Physics contribution
Forward or inversely planned segmental multileaf collimator IMRT and sequential tomotherapy to treat multiple dominant intraprostatic lesions of prostate cancer to 90 Gy

Presented October 1998 at the 40th Annual Meeting of ASTRO, Phoenix, Arizona.
https://doi.org/10.1016/S0360-3016(01)01643-1Get rights and content

Abstract

Purpose: To investigate the technical feasibility of using forward or inversely planned segmental multileaf collimator (SMLC) intensity-modulated radiotherapy and sequential tomotherapy (ST) to escalate to a dose of 90 Gy to multiple dominant intraprostatic lesions within the prostate gland while delivering a dose of 75.6 Gy to the remaining prostate.

Methods and Materials: A selected case with one dominant intraprostatic lesion located at the left base and a second dominant intraprostatic lesion at the right apex of the prostate was planned using three different intensity modulation techniques. Two plans were generated with inverse treatment planning, using either SMLC or ST with a special multivane collimator. The third plan also employed SMLC but was generated using forward planning. All three plans were compared based on dose-volume histograms, isodose distributions, and doses to sensitive normal structures.

Results: All three plans meet and exceed the desired dose constraints, limiting doses to the rectum and bladder to an estimated RTOG Grade 2 complication rate of <10%. The ST plan achieved the best dose conformality, whereas the inverse SMLC plan gave the lowest dose to the rectal wall, and the forward SMLC plan obtained the best dose homogeneity inside the targets.

Conclusions: Using any of the three intensity-modulated techniques, it is technically feasible to concurrently treat multiple selected high-risk regions within the prostate to 90 Gy and the remaining prostate to 75.6 Gy, while keeping the doses to the rectum and the bladder significantly lower than those associated with a Grade 2 complication rate of 10%.

Introduction

Several clinical studies (1) of radiation therapy for prostate carcinoma have indicated a positive correlation between high radiation dose and improved local control. A retrospective review conducted by Pollack and Zagars (2) for 938 patients with prostate cancer treated with a radiation dose from 60 Gy to 78 Gy indicated that the dose to the prostate is a critical factor for biochemical control of prostate cancer. A study of 743 patients from Memorial Sloan-Kettering Cancer Center (3) also demonstrated that both the initial clinical response and the long-term tumor control were dose dependent.

By increasing the radiation dose, however, the risk of developing late gastrointestinal and genitourinary complications also increases 1, 4, 5. Sandler et al. (4) analyzed the chronic rectal morbidity observed in a large group of patients undergoing radiotherapy for prostate cancer. They found that dose was the most significant factor associated with risk of Grade 3 or 4 gastrointestinal complications. A group from the Netherlands (1) analyzed data from 130 patients with localized prostate cancer who were treated with conformal radiotherapy in a dose escalation protocol (70–78 Gy, 2 Gy/fraction). Their data indicated that severe rectal bleeding is related to both radiation dose and volume. According to this study, patients with 40% of the rectal wall volume receiving at least 65 Gy, 30% of the rectal wall receiving at least 70 Gy, or more than 5% of the rectal wall receiving 75 Gy are at higher risk of developing severe rectal bleeding than patients in whom these volumes are smaller. A study from Fox Chase (5) concluded that treatment techniques that reduce the total dose to the anterior rectal wall have reduced the incidence of late rectal morbidity. Recently, Zelefsky et al. (6) compared acute and late toxicity of high-dose (81 Gy) radiation for prostate cancer, delivered either by conventional three-dimensional conformal radiotherapy or intensity-modulated radiotherapy (IMRT). The results showed that the 2-year actuarial risk of Grade 2 bleeding was 2% for IMRT and 10% for conventional three-dimensional conformal radiotherapy.

Dose is a critical factor for the cure of prostate cancer, yet it is the most significant factor associated with the complication rate. Because of the anatomic relationship between the prostate and the rectum, further dose escalation also requires new treatment schemes. To address both issues, new treatment techniques that can generate high-dose conformality are necessary. We previously (7) described a treatment scheme that can be used to deliver 90 Gy to a single dominant intraprostatic lesion (DIL) and concomitantly treat the entire prostate to 73.8 Gy while keeping the dose to the rectum and bladder below tolerance. In this treatment scheme, 18 segmental multileaf collimator (SMLC) shapes were manually designed at seven gantry angles, requiring significant treatment planning expertise. A computer-optimized inverse planning technique using intensity modulation might be expected to produce a similar or improved treatment plan with less effort.

In this article, we investigate a rather challenging case of prostate carcinoma with two DILs, one located at the left base and the second one located at the right apex of the prostate. This case represents a difficult situation, because one DIL involves the most distal portion of the gland on the right, and the other lesion involves the most proximal portion of the gland on the left. The goal is to treat the DILs to 90 Gy while concurrently treating the entire prostate to 75.6 Gy (1.8 Gy higher than the previous paper described [7]). The planning techniques used for this case include an inversely planned SMLC IMRT, a sequential tomotherapy (ST) IMRT, and a forward planned SMLC IMRT.

Section snippets

Prescription

The patient was positioned supine, immobilized in an alpha cradle (MED-TEC, Orange City, IA), and a treatment planning CT scan was obtained at 0.5-cm intervals using a urethrogram to assist in defining the inferior border of the prostate gland (8). The patient was immobilized, given a CT scan, and simulated with the rectum empty and the bladder full. Oral hydration was performed, and a “Fleet” enema was administered 30 min before the CT scanning procedure to empty the rectum and fill the

Results

Figures 6a–c show the dose distributions for the different plans, displayed in axial, coronal, and sagittal images. The isodose lines are displayed on an absolute dose scale ranging from 30 Gy to 90 Gy. The contour of one of the DILs, located at the right apex, is shown in solid orange. The contour of the PTV is shown in solid red in Figs. 6a and 6b, whereas in Fig. 6c the PTV is shown as a pink line. This inconsistency is due to the use of different treatment planning systems. With visual

Discussion

This study is based on a selective, but representative, difficult case, where one DIL is located at the most distal portion of the gland on the right, and the other DIL is located at the most proximal portion of the gland on the left. Based on our clinical experience and on a number of our previous studies 8, 9, 12, which compared various treatment techniques on a number of patients, we found that the conclusion about the relative merit of a treatment technique can be generalized to a majority

Conclusions

We have demonstrated that by using any of the three different intensity modulation schemes, it is possible to concurrently deliver 90 Gy to selective high-risk regions within the prostate gland while treating the rest of the gland to 75.6 Gy. For all three plans, the rectum and bladder doses were significantly lower than those that might be associated with a Grade 2 complication rate of 10%. It remains to be determined whether this treatment scheme will improve the local control and ultimately

Acknowledgements

The authors would like to thank Dr. Mark Carol, Bruce Curran, and Robert Hill from NOMOS Corporation for valuable discussion and help in implementing IMRT at our clinic. The authors would like to thank Dr. Jenny Hai for providing the results of her study on the use of implanted markers for prostate localization.

References (22)

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This research was partially supported by NOMOS Corporation.

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