered by an average of at least 1% for each day's extension of treatment duration. However, the data are slightly more consistent with an average delay of as long as 30 days before onset of accelerated repopulation, with a consequent increase to an average of 0.73 Gy per day for the value of the compensatory dose. The alpha/beta ratio for this tumor is high enough that the effect of fraction size on the probability of local control can be ignored; a precise estimate is not possible because the best value for beta was close to zero. After accounting for the significant variables studied (treatment time, T-stage, N-stage), the dose-response curves for tumor control were still shallow, suggesting that there are additional causes for heterogeneity of responses among these tumors.

CONCLUSIONS

Total dose is important to treatment outcome: After accounting for other treatment variables, there is about a 2% per Gy increase in probability of tumor control over the ranges of control commonly achieved. Overall treatment duration is important. There is at least a 1% per day decrease in tumor control probability if delivery of a constant total dose is prolonged, requiring a compensatory increase in dose by 0.5-0.7 Gy per day to achieve a constant rate of tumor control. Fraction size is not, of itself, an important factor in the response of primary carcinoma of the tonsil. If a tumor has demonstrated a capacity for metastatic spread to lymph nodes, a higher total dose should be considered to achieve control rates at the primary site equivalent to those in node negative patients. Even after accounting for variables such as tumor stage, total dose, and overall treatment duration, there is sufficient heterogeneity in other undocumented determinants of tumor control to cause the tumor control probability curve to be a shallow function of dose.

The kinetics of repair for sublethal radiation-induced damage in the pig epidermis: an interpretation based on a fast and a slow component of repair

The kinetics of repair of sublethal damage were studied for the epidermis of the pig after beta-irradiation. Total doses were given as 3 or 4 fractions with interfraction intervals that ranged from 0.08 h-8.0 h. Three different methods were used to analyse the results: (a) the incomplete repair (IR)-model; (b) theoretical curves showing changes in iso-effect doses, based on the IR-model, were compared with experimentally derived iso-effect doses; and (c) the percentage unrepaired dose was calculated using differences in the iso-effect doses (ED50) for moist desquamation for the various interfraction intervals. Although mono-exponential kinetics are assumed in the IR-model, the half time for repair (T1/2) was not unique, but increased with the increase in interfraction interval. For 3 fractions, with an interfraction interval of 0.25 h, a T1/2 value of 0.27 h (0.19-0.43 h) was obtained, while an interval of 4 h gave a significantly higher T1/2 value of 1.40 h (0.01-2.28 h). Similar results were obtained with the 4-fraction schedule. The experimentally derived iso-effect doses for short interfraction intervals were above the theoretical curve indicating a faster rate of repair than anticipated, based on the IR-model. Iso-effect doses for longer interfraction intervals were below the theoretical curve suggesting a slower rate of repair. For both the 3- and 4-fraction data the percentage unrepaired dose, as a function of the interfraction interval, was significantly better fitted by a bi-exponential equation than a mono-exponential equation (p less than 0.05). Two distinct components of repair were resolved suggesting a fast and a slow component of repair with T1/2 values of approximately 0.14 h and of approximately 2.7 h, respectively. Thus all three methods of analysis suggest that the repair of sublethal radiation-induced damage to pig skin are better explained by bi-exponential rather than mono-exponential kinetics.

Influence of treatment time on the response of rat rhabdomyosarcoma R1H to fractionated irradiation

The aim of this work was to study influence of overall treatment time on response of an experimental tumour to fractionated radiotherapy. Rhabdomyosarcoma R1H of the rat was treated by fractionated irradiation applying 30 fractions in different overall treatment times of 10, 18.5, 29, 39, and 67 days. The doses per fraction ranged from 1.50 to 2.67 Gy. Tumour response was assessed by net growth delay and by an in vitro colony assay. The results indicate that repopulation of the tumour with clonogenic tumour cells slowed down during treatment leading to an average doubling time of clonogenic tumour cells of 8 +/- 1 days, which is considerably longer than that of unirradiated tumours (3 days). The repopulation rate seemed to increase after about 3 weeks of treatment with a doubling time of tumour clonogens of 5 +/- 1 days, which was still lower than in control tumours.

Repopulation in human tumors. A biological background for fractionation in radiotherapy

Tumor repopulation plays a major role in the response of tumors to radiotherapy or chemotherapy but its degree varies widely among human tumors. Its mechanism has not been elucidated, however it is unlikely that repopulation can be simply explained by the greater availability of oxygen and nutrients resulting from the reduction in tumor mass. In normal tissues, such as bone marrow, treated cells release stimulating factors which are able to recruit quiescent cells into proliferation. Similar mechanisms have been observed in experimental tumors and their existence is probable in human tumors. A low pretreatment proliferation index and low cell density indicate that the control mechanisms which regulate cell proliferation in normal tissues are still partially operating in these tumors. In such tumors an increase in the level of cell proliferation during or after treatment has often, but not always, been observed. There is not yet a clear guide for predicting tumor proliferation rate under treatment and further investigations are needed.

Accelerated radiotherapy followed by chemotherapy for locally recurrent small-cell carcinoma of the lung. A phase II study of Cancer and Leukemia Group B

Recurrent or persistent small-cell carcinoma of the lung (SCCL) after chemotherapy (CT) alone has shown a poor response to conventional salvage radiotherapy (RT). Accelerated RT is judged more effective than conventional RT for rapidly growing tumors such as SCCL. The objectives of this study were: to determine the tolerability of accelerated RT; and to test the ability of accelerated RT plus CT to achieve local tumor control (LTC) of SCCL recurrent after CT. Patients whose localized tumor was not controlled were selected from Arm III of the Cancer and Leukemia Group B (CALGB) protocol 8083 (Proc. ASCO 2:230, 1984) as eligible for this study. The program of accelerated RT consisted of the delivery of 50.1 Gray (Gy) in 30 fractions over a period of 21 days to the chest. New chemotherapy different from the first began 2 weeks after the completion of RT and was repeated every 3 weeks for 18 months (M). Of 29 potentially eligible patients with locally recurrent SCCL after the first line CT alone from Arm III of the CALBG protocol 8083, 12 were enrolled initially in this study. The analysis of LTC included 11 patients excluding one patient who died 4 weeks after the start of RT from liver metastases. The LTC achieved was as follow: complete remission in 8/11 (72%) and partial remission in 3/11 patients. None of the patients was converted to CR by subsequent chemotherapy. Survival ranged from 2 to 20 M, with a median survival time of 6 M. Tolerance to the subsequent CT, normal tissue reaction to accelerated RT, and the theoretical advantage of accelerated RT over conventional RT for SCCL were evaluated.

Potential for increasing the differential response between tumors and normal tissues: can proliferation rate be used?

Rapid proliferation of malignant cells has not previously been emphasized as a major source of failure to control tumors. Evidence is presented that the effective doubling times of clonogenic cells in human tumors during multifraction radiotherapy are in the range of a few days, that is, similar to the pre-treatment Potential Doubling Times and much shorter than Volume Doubling Times. Evidence from animal tumor studies leads to the same conclusion. Accelerated fractionation should be considered for individual human tumors whose LI is measured (e.g., by flow cytometry and the BUdR antibody) and found to be too high.

Co-cultured transformed and untransformed C3H 10T1/2 cells: preferential killing of transformed cells by low dose rate irradiation

Co-cultured C3H 10T1/2 cells, in which transformed cells were grown as discrete colonies on top of density-inhibited monolayers of untransformed cells, were used to determine the potential usefulness of a short term assay system for the study of differential radiation effects as they may apply to cell populations with differing turnover rates, but in close physical contact. Mixed cultures were exposed to either an acute dose of 20 Gy of Cs-137 gamma rays, or a dose of 72 Gy delivered at a low dose rate of 0.34 Gy per hour. These treatments resulted in approximately equal levels of damage to the untransformed monolayers. At 10-day intervals after treatment, representative flasks from each dose group were examined for evidence of degeneration, and subsequent regrowth, if any, of the transformed colonies and untransformed monolayers. For comparable amounts of visible damage to the untransformed monolayers, the low dose rate irradiation was more effective at delaying regrowth of, or even eradicating, transformed colonies. These results are consistent with expectations based on previous results in which dose-rate or dose fractionation isoeffect curves were compared for these two cell types, grown independently in plateau phase cultures.

Rapid hyperfractionated radiotherapy. Clinical results in 178 advanced squamous cell carcinomas of the head and neck

The authors present a series of 178 patients with Stage III or IV squamous cell carcinoma of the head and neck treated by rapid irradiation using multiple and small fractions per day. An initial group of 91 patients (G1) received a total dose of 72 Gy in 80 sessions and 10 days, according to the following split course schedule: J1 to J5, 36 Gy in 40 sessions, eight daily fractions of .9 Gy separated by 2 hours; J6 to J20, rest period; J21 to J25, same as in J1 except that the spinal cord was shielded. This protocol was altered for the following 87 patients (G2) by lessening the total dose to 60 to 66 Gy and the number of fractions to 60. The rest period was lengthened to 4 weeks. All patients but five completed the whole program and the minimal follow-up period was 24 months. At the end of irradiation, 121 patients achieved a total remission, but local recurrences occurred in 56%. Moreover, acute intolerance was considered as severe in 34% of G1 patients, and included extensive mucosal necrosis and bleeding. Although this rate was significantly reduced in G2 patients, late complications were observed in 20 of the 25 survivors, and included trismus, cervical sclerosis, and recurrent laryngeal edema. The crude survival rate is 13% at 2 years. Although this study was not randomized, this particular type of accelerated and hyperfractionated combination of irradiation did not really improve the clinical results in advanced carcinoma of the head and neck. Other schedules and probably other tumors, less extended, should be tested.

Biologic basis for altered fractionation schemes

Conventional is commonly not universally correct, and so with dose fractionation in radiotherapy. Fractionation spares slowly responding tissues more than tissues and tumors that show an early response, suggesting that therapeutic gains may be further increased by reducing fractional doses below 1.8 to 2 Gy. The overall duration of a course of radiotherapy should not be the same for all tumors in all sites because the time of onset of regeneration after the start of radiotherapy varies from tissue to tissue and among tumors. Although growth kinetics and dose-response characteristics are known to vary, inability to identify and quantify them prospectively frustrates rational selection of patients for individualized fractionation regimens. In general, curative radiotherapy should be delivered in as short an overall time as possible using the smallest practical dose per fraction. Although 2 Gy, 5 times per week may be a reasonable "average" treatment, greater individualization should be a research goal.

What is known about tumour proliferation rates to choose between accelerated fractionation or hyperfractionation?

Repopulation of surviving clonogenic tumour cells during fractionated radiotherapy is one of the crucial factors determining cure probability in radiotherapy. Clinical and experimental data suggest that repopulation rates vary considerably between different tumours but may be similar to the cell production rates in the untreated tumour. For those tumours which repopulate fast, such as squamous cell carcinomas, bladder cancer and colorectal carcinomas, accelerated fractionation may be indicated.

Acute normal tissue tolerance to 7 day per week accelerated fractionation

Fifty-six sites in 49 patients were irradiated by a 7 day/week accelerated fractionation schedule to conventional tumor doses. Daily doses were 180 to 200 rad in 85% of sites. Patients were analyzed for normal tissue tolerance. A 7 day/week accelerated fractionation appears clinically tolerable at 180 rad per fraction.

Fractionation in radiation therapy: theoretical basis, experimental, and clinical studies

In summary, there is a growing body of clinical evidence along with laboratory studies that in rapidly growing tumors, small doses per fraction given over a short time interval, as in hyperfractionation or accelerated fractionation, may increase the therapeutic ratio. For slowly growing tumors, small doses per fraction may still have an advantage, but they need not be compressed into a short time interval. Finally, for certain "radioresistant" tumors, with a large shoulder to the survival curve, there may be a clinical advantage to larger doses per fraction, but this will have to be weighed against the potential long-term chronic toxicity which increases rapidly with large doses per fraction.

Keynote address: the scientific basis of the present and future practice of clinical radiotherapy

At mid-century radiotherapy was more an art than a science, but is presently based on radiobiological parameters and cell kinetics. This close interaction between basic scientific principles and clinical practice has been made possible because one can correlate quantitatively doses of irradiation with observed responses. First, a short historical review will be made because it gives a perspective for the understanding both of progress made and prevailing misconceptions. The important radiobiological parameters and cell kinetics will then be discussed in some detail to demonstrate that they should be thoroughly understood in their relationship to radiotherapy. The overall treatment planning must be based on the clinical applications of the main radiobiological parameters. The combined treatment with surgery, either pre- or postoperatively, and multiple daily fractionations will be used as examples. The teaching of radiobiology should be considerably expanded, not only for its own scientific merit but also to show how it applies to clinical situations. This should be reflected in the expansion of the board examination.

Accelerated fractionation vs hyperfractionation: rationales for several treatments per day

Treatment with several doses per day offers the prospect of a significant therapeutic gain using readily available low LET beams. These regimens can be classified as either accelerated fractionation or hyperfractionation according to their rationales. With accelerated fractionation a conventional number of dose fractions is delivered in a significantly shortened overall treatment time in order to reduce the opportunity for tumor cell regeneration during treatment. With hyperfractionation, on the other hand, a large number of significantly reduced dose fractions is used to give a greater total dose in a conventional overall treatment time. The rationale for this strategy is threefold: 1) increased opportunity for tumor cell redistribution and reoxygenation between dose fractions: 2) a possibly lower oxygen enhancement ratio with small incremental doses; and 3) different sparing of late reacting normal tissues with small dose fractions. A review of the published clinical experience with multiple fractions per day treatment reveals few studies of either pure accelerated fractionation or hyperfractionation since both are limited by acute normal tissue reactions. This has led to a variety of hybrid regimens, some of which have no clear rationale. The choice between accelerated fractionation and hyperfractionation is determined by the regenerative capability of tumor clonogens during treatment. A method of selection based on potential doubling times is presented.

Non-effectivity of roentgen irradiation in an in vivo rapidly growing murine lymphosarcoma

The survival time of mice bearing a rapidly growing murine lymphosarcoma (cell doubling time 9.6 h) is hardly influenced by roentgen irradiation with fractions of 2 Gy, not even if delivered according to superfractionated schedules. Growth kinetic measurements reveal an almost complete mitotic block 6 h after exposure, followed by a large overshoot in cell proliferative activity. The dose-response curve for in vivo irradiated cells has a D0 of 1.9 and lacks a shoulder. Hence, repair of sublethal damage must be of restricted significance. The extremely strong proliferative capability appears to lead to rapid repopulation of the tumour. Typically, surviving cells show a high content of RNA and protein.

Radiosensitivity of human B-lymphocytic lymphomas in vitro

The radiosensitivity of four human B-lymphocytic lymphoma cell lines has been studied. For all lines the Do values were in the range 1.3 to 1.8 Gy. None of the survival curves had an appreciable shoulder. The extrapolation numbers varied in the range 1.0 to 1.2. Thus, the cell lines had a low capacity to accumulate sublethal damage. Furthermore, split dose experiments showed that no line had the capacity to repair sublethal damage. Taken together with earlier published experimental and clinical observations the results indicate that the use of an increased number of fractions in radiotherapy of B-lymphocytic lymphomas might be of great benefit to the patients. A schedule with an increased number of fractions will probably be as efficient in killing the tumour cells as previously used schedules but the tolerance of adjacent normal tissue will probably increase.

Tumor radioresistance in clinical radiotherapy

Tumor radioresistance in clinical radiotherapy implies failure to achieve loco-regional disease control with radiation doses producing an acceptable degree of morbidity. Such radioresistance may be a result of many different causes (biological and technical) which are reviewed in terms of possible remedial actions. Dose response relationships for human cancers suggest that in many sites, tumors are heterogenous with respect to their cure-limiting characteristics. The case is developed that unless the predominant cure-limiting factor can be predicted, little benefit may be seen in trails of new treatment strategies using heterogeneous tumor populations. The fundamental problem of clinical radioresistance is therefore perceived as the inability to predictively identify its cause in the individual patient.

Optimum fractionation for irradiation of carcinoma of the bladder. Experiments based on one case

A patient with multiple metastases of transitional cell carcinoma of the bladder to the skin and subcutaneous tissues proved a model where most of the biologic and host-related factors were constant. The effects of varying the physical factors of radiation dose and time were observed. The results indicate that fractionated irradiation at intervals of 48 h was more effective than at intervals of 24 h and that superfractionation (intervals of 5 h) was not an effective method for the treatment of this tumor. The possible reasons for this effect and the implication for the treatment of bladder carcinoma are discussed.

Radiation therapy in Burkitt's lymphoma. Long term results

In 40 of 61 irradiated patients with Burkitt's lymphoma the long term results were evaluated. Five patients were primarily irradiated while the remaining patients had late recurrences or were chemotherapy failures. Seven patients were alive and free of recurrence after 6 months and 4 after 24 months.